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The medullary projections of afferent bronchopulmonary C fibres in the cat as shown by antidromic mapping
Abstract
1. The activity of eighty-seven bronchopulmonary vagal afferent neurones with unmyelinated axons (C fibres) was recorded extracellularly in the nodose ganglia of decerebrate, paralysed and artificially ventilated cats. On the basis of their response latencies following the right atrial injection of capsaicin or phenyldiguanide, the cells were classified as having their receptor endings within the reach of pulmonary (latency less than 3.5 s) or bronchial (latency above 3.5 s) circulation. 2. Pulmonary and bronchial receptor cells differed only slightly in their response characteristics (firing rate, burst duration) and the conduction velocity of their peripheral axons. Bronchial C fibres represented about 70% of the population studied. 3. The medullary distributions of the central branches of six pulmonary and six bronchial C fibres were determined by means of the antidromic mapping technique. The two receptor subtypes did not differ in their central projection patterns. 4. Rostral to the obex, the central branches of the bronchopulmonary C fibres were localized within the medial portions of the nucleus tractus solitarii (NTS) and area postrema, and were most densely distributed along the borders of the parvicellular subnucleus of the NTS. Caudal to the obex, the most dense branching was found in the dorsal portion of the commissural subnucleus. Projections to the contralateral NTS were found, but these were of a much lower density. 5. The central distribution of bronchopulmonary C fibres is compared to the projection patterns of vagal and glossopharyngeal afferents of other modalities that are involved in respiratory and cardiovascular control. This is discussed in relation to the concept of a modality-specific organization of the NTS.
... The medullary projections of afferent pulmonary C-fibres and bronchial C-fibres were antidromically mapped for the first time by Kubin et al. (1991). The two receptor subtypes did not differ in their central projection pattern. ...
... Pulmonary C-fibres have scant or no input to the lateral, ventrolateral or ventral subnuclei unlike their myelinated counterparts, which have some degree of projection to these subnuclei (Kalia & Richter 1985). The technically difficult study of Kubin et al. (1991) successfully tracked a small sample of 12 fibres out of an estimated 10,000 bronchopulmonary C-fibre population in the cat. However there is reason to believe that this sample is representative. ...
... Injections of cobalt abolish the reflex if applied focally to this site (Bonham & load 1991). The anatomical location of this site agrees strikingly with the caudal site described by Kubin et al. (1991); however whether a rostral projection site is present in the rat is unknown. Verberne and Guyenet (1992) have demonstrated that a bilateral block of excitatory amino acid transmission in the lateral commissural nuclei of the rat attenuates the hypotension of the pulmonary chemoreflex. ...
This thesis explores the central organization of the reflex associated with pulmonary C-fibre stimulation. In particular the circulatory and respiratory changes associated with the pulmonary chemoreflex are analyzed. Electrophysiological techniques are used to dissect the medullary networks involved in elaborating the cardiac vagal changes that occur in response to phenylbiguanide administered into the pulmonary circulation. The primary objective is to explain why the cardioinhibition of the pulmonary chemoreflex is unaffected by central respiratory activity. Results from experiments performed on cat, rat, rabbit and dogfish are described. Two different populations of cardiac vagal preganglionic neurones are described in the cat. One population displays respiratory related activity and the other, tonic non- respiratory related activity. Both populations are implicated in the pulmonary chemoreflex. The results of vagal stimulation experiments, performed with anodal block, demonstrate that both populations of cardiac vagal preganglionic neurones have chronotropic action in the cat, rat and rabbit. A hypothesis, based on the existence of two populations of cardioinhibitory vagal preganglionic neurones, is advanced to interpret certain features of the vagal control of the heart which hitherto, have eluded explanation. A philosophy is presented that endeavours to examine the possible evolutionary physiology of such a system.
... Functional studies suggest that pulmonary C fibers mediating the pulmonary C fiber reflex -consisting of rapid shallow breathing (which may be preceded by apnea), bradycardia, and hypotension -act on yet another population of target neurons. Based on functional studies, some of these target neurons are located in a discrete region caudal to the obex within the dorsomedial aspect of the commissural nucleus of the nTS [72,73]. Antidromic mapping studies in cats suggest that bronchial and pulmonary C fibers also innervate a region rostral to the obex in the medial portions of the nTS and area postrema, with dense innervation of the borders of the gelatinosus nucleus (sg) (or parvicellular nucleus) throughout its rostrocaudal extent [73]. ...
... Based on functional studies, some of these target neurons are located in a discrete region caudal to the obex within the dorsomedial aspect of the commissural nucleus of the nTS [72,73]. Antidromic mapping studies in cats suggest that bronchial and pulmonary C fibers also innervate a region rostral to the obex in the medial portions of the nTS and area postrema, with dense innervation of the borders of the gelatinosus nucleus (sg) (or parvicellular nucleus) throughout its rostrocaudal extent [73]. Careful anatomical studies indicate further that -in addition to the medial, gelatinosus, and commissural nuclei -unmyelinated C fibers project to the dorsal subnucleus [74]. ...
... In contrast to SAR fibers, no bronchopulmonary C fiber projections have been observed in the intermediate or the ventrolateral subnuclei, consistent with the observation that neurons excited by electrical stimulation of nonmyelinated pulmonary vagal fibers are restricted to medial regions of the nTS [75]. Collectively, these studies have led to the proposal of a 'modality-specific (and thus functional) organization of the vagal primary afferent projections to the nTS' [73]. ...
Bronchopulmonary inflammation, such as that associated with asthma, activates afferent neural pathways. We recently demonstrated that localized inflammation in the lungs, induced by intratracheal administration of ovalbumin in ovalbumin-preimmunized mice (an animal model of asthma) results in activation of the dorsolateral part of the nucleus of the solitary tract, a major target of vagal afferent fibers innervating the lungs and airways. Activation of the nucleus of the solitary tract was evident in the absence of activation of the area postrema, a circumventricular organ, consistent with the hypothesis that localized inflammation in the bronchopulmonary system can signal to the central nervous system via specific neural pathways, in the absence of circulating proinflammatory mediators. The pattern of brain activation in ovalbumin-challenged mice differs from the pattern of activation in mice challenged with heat-killed Mycobacterium vaccae, suggesting that qualitative aspects of bronchopulmonary inflammation determine the overall pattern of brain activation. The mechanisms through which localized bronchopulmonary inflammation signals to the central nervous system is poorly understood, but appears to involve both vagal and spinal afferent pathways. In this chapter, we review our current understanding of the anatomical pathways through which localized inflammation in the bronchopulmonary system influences central nervous system function.
... Aluch less is known about the central pathways and neurotransmitters whereby stimulation of these endings is transformed into integrated ventilatory, bronchomotor and cardiovascular changes. In the cat, Kubin et al. (1991) localized central axonal projections of 'bronchopulmonary' C fibre afferents to the caudomedial comimissural nucleus of the nucleus tractus solitarii (NTS) caudal to the obex and also in the parvicellular nucleus of the NTS rostral to the obex; the terminal afferents were located by stimulating in the NTS to antidromically activate cell bodies of the afferent fibres in the nodose ganglion; the cell bodies were identified by increases in activity in response to right atrial injections of phenylbiguanide or capsaicin (Kubin et al. 1991). We have previously shown in the rat that neurones limited to the caudomedial commissural NTS are essential for the full expression of the rapid shallow breathing produced by activation of C fibre endings; microinjection of picomolar amounts of the broad spectrum excitatory amnino acid, DL-homocysteic acid, into this site mimicked the rapid shallow breathing elicited by right atrial injections of phenylbiguanide, and transient interruption of synaptic transmission by microinjection of cobalt chloride into the site blocked it (Bonham &.Joad, 1991). ...
... Aluch less is known about the central pathways and neurotransmitters whereby stimulation of these endings is transformed into integrated ventilatory, bronchomotor and cardiovascular changes. In the cat, Kubin et al. (1991) localized central axonal projections of 'bronchopulmonary' C fibre afferents to the caudomedial comimissural nucleus of the nucleus tractus solitarii (NTS) caudal to the obex and also in the parvicellular nucleus of the NTS rostral to the obex; the terminal afferents were located by stimulating in the NTS to antidromically activate cell bodies of the afferent fibres in the nodose ganglion; the cell bodies were identified by increases in activity in response to right atrial injections of phenylbiguanide or capsaicin (Kubin et al. 1991). We have previously shown in the rat that neurones limited to the caudomedial commissural NTS are essential for the full expression of the rapid shallow breathing produced by activation of C fibre endings; microinjection of picomolar amounts of the broad spectrum excitatory amnino acid, DL-homocysteic acid, into this site mimicked the rapid shallow breathing elicited by right atrial injections of phenylbiguanide, and transient interruption of synaptic transmission by microinjection of cobalt chloride into the site blocked it (Bonham &.Joad, 1991). ...
... The differences in the peak increases agree with findings in the cat by Kubin and co-workers, who used antidromic mapping in the NTS to identify termination of C fibre neurones, which were recorded in the nodose ganglion and identified by their responses to phenylbiguanide or capsaicin. They showed that right atrial injections of capsaicin or phenylbiguanide produced increases in the activity of C fibre neurones supplied by the pulmonary circulation (as established by onset of the increases of < 3-5 s from the time of the injection) that were twice as large as the increases in activity of C fibre neurones supplied by the systemic circulation (as established by onset of the increases by > 3-5 s from the time of the injection) (Kubin et al. 1991). While our findings suggest that the neurones in the NTS region explored in this study and shown previously to contain neurones necessary for the production of rapid * Pre-KYN S Post-KYN T * QUIS NMDA Figure 10. ...
1. We sought first to determine whether neurones in caudomedial aspects of commissural nucleus tractus solitarii (NTS) received input from cardiopulmonary C fibre endings supplied by the pulmonary versus systemic circulation. We then examined the role of N-methyl-D-aspartate (NMDA) and non-NMDA receptors in transmitting cardiopulmonary C fibre input to such NTS neurones. 2. Extracellular NTS unit activity, phrenic nerve activity and arterial blood pressure were recorded in urethane-anaesthetized rats. Unit responses to right atrial and left ventricular phenylbiguanide injections were compared in rats with arterial baroreceptors, carotid chemo-receptors and subdiaphragmatic vagal inputs eliminated. Right atrial phenylbiguanide injections produced greater peak responses (27 +/- 11 impulses s-1) than did left ventricular injections (11 +/- 3 impulses s-1) (n = 9). 3. The non-NMDA receptor agonist quisqualic acid (QUIS) and NMDA were ionophoresed onto NTS neurones that were synaptically activated by right atrial phenylbiguanide injection. Responses were compared before and during ionophoresis of the non-NMDA receptor antagonist 2,3-dihydroxy-6-nitro-7-sulphamoylbenzo(f)quinoxaline (NBQX), the NMDA receptor antagonist DL-2-amino-5-phosphovaleric acid (AP5), or the broad spectrum antagonist kynurenic acid (KYN). 4. NBQX, which blocked QUIS-but spared NMDA-evoked responses, significantly attenuated synaptic activation by 65% (n = 9). AP5, which blocked NMDA- but spared QUIS-evoked responses, did not significantly diminish synaptic activation (11%; n = 7). KYN, which blocked QUIS- and NMDA-evoked responses, decreased synaptic activation by 70% (n = 9). 5. The results suggest that input from cardiopulmonary C fibre endings, primarily supplied by the pulmonary circulation, is transmitted to this commissural NTS region largely via non-NMDA receptors.
... Distinct airway sensors send their axons to the separate NTS subregions. In particular, slowly adapting receptors terminate rostral to obex, primarily in dorsal, lateral, and ventral subnucleus (Berger and Dick 1987;Davies et al. 1987); irritant rapidly adapting receptors terminate mostly in the commissural and medial subnucleus caudal to the obex (Davies and Kubin 1986;Kubin and Davies 1988;Lipski et al. 1991); and bronchopulmonary C fibers terminate in the medial and parvicellular subregions as well as caudally in the dorsal commissural NTS (Kubin et al. 1991). Most recently, novel sensory receptors located primarily in the large airways have been identified and termed "cough receptors" (Mazzone et al. 2005) and are proposed to project to medial, intermediate, central, dorsomedial, and ventrolateral NTS subnuclei (Canning and Mori 2010). ...
... The micropipette tips were directed to the commissural subnucleus of the cNTS (Claps and Torrealba 1988), and this area was significantly covered by our microinjections in all cats (Fig. 1). The cNTS is an important site for pulmonary C-fiber (Kubin et al. 1991) and rapidly adapting receptor (Davies and Kubin 1986;Kubin and Davies 1988;Lipski et al. 1991) interneurons. The nonspecific activity of kyn supports the concept that second-order interneurons mediating the sensory effects of these receptor types are not important in the neurogenesis of mechanically induced tracheobronchial cough in the anesthetized cat. ...
The importance of neurons in the solitary tract nucleus (NTS) in the production of coughing was tested by microinjections of the non-specific glutamate receptor antagonist kynurenic acid (kyn; 100 mM in artificial cerebrospinal fluid) in 15 adult spontaneously breathing anesthetized cats. Repetitive coughing was elicited by mechanical stimulation of the intrathoracic airway. Electromyograms (EMG) were recorded from inspiratory parasternal and expiratory transversus abdominis muscles (ABD). Bilateral microinjections of kyn into the NTS rostral to obex (55±4 nl total in two locations; n=6 or 110±4 nl total in four locations; n=5), primarily the ventrolateral subnucleus, reduced cough number and expiratory cough efforts (amplitudes of ABD EMG and maxima of esophageal pressure) compared to control. These microinjections also markedly prolonged the inspiratory phase, all cough related EMG activation, and the total cough cycle duration as well as some other cough related time intervals. In response to microinjections of kyn into the NTS rostral to the obex respiratory rate decreased, and there were increases in the durations of the inspiratory and post-inspiratory phases and mean blood pressure. However, bilateral microinjections of kyn into the NTS caudal to obex as well as control vehicle microinjections in the NTS location rostral to obex had no effect on coughing or cardiorespiratory variables. These results are consistent with the existence of a critical component of the cough rhythmogenic circuit located in the rostral ventral and lateral NTS. Neuronal structures of the rostral NTS are significantly involved specifically in the regulation of the cough magnitude and phase timing.
... Intriguingly, the dorsal respiratory group, which contains second order neurons that control respiration, is located in the lateral NTS (Saether et al., 1987;Speck and Feldman, 1982). Similar experiments in Npy2r-ires-Cre mice revealed that Npy2r neurons instead innervated a different NTS region, with fibers predominantly emerging in the medial posterior aspects of the vagal NTS and area postrema, regions known to receive pulmonary C fiber input (Kubin et al., 1991(Kubin et al., , 2006. We quantified innervation density along the medial-lateral axis of the NTS and in the area postrema by calculating the area of red fibers and normalizing to the area of green fibers. ...
... Npy2r neurons are largely slow-conducting C fibers, express the capsaicin receptor TRPV1, and respond to capsaicin in single neuron imaging experiments involving acute cultures of nodose/jugular ganglia. Centrally, Npy2r neurons target a medial posterior region of the NTS that receives pulmonary C fiber input (Kubin et al., 1991), and this region is distinct from the innervation zone of P2ry1 neurons. Optogenetic activation of Npy2r neurons caused rapid and shallow breathing, a respiratory effect reminiscent of certain pulmonary defense responses (Coleridge and Coleridge, 2011). ...
Breathing is essential for survival and under precise neural control. The vagus nerve is a major conduit between lung and brain required for normal respiration. Here, we identify two populations of mouse vagus nerve afferents (P2ry1, Npy2r), each a few hundred neurons, that exert powerful and opposing effects on breathing. Genetically guided anatomical mapping revealed that these neurons densely innervate the lung and send long-range projections to different brainstem targets. Npy2r neurons are largely slow-conducting C fibers, while P2ry1 neurons are largely fast-conducting A fibers that contact pulmonary endocrine cells (neuroepithelial bodies). Optogenetic stimulation of P2ry1 neurons acutely silences respiration, trapping animals in exhalation, while stimulating Npy2r neurons causes rapid, shallow breathing. Activating P2ry1 neurons did not impact heart rate or gastric pressure, other autonomic functions under vagal control. Thus, the vagus nerve contains intermingled sensory neurons constituting genetically definable labeled lines with different anatomical connections and physiological roles.
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... This observation was rather surprising insofar as BötC and preBötC neurons are generally believed to be critically involved in respiratory rhythm generation (Smith et al., 1991;Gray et al., 1999) and a variety of respiratory afferent pathways relayed through SolC and SolM alter respiratory frequency (Kubin et al., 2006). These include peripheral chemoreceptor input from the carotid body (Finley and Katz, 1992;Ciriello et al., 1994), broncho-pulmonary C-fiber afferents (Kubin et al., 1991;Paton, 1998;Moreira et al., 2007a), and pulmonary rapidly adapting stretch receptors (RAR; Kubin and Davies, 1988;Ezure et al., 1991;Otake et al., 2001). Hence, sensory afferent input that is relayed via SolC and caudal SolM must influence inspiratory rhythm generating circuits in the preBötC principally via indirect links. ...
... Moreira et al. (2007a) have argued that the arVRG induced bradypnea may be mediated by activation of rostrally directed inhibitory inputs to the RTN and that this pathway likely mediates at least a portion of the higher order processing of bronchopulmonary C-fiber afferents within the ventrolateral medulla. The latter include projections to the rVRG from a broad region of the caudal NTS that receives bronchopulmonary C-fiber input (Kubin et al., 1991). ...
A substantial array of respiratory, cardiovascular, visceral and somatic afferents are relayed via the nucleus of the solitary tract (NTS) to the brainstem (and forebrain). Despite some degree of overlap within the NTS, specificity is maintained in central respiratory reflexes driven by second order afferent relay neurons in the NTS. While the topographic arrangement of respiratory-related afferents targeting the NTS has been extensively investigated, their higher order brainstem targets beyond the NTS has only rarely been defined with any precision. Nonetheless, the various brainstem circuits serving blood gas homeostasis and airway protective reflexes must clearly receive a differential innervation from the NTS in order to evoke stimulus appropriate behavioral responses. Accordingly, we have examined the question of which specific NTS nuclei project to particular compartments within the ventral respiratory column (VRC) of the ventrolateral medulla. Our analyses of NTS labeling after retrograde tracer injections in the VRC and the nearby neuronal groups controlling autonomic function indicate a significant distinction between projections to the Bötzinger complex and preBötzinger complex compared to the remainder of the VRC. Specifically, the caudomedial NTS, including caudal portions of the medial solitary nucleus and the commissural division of NTS project relatively densely to the region of the retrotrapezoid nucleus and rostral ventrolateral medullary nucleus as well as to the rostral ventral respiratory group while avoiding the intervening Bötzinger and preBötzinger complexes. Area postrema appears to demonstrate a pattern of projections similar to that of caudal medial and commissural NTS nuclei. Other, less pronounced differential projections of lateral NTS nuclei to the various VRC compartments are additionally noted.
... C-fibers terminate within the medial ipsilateral and contralateral NTS. This bilateral innervation is significant, as silencing the caudal contralateral NTS pathway impairs the reflexive breathing changes that are associated with Cfiber excitation (126,142). The second-order NTS neurons for C-fibers remain undefined. ...
The rhythmicity of breath is vital for normal physiology. Even so, breathing is enriched with multifunctionality. External signals constantly change breathing, stopping it when under water or deepening it during exertion. Internal cues utilize breath to express emotions such as sighs of frustration and yawns of boredom. Breathing harmonizes with other actions that use our mouth and throat, including speech, chewing, and swallowing. In addition, our perception of breathing intensity can dictate how we feel, such as during the slow breathing of calming meditation and anxiety-inducing hyperventilation. Heartbeat originates from a peripheral pacemaker in the heart, but the automation of breathing arises from neural clusters within the brainstem, enabling interaction with other brain areas and thus multifunctionality. Here, we document how the recent transformation of cellular and molecular tools has contributed to our appreciation of the diversity of neuronal types in the breathing control circuit and how they confer the multifunctionality of breathing.
Expected final online publication date for the Annual Review of Physiology, Volume 85 is February 2023. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.
... The number of putative synapses, quantified relative to NTS subnuclei/subregion, were counted, and are displayed in the graph (g). The purple data points represent close associations between pre-and postsynaptic proteins and a stomach vagal afferent (n = 3) whilst the red points represent close associations between both these elements at postsynaptic neurons (n = 1 (Kubin, Kimura, & Davies, 1991). These factors may lead to differences in uptake of different tracers at sensory terminal endings in the same organ (Akgul Caglar et al., 2021;Saleeba, Dempsey, Le, Goodchild, & McMullan, 2019) and skew interpretations of results. ...
Anatomical tracing studies examining the vagal system can conflate details of sensory afferent and motor efferent neurons. Here, we used a serotype of adeno-associated virus that transports retrogradely and exhibits selective tropism for vagal afferents, to map their soma location and central termination sites within the nucleus of the solitary tract (NTS). We examined the vagal sensory afferents innervating the trachea, duodenum, stomach, or heart, and in some animals, from two organs concurrently. We observed no obvious somatotopy in the somata distribution within the nodose ganglion. The central termination patterns of afferents from different organs within the NTS overlap substantially. Convergence of vagal afferent inputs from different organs onto single NTS neurons is observed. Abdominal and thoracic afferents terminate throughout the NTS, including in the rostral NTS, where the 7th cranial nerve inputs are known to synapse. To address whether the axonal labeling produced by viral transduction is so widespread because it fills axons traveling to their targets, and not just terminal fields, we labeled pre and postsynaptic elements of vagal afferents in the NTS . Vagal afferents form multiple putative synapses as they course through the NTS, with each vagal afferent neuron distributing sensory signals to multiple second-order NTS neurons. We observe little selectivity between vagal afferents from different visceral targets and NTS neurons with common neurochemical phenotypes, with afferents from different organs making close appositions with the same NTS neuron. We conclude that specific viscerosensory information is distributed widely within the NTS and that the coding of this input is probably determined by the intrinsic properties and projections of the second-order neuron.
... c i) Sterile lung injury (SLI) can trigger local immune activation within the lung parenchyma [1][2][3][4] , but may also directly activate vagal pathways projecting to the nucleus tractus solitarii (nTS), and area postrema (AP) [5][6][7][8][9][10][11] . ii) If injury to the lung is severe, a systemic in ammatory response can occur 3,12-17 . ...
... This pathway: i) activates radial-glia in the dbCC and FS, which is a target for the immune response to SLI, ii) is mediated by IL-1β and COX-1/2 within the glial-barrier separating peripheral circulation from autonomic control circuits in the dmbrainstem, and iii) contributes to the increase in post-sigh apneas following SLI. Together this suggests P → C I-comm pathways utilized by sterile immune challenges target AP border radial-glia and may (Cavarra et al., 2004;Matute-Bello et al., 2008;Hoshino et al., 2009;Ryzhavskii and Lebed, 2016), but may also directly activate vagal pathways that project to the nucleus tractus solitarii (nTS), and area postrema (AP) (Florez and Borison, 1967;Coleridge et al., 1976;Kubin et al., 1991;Srinivasan et al., 1993;Coleridge and Coleridge, 1994;Schelegle et al., 2001;Jung et al., 2018). ii. ...
The pathways for peripheral-to-central immune communication (P→C I-comm) following sterile lung injury (SLI) are unknown. SLI evokes systemic and central inflammation, which alters central respiratory control and viscerosensory transmission in the nucleus tractus solitarii (nTS). These functional changes coincide with increased interleukin-1 beta (IL-1β) in the area postrema, a sensory circumventricular organ that connects P→C I-comm to brainstem circuits that control homeostasis. We hypothesize that IL-1β and its downstream transcriptional target, cyclooxygenase-2 (COX-2), mediate P→C I-comm in the nTS. In a rodent model of SLI induced by intratracheal bleomycin (Bleo), the sigh frequency and duration of post-sigh apnea increased in Bleo- compared to saline- treated rats one week after injury. This SLI-dependent change in respiratory control occurred concurrently with augmented IL-1β and COX-2 immunoreactivity (IR) in the funiculus separans (FS), a barrier between the AP and the brainstem. At this barrier, increases in IL-1β and COX-2 IR were confined to processes that stained for glial fibrillary acidic protein (GFAP) and that projected basolaterally to the nTS. Further, FS radial-glia did not express TNF-α or IL-6 following SLI. To test our hypothesis, we blocked central COX-1/2 activity by intracerebroventricular (ICV) infusion of Indomethacin (Ind). Continuous ICV Ind treatment prevented Bleo-dependent increases in GFAP+ and IL-1β+ IR, and restored characteristics of sighs that reset the rhythm. These data indicate that changes in sighs following SLI depend partially on activation of a central COX-dependent P→C I-comm via radial-glia of the FS.
... Les résultats d'autres études conduites chez d'autres espèces animales retrouvent également cette notion que les différents types de fibres C peuvent avoir des effets opposés sur la toux (Belvisi and Bolser, 2002;Lee and Pisarri, 2001;Ohi et al., 2004;Tatar et al., 1994;Tatar et al., 1988). (Bonham and Joad, 1991;Davies and Kubin, 1986;Ezure et al., 1991;Kubin et al., 2006;Kubin et al., 1991;Lipski et al., 1991;Mazzone and Canning, 2002). Les SARs quant à eux se projettent essentiellement du même côté que leur origine vagale, dans les noyaux latéral et interstitiel (Bonham and McCrimmon, 1990;Davies et al., 1987;Ezure et al., 2002;Kalia and Richter, 1985a, b;Kubin et al., 2006). ...
Contexte – La stimulation de la muqueuse trachéale peut provoquer une réponse défensive à type de toux. La finalité de cette réponse est la protection des voies aériennes via la clairance du mucus et des particules déposées au sein de l’arbre trachéo-bronchique. La stimulation de la muqueuse nasale est à l’origine de réponses défensives n’incluant toutefois pas la toux. La modulation de la toux suggère de possibles interactions centrales des afférences provenant de localisations anatomiques distinctes. Objectif – Déterminer si une stimulation trachéale mécanique discriminante est capable de provoquer une toux lors d’une apnée provoquée par l’instillation nasale d’eau distillée. Méthodes – Douze lapins anesthésiés et trachéotomisés ont été étudiés. Les stimulations trachéales mécaniques ont été réalisées dans 3 conditions : contrôle, après instillation nasale de sérum physiologique, et lors d’apnée suite à l’instillation d’eau distillée. Résultats – Les paramètres ventilatoires de références ne sont pas différents dans les 3 conditions. Un total de 171 stimulations trachéales a été réalisé. Lors de l’apnée, 81% des réponses sont des réflexes expiratoires et le pic de débit expiré est inférieur (p < 0.0001) à celui observé dans les conditions contrôle et sérum physiologique. L’incidence des réponses comprenant un réflexe de toux est plus faible (p < 0,0001) en cas d’instillation d’eau distillée que dans les 2 autres conditions.Conclusion – La stimulation nasale par l’eau distillée désensibilise les réflexes de défense respiratoire faisant suite à une stimulation trachéale mécanique
... The modulation of neuronal functions in the area of the commissural sub-nucleus of the NTS by codeine did not result in an alteration of tracheobronchial cough in anesthetized cats (Figs. 2 and 3). This area includes 2nd order neurons that are part of the afferent pathways from rapidly adapting receptors and C-fibers (Davies and Kubin, 1986;Kubin et al., 1991). However, the role of RARs in the production of cough has recently been called into question (Canning and Mori, 2010). ...
The modulation of cough by microinjections of codeine in 3 medullary regions, the solitary tract nucleus rostral to the obex (rNTS), caudal to the obex (cNTS) and the lateral tegmental field (FTL) was studied. Experiments were performed on 27 anesthetized spontaneously breathing cats. Electromyograms (EMG) were recorded from the sternal diaphragm and expiratory muscles (transversus abdominis and/or obliquus externus; ABD). Repetitive coughing was elicited by mechanical stimulation of the intrathoracic airways. Bilateral microinjections of codeine (3.3 or 33 mM, 54 ± 16 nl per injection) in the cNTS had no effect on cough, while those in the rNTS and in the FTL reduced coughing. Bilateral microinjections into the rNTS (3.3 mM codeine, 34 ± 1 nl per injection) reduced the number of cough responses by 24% (P < 0.05), amplitudes of diaphragm EMG by 19% (P < 0.01), of ABD EMG by 49% (P < 0.001) and of expiratory esophageal pressure by 56% (P < 0.001). Bilateral microinjections into the FTL (33 mM codeine, 33 ± 3 nl per injection) induced reductions in cough expiratory as well as inspiratory EMG amplitudes (ABD by 60% and diaphragm by 34%; P < 0.01) and esophageal pressure amplitudes (expiratory by 55% and inspiratory by 26%; P < 0.001 and 0.01, respectively). Microinjections of vehicle did not significantly alter coughing. Breathing was not affected by microinjections of codeine. These results suggest that: 1) codeine acts within the rNTS and the FTL to reduce cough in the cat, 2) the neuronal circuits in these target areas have unequal sensitivity to codeine and/or they have differential effects on spatiotemporal control of cough, 3) the cNTS has a limited role in the cough suppression induced by codeine in cats.
... The central representation of jugular and nodose airway afferents has been investigated using a variety of physiological and neuroanatomical techniques [14,56e59]. Such studies have described in detail a primary termination site for airway vagal afferents in the brainstem nucleus of the solitary tract (nTS) and not surprisingly this nucleus has underpinned a substantial body of work on airway sensory neurotransmission [1, 36,41,51,61,66,85]. However, our own tracing studies from the airways using a novel Herpes Simplex Virus 1 strain H129 (HSV1 H129) anterograde transynaptic tracer recently described a second primary termination site for airway vagal afferents in the brainstem paratrigeminal nucleus (Pa5) [56,58,59]. ...
The airway sensory nervous system is composed of two anatomically distinct processing pathways that allow for the production of respiratory reflexes and voluntary evoked respiratory behaviours in response to sensing an airway irritation. Disordered sensory processing is a hallmark feature of many pulmonary disorders and results in the development of cough hypersensitivity syndrome, characterised by chronic cough and a persistent urge-to-cough in affected individuals. However, the mechanism underpinning how the airway sensory circuits become disordered, especially at the level of the central nervous system, is not well understood. In this mini-review we present well-defined mechanisms that lead to the development of chronic pain as a framework to explore the evidence that cough disorders may manifest due to neuroplasticity and sensitisation of important components of the airway sensory circuitry in the brain. We highlight recent discoveries of how airway sensory processing occurs in the brain in health and disease and additionally suggest areas where gaps exist in our current knowledge on the topic, with the goal of providing a better understanding of how airway circuits become dysfunctional in disease. This may in turn help identify novel therapeutic targets for restoring normal airway sensory processing and alleviating excessive cough.
... Nerve fibers involved in coughing travel via the superior laryngeal nerve to cell bodies located in the jugular ganglia, or the recurrent nerve to cell bodies in the nodose ganglia, respectively and project to second-order neurons located in the NTS of the medulla oblongata (Canning et al. 2004;Hewitt et al. 2016;Muroi et al. 2013;Mazzone and Canning 2002;McGovern et al. 2015b;Driessen et al. 2015) (Fig. 5). Myelinated and unmyelinated lung afferents project bilaterally but with an ipsilateral predominance, to second-order neurons located in the ventrolateral subnucleus (probably receiving input from SARs; Kalia and Richter 1985) and in the rostral, medial and the dorsal commissural subnucleus of the NTS as has been shown in cats (Kubin et al. 1991), dogs, ferrets, rats (Haxhiu and Loewy 1996) and guinea pigs (Mazzone and Canning 2002). Interestingly, the possibility has been suggested that a single nerve fiber may give several terminal branches that terminate in different NTS subnuclei (Bellingham and Lipski 1992). ...
Inflammatory lung diseases are associated with bronchospasm, cough, dyspnea and airway hyperreactivity. The majority of these symptoms cannot be primarily explained by immune cell infiltration. Evidence has been provided that vagal efferent and afferent neurons play a pivotal role in this regard. Their functions can be altered by inflammatory mediators that induce long-lasting changes in vagal nerve activity and gene expression in both peripheral and central neurons, providing new targets for treatment of pulmonary inflammatory diseases.
... This strong tendency to receive multi-afferent ST inputs was even more unexpected given the absolute segregation of myelinated and unmyelinated afferent messages at NTS-CeA neurons. In the physical layout within the NTS, myelinated and unmyelinated ST axons branch to spread their terminal fields broadly crossing anatomical subregions (Kubin et al. 1991;Kubin et al. 2006) and covering similar areas of NTS with no discernible spatial regionalization. Despite close proximity, adjacent second-order neurons received only one ST TRPV1 phenotype (McDougall & Andresen, 2013). ...
Key points:
Emotions are accompanied by concordant changes in visceral function, including cardiac output, respiration and digestion. One major forebrain integrator of emotional responses, the amygdala, is considered to rely on embedded visceral afferent information, although few details are known. In the present study, we retrogradely transported dye from the central nucleus of the amygdala (CeA) to identify CeA-projecting nucleus of the solitary tract (NTS) neurons for synaptic characterization and compared them with unlabelled, near-neighboor NTS neurons. Solitary tract (ST) afferents converged onto NTS-CeA second-order sensory neurons in greater numbers, as well as indirectly via polysynaptic pathways. Unexpectedly, all mono- and polysynaptic ST afferent pathways to NTS-CeA neurons were organized exclusively as either transient receptor potential cation channel subfamily V member 1 (TRPV1)-sensitive or TRPV1-resistant, regardless of whether intervening neurons were excitatory or inhibitory. This strict sorting provides viscerosensory signals to CeA about visceral conditions with respect to being either 'normal' via A-fibres or 'alarm' via TRPV1 expressing C-fibres and, accordingly, this pathway organization probably encodes interoceptive status.
Abstract:
Emotional state is impacted by changes in visceral function, including blood pressure, breathing and digestion. A main line of viscerosensory information processing occurs first in the nucleus of the solitary tract (NTS). In the present study conducted in rats, we examined the synaptic characteristics of visceral afferent pathways to the central nucleus of the amygdala (CeA) in brainstem slices by recording from retrogradely labelled NTS projection neurons. We simultaneously recorded neuron pairs: one dye positive (i.e. NTS-CeA) and a second unlabelled neighbour. Graded shocks to the solitary tract (ST) always (93%) triggered EPSCs at CeA projecting NTS neurons. Half of the NTS-CeA neurons received at least one primary afferent input (classed 'second order') indicating that viscerosensory information arrives at the CeA conveyed via a pathway involving as few as two synapses. The remaining NTS-CeA neurons received viscerosensory input only via polysynaptic pathways. By contrast, ∼3/4 of unlabelled neighbouring neurons were directly connected to ST. NTS-CeA neurons received greater numbers of ST-related inputs compared to unlabelled NTS neurons, indicating that highly convergent viscerosensory signals reach the CeA. Remarkably, despite multifibre convergence, all single NTS-CeA neurons received inputs derived from only unmyelinated afferents [transient receptor potential cation channel subfamily V member 1 (TRPV1) expressing C-fibres] or only non-TRPV1 ST afferent inputs, and never a combination of both. Such segregation means that visceral afferent information followed separate lines to reach the CeA. Their very different physiological activation profiles mean that these parallel visceral afferent pathways encode viscerosensory signals to the amygdala that may provide interoceptive assessments to impact on behaviours.
... C fibres are sensitive to venous congestion, exertional capillary hypertension and to inhaled irritants. C fibres project bilaterally to the medial NTS, the parvicellular subnucleus, the dorsal aspects of the commisural subnucleus, and the area postrema, with an ipsilateral predominance, and act to excite a cell population with unknown projections (Kalia & Mesulam, 1980b;Kalia & Sullivan, 1982;Kubin et al., 1991;Hines et al., 1994;Wilson et al., 1996). The activation of pulmonary C fibres leads to excitation in VRG expiratory areas, the inhibition of inspiration, bronchoconstriction, airway hypersecretion and cardiovascular depression (Paintal, 1973;Coleridge & Coleridge, 1984;Coleridge & Coleridge, 1986;Paton, 1997;Wilson & Bonham, 1997). ...
... The bronopulmonary C-fibre afferents terminate in the medial area and in the dorsal part of the commissural area of the nucleus of the solitary tract, and in the area postrema. These projections are mediated by glutamate acting on non-NMDA receptors (Kubin et al. 1991). ...
... At a delay consistent with a polysynaptic ascending sensory circuit, the parabrachial nuclei, Kölliker-Fuse nuclei, locus coeruleus, periaqueductal grey and neighbouring regions of the pons and midbrain are infected subsequent to inoculation of the trachea (McGovern et al., 2012a). There is converging evidence supporting a primary role for the nucleus of the solitary tract in the transmission of afferent inputs from the airways (Atoji et al., 2005; Haxhiu and Loewy, 1996; Kubin et al., 1991; Perez Fontan and Velloff, 2001), and it seems very likely that this region is part of the circuitry contributing to urge-to-cough. The spinal trigeminal nucleus has been implicated in visceroafference (Ma et al., 2007; Saxon and Hopkins, 2006), and is another candidate region for the transmission of afferent inputs with relevance for the representation of urge-tocough . ...
... Thus, cardiovascular investigators produced diagrams representing locations of NTS neurons involved in cardiovascular regulation (Ciriello and Calaresu, 1981; Donoghue et al., 1981b; Chan and Sawchenko, 1998; Corbett et al., 2005). Respiratory (Kubin et al., 1991; Haxhiu and Loewy, 1996) or gastrointestinal (Altschuler et al., 1991; Babic et al., 2009 ) investigators also diagrammed associated functional zones. The results of these targeted distributions, however, are remarkably similar to each other, and the resemblance suggests that these afferent-linked functional regions largely overlap and correspond to anatomic sub regions with fuzzy boundaries and co-mingled neurons of different functions. ...
The brainstem nucleus of the solitary tract (NTS) holds the first central neurons in major homeostatic reflex pathways. These homeostatic reflexes regulate and coordinate multiple organ systems from gastrointestinal to cardiopulmonary functions. The core of many of these pathways arise from cranial visceral afferent neurons that enter the brain as the solitary tract (ST) with more than two-thirds arising from the gastrointestinal system. About one quarter of ST afferents have myelinated axons but the majority are classed as unmyelinated C-fibers. All ST afferents release the fast neurotransmitter glutamate with remarkably similar, high-probability release characteristics. Second order NTS neurons receive surprisingly limited primary afferent information with one or two individual inputs converging on single second order NTS neurons. A- and C-fiber afferents never mix at NTS second order neurons. Many transmitters modify the basic glutamatergic excitatory postsynaptic current often by reducing glutamate release or interrupting terminal depolarization. Thus, a distinguishing feature of ST transmission is presynaptic expression of G-protein coupled receptors for peptides common to peripheral or forebrain (e.g., hypothalamus) neuron sources. Presynaptic receptors for angiotensin (AT1), vasopressin (V1a), oxytocin, opioid (MOR), ghrelin (GHSR1), and cholecystokinin differentially control glutamate release on particular subsets of neurons with most other ST afferents unaffected. Lastly, lipid-like signals are transduced by two key ST presynaptic receptors, the transient receptor potential vanilloid type 1 and the cannabinoid receptor that oppositely control glutamate release. Increasing evidence suggests that peripheral nervous signaling mechanisms are repurposed at central terminals to control excitation and are major sites of signal integration of peripheral and central inputs particularly from the hypothalamus.
... Most of the work on central terminations of airway sensory nerves has been carried out in cats and rats. Bronchopulmonary C-fibers and RARs terminate extensively and often bilaterally in the nucleus of the solitary tract (nTS), particularly in the commissural and medial subnuclei (Davies and Kubin 1986;Kalia and Richter 1988;Bonham and Joad 1991;Ezure et al. 1991;Kubin et al. 1991;Lipski et al. 1991;Otake et al. 1992;Mazzone and Canning 2002a, b;Kubin et al. 2006). SARs terminate primarily ipsilateral to their vagal origin, rostral to obex in the lateral and interstitial subnuclei (Kalia and Richter 1985;Davies et al. 1987a,b;Bonham and McCrimmon 1990;Ezure et al. 2002;Kubin et al. 2006). ...
The lung, like many other organs, is innervated by a variety of sensory nerves and by nerves of the parasympathetic and sympathetic nervous systems that regulate the function of cells within the respiratory tract. Activation of sensory nerves by both mechanical and chemical stimuli elicits a number of defensive reflexes, including cough, altered breathing pattern, and altered autonomic drive, which are important for normal lung homeostasis. However, diseases that afflict the lung are associated with altered reflexes, resulting in a variety of symptoms, including increased cough, dyspnea, airways obstruction, and bronchial hyperresponsiveness. This review summarizes the current knowledge concerning the physiological role of different sensory nerve subtypes that innervate the lung, the factors which lead to their activation, and pharmacological approaches that have been used to interrogate the function of these nerves. This information may potentially facilitate the identification of novel drug targets for the treatment of respiratory disorders such as cough, asthma, and chronic obstructive pulmonary disease.
... Most of the work on central terminations of airway sensory nerves has been carried out in cats and rats. Bronchopulmonary C-fibers and RARs terminate extensively and often bilaterally in nucleus tractus solitarius (nTS), particularly in the commissural and medial subnuclei (Bonham and Joad 1991; Davies and Kubin 1986; Ezure et al. 1991; Kalia and Richter 1988; Kubin et al. 1991, 2006; Lipski et al. 1991; Mazzone and Canning 2002; Otake et al. 1992). SARs terminate primarily ipsilateral to their vagal origin, rostral to obex in the lateral and interstitial subnuclei (Bonham and McCrimmon 1990; Davies et al. 1987; Ezure et al. 2002; Kalia and Richter 1985; Kubin et al. 2006). ...
The afferent nerves regulating cough have been reasonably well defined. The selective effects of general anesthesia on C-fiber-dependent cough and the opposing effects of C-fiber subtypes in cough have led to some uncertainty about their regulation of this defensive reflex. But a role for C-fibers in cough seems almost certain, given the unique pharmacological properties of these unmyelinated vagal afferent nerves and the ability of many C-fiber-selective stimulants to evoke cough. The role of myelinated laryngeal, tracheal, and bronchial afferent nerve subtypes that can be activated by punctate mechanical stimuli, inhaled particulates, accumulated secretions, and acid has also been demonstrated. These "cough receptors" are distinct from the slowly and rapidly adapting intrapulmonary stretch receptors responding to lung inflation. Indeed, intrapulmonary rapidly and slowly adapting receptors and pulmonary C-fibers may play no role or a nonessential role in cough, or might even actively inhibit cough upon activation. A critical review of the studies of the afferent nerve subtypes most often implicated in cough is provided.
... One central site of action of these 5-HT 1A and 5- HT 1B/1D receptor ligands could be the nucleus tractus solitarius. This is the site of termination of bronchial and pulmonary C-fibres (Kubin et al., 1991) which are being excited by capsaicin inhalation (see Coleridge et al., 1989) to produce the bronchoconstriction observed. Furthermore, ionophorectic application of the 5-HT 1A receptor agonist, 8-OH-DPAT, has been shown to cause excitation in 43% of nucleus tractus solitarius neurones that receive an afferent vagal input (Wang et al., 1997). ...
The effects of intracisternal (i.c) injections of the 5-HT1A receptor agonists, buspirone and 8-OH-DPAT, and the antagonists WAY-100635; and (-)-pindolol, the 5-HT1B/1D receptor agonist sumatriptan and antagonist GR127935, the 5-HT2 receptor agonist DOI and the antagonist cinanserin, the 5-HT3 receptor antagonist granisetron, the alpha-adrenoceptor agonist clonidine and the antagonist idazoxan, the D2 receptor antagonists (-)-sulpiride and the 5-HT uptake inhibitor fluoxetine on capsaicin-evoked increase in tracheal inflation pressure (bronchoconstriction) were investigated in alpha-chloralose anaesthetised, neuromuscularly blocked, artificially ventilated guinea-pigs. Buspirone, 8-OH-DPAT and fluoxetine significantly potentiated while WAY-100635 (-)-pindolol and sumatriptan attenuated the evoked bronchoconstriction when applied i.c. Granisetron attenuated the response when applied i.v. but not when given i.c. The 5-HT2, alpha2-adrenoceptor and D2 dopamine receptor ligands did not have any significant effect on the evoked bronchoconstriction. Pretreatment i.v. with WAY-100635 alone had no effect on the capsaicin-evoked bronchoconstriction but blocked the potentiating action of i.c. buspirone. The effects of sumatriptan could be completely blocked by pretreatment i.v. with GR127935. Only DOI, in the presence (i.v.) of the peripheral acting 5-HT2 receptor antagonist BW501C67, caused a significant increase in baseline tracheal inflation pressure. It is concluded that activation of central 5-HT1A and 5-HT1B/1D receptors have opposing roles, facilitation and inhibition respectively, on the reflex activation of bronchoconstrictor vagal preganglionic neurones.
Bicuculline and saclofen were microinjected into the rostral (rNTS) and caudal nucleus of the solitary tract (cNTS) in 17 anesthetized cats. Electromyograms (EMGs) of the diaphragm (DIA) and abdominal muscles (ABD), esophageal pressures (EP), and blood pressure were recorded and analyzed. Bilateral microinjections of 1mM bicuculline in the rNTS significantly reduced the number of coughs (CN), amplitudes of DIA and ABD EMG, inspiratory and expiratory EP, and prolonged the duration of the cough expiratory phase (CTE) as well as the total cough cycle duration (CTtot). Bilateral microinjections of 2mM saclofen reduced only cough expiratory efforts. Bilateral microinjection of bicuculline in the cNTS significantly reduced CN and amplitudes of ABD EMG and elongated CTE and CTtot. Bilateral microinjections of saclofen in cNTS had no significant effect on analyzed cough parameters. Our results confirm a different GABAergic inhibitory system in the rNTS and cNTS acting on mechanically induced cough in cats.
This chapter broadly reviews cardiopulmonary sympathetic and vagal sensors and their reflex functions during physiologic and pathophysiologic processes. Mechanosensory operating mechanisms, including their central projections, are described under multiple sensor theory. In addition, ways to interpret evidence surrounding several controversial issues are provided, with detailed reasoning on how conclusions are derived. Cardiopulmonary sensory roles in breathing control and the development of symptoms and signs and pathophysiologic processes in cardiopulmonary diseases (such as cough and neuroimmune interaction) also are discussed.
The lower airways (larynx to alveoli) are protected by a complex array of neural networks that regulate respiration and airway function. Harmful stimuli trigger defensive responses such as apnea, cough and bronchospasm by activating a subpopulation of sensory afferent nerves (termed nociceptors) which are found throughout the airways. Airway nociceptive fibers are projected from the nodose vagal ganglia, the jugular vagal ganglia and the dorsal root ganglia, which are derived from distinct embryological sources: the former from the epibranchial placodes, the latter two from the neural crest. Embryological source determines nociceptive gene expression of receptors and neurotransmitters and recent evidence suggests that placode- and neural crest-derived nociceptors have distinct stimuli sensitivity, innervation patterns and functions. Improved understanding of the function of each subset in specific reflexes has substantial implications for therapeutic targeting of the neuronal components of airway disease such as asthma, viral infections and chronic obstructive pulmonary disease.
The ATP-sensitive P2X2 ionotropic receptor plays a critical role in a number of signal processes including taste and hearing, carotid body detection of hypoxia, the exercise pressor reflex and sensory transduction of mechanical stimuli in the airways and bladder. Elucidation of the role of P2X2 has been hindered by the lack of selective tools. In particular, detection of P2X2 using established pharmacological and biochemical techniques yields dramatically different expression patterns, particularly in the peripheral and central nervous systems. Here, we have developed a knock-in P2X2-cre mouse, which we crossed with a cre-sensitive tdTomato reporter mouse to determine P2X2 expression. P2X2 was found in more than 80% of nodose vagal afferent neurons, but not in jugular vagal afferent neurons. Reporter expression correlated in vagal neurons with sensitivity to α,β methylene ATP (αβmATP). P2X2 was expressed in 75% of petrosal afferents, but only 12% and 4% of dorsal root ganglia (DRG) and trigeminal afferents, respectively. P2X2 expression was limited to very few cell types systemically. Together with the central terminals of P2X2-expressing afferents, reporter expression in the CNS was mainly found in brainstem neurons projecting mossy fibers to the cerebellum, with little expression in the hippocampus or cortex. The structure of peripheral terminals of P2X2-expressing afferents was demonstrated in the tongue (taste buds), carotid body, trachea and esophagus. P2X2 was observed in hair cells and support cells in the cochlear, but not in spiral afferent neurons. This mouse strain provides a novel approach to the identification and manipulation of P2X2-expressing cell types.
Cough is an adverse effect that may hinder the delivery of drugs into the lungs. Chemical or mechanical stimulants activate the transient receptor potential in some airway afferent nerves (C or A fibres) to trigger cough. Types of inhaler device and drug, dose, excipients, formulation characteristics including pH, tonicity, aerosol output and particle size may trigger cough by stimulating the cough receptors. Release of inflammatory mediators may increase the sensitivity of the cough receptors to stimulants. The cough‐provoking effect of aerosols is enhanced by bronchoconstriction in diseased airways and reduces drug deposition in the target pulmonary regions. In this article, we review the factors by which inhalation products may cause cough.
Vagal afferent sensory nerves, originating in jugular and nodose ganglia, are composed of functionally distinct subsets whose activation evokes distinct thoracic and abdominal reflex responses. We used Cre-expressing mouse strains to identify specific vagal afferent populations and map their central projections within the brainstem. We show that Pirt is expressed in virtually all vagal afferents; whereas, 5-HT3 is expressed only in nodose neurons, with little expression in jugular neurons. Transient receptor potential vanilloid 1 (TRPV1), the capsaicin receptor, is expressed in a subset of small nodose and jugular neurons. Tac1, the gene for tachykinins, is expressed predominantly in jugular neurons, some of which also express TRPV1. Vagal fibers project centrally to the nucleus tractus solitarius (nTS), paratrigeminal complex, area postrema, and to a limited extent the dorsal motor nucleus of the vagus. nTS subnuclei preferentially receive projections by specific afferent subsets, with TRPV1+ fibers terminating in medial and dorsal regions predominantly caudal of obex, whereas TRPV1- fibers terminate in ventral and lateral regions throughout the rostral-caudal aspect of the medulla. Many vagal Tac1+ afferents (mostly derived from the jugular ganglion) terminate in the nTS. The paratrigeminal complex was the target of multiple vagal afferent subsets. Importantly, lung-specific TRPV1+ and Tac1+ afferent terminations were restricted to the caudal medial nTS, with no innervation of other medulla regions. In summary, this study identifies the specific medulla regions innervated by vagal afferent subsets. The distinct terminations provide a neuroanatomic substrate for the diverse range of reflexes initiated by vagal afferent activation.
This review analyses the knowledge about the incidence of transient apnoeic spells, induced by substances which activate vagal chemically sensitive afferents. It considers the specificity and expression of appropriate receptors, and relevant research on pontomedullary circuits contributing to a cessation of respiration. Insight is gained into an excitatory drive of 5-HT1A serotonin receptors in overcoming opioid-induced respiratory inhibition.
We have addressed the hypothesis that the opposing effects of bronchopulmonary C-fiber activation on cough are attributable to the activation of C-fiber subtypes. Coughing was evoked in anesthetized guinea pigs by citric acid (0.001-2M) applied topically in 100 µL aliquots to the tracheal mucosa. In control preparations, citric acid evoked 10±1 coughs cumulatively. Selective activation of the pulmonary C-fibers arising from the nodose ganglia with either aerosols or continuous intravenous infusion of adenosine or the 5-HT3 receptor selective agonist 2-methyl-5-HT nearly abolished coughing evoked subsequently by topical citric acid challenge. Delivering adenosine or 2-methyl-5-HT directly to the tracheal mucosa (where few if any nodose C-fibers terminate) was without effect on citric acid evoked cough. These actions of pulmonary administration of adenosine and 2-methyl-5-HT were accompanied by an increase in respiratory rate, but it is unlikely that the change in respiratory pattern caused the decrease in coughing, as the rapidly adapting receptor stimulant histamine also produced a marked tachypnea but was without effect on cough. In awake guinea pigs, adenosine failed to evoke coughing but reduced coughing induced by the nonselective C-fiber stimulant capsaicin. We conclude that bronchopulmonary C-fiber subtypes in guinea pigs have opposing effects on cough, with airway C-fibers arising from the jugular ganglia initiating and/ or sensitizing the cough reflex, and the intrapulmonary C-fibers arising from the nodose ganglia actively inhibiting cough upon activation.
Postganglionic sympathetic and parasympathetic nerves innervate the airways and lungs of all vertebrates. When activated, these nerves can initiate airway smooth muscle contraction or relaxation, pulmonary or bronchial vascular smooth muscle relaxation and mucus secretion. The often opposing effects of the multiple postganglionic autonomic pathways projecting to the airways and lungs is regulated by many brainstem and midbrain structures. The origin of bronchopulmonary sympathetic and parasympathetic preganglionic nerves are described as are the actions and interactions between brainstem subnuclei controlling autonomic outflow to the airways and the pulmonary and bronchial vasculature. © Oxford University Press, Inc. 2011, 1990. All rights reserved.
This review introduces the potential involvement of the pedunculopontine tegmental nucleus (PPN) in sudden infant death syndrome (SIDS). SIDS is most likely to occur during sleep, and subcortical structures of the brain are well known to be involved in sleep. With this in mind, particular attention has been given to the relationship between these subcortical structures and SIDS. The disturbances of muscle tone suppression during rapid eye movement sleep are postulated to be implicated in the occurrence of SIDS. Author is interested in the PPN, because the PPN is not only involved in the control of muscle tone but also in the subcortical activation system. Ultimately, a hypothesis emerged that dysfunction of the PPN is involved in the occurrence of SIDS. Four possibilities for the PPN to produce SIDS are raised: (1) insufficient PPN activation in rapid eye movement sleep produces an excess of regular respiration, (2) insufficient PPN activation in rapid eye movement sleep decreases subcortical activation, (3) insufficient PPN activation in rapid eye movement sleep elicits an interruption in respiration, and (4) inadequate PPN activation in non-rapid eye movement sleep suppresses respiration.
The primary function of respiration is to exchange gases between the external environment and the internal milieu of the organism. Gas exchange occurs through coordinated action of the respiratory and cardiovascular systems. In the mammal, the respiratory system controls ventilation of the lung, while the cardiovascular system transports O2 and CO2 between the pulmonary and systemic capillaries. These processes adjust to varying physiological circumstances to maintain homeostasis (cf. Chap. 108).
Aims:
Volatile anesthetics have distinct odors and some are irritating to the upper airway and may cause cough and laryngospasm, which may result, in part, from stimulation of C-fiber reflex. Local exposure of such anesthetics increases the sensitivity of capsaicin-sensitive laryngeal C-fiber endings compatible with airway irritability presumably by activation of transient receptor potential (TRP) ion channels, but the physiological relevance of this sensitization transmitted to the higher-order neurons in the central reflex pathway and output is unknown.
Main methods:
In anesthetized young guinea pigs, baseline and left atrial capsaicin evoked changes in the extracellular unit activity of laryngeal C-fiber-activated neurons in the nucleus tractus solitarii (NTS) and phrenic nerve activity were compared between irritant (desflurane) and non-irritant (sevoflurane) anesthetic gas exposure to the isolated larynx.
Key findings:
Desflurane significantly augmented the peak and duration (p<0.01) of the NTS neuronal responses and the prolongation of expiratory time (p=0.017). The effect was enhanced by iontophoretic application of the TRPA1 agonist allyl-isothiocyanate (p<0.05), inhibited by TRPA1 antagonist HC-030031 (p<0.01), but not by TRPV1 antagonist BCTC. Sevoflurane did not affect the central pathway.
Significance:
Thus, the sensitization of the laryngeal C-fiber endings by irritant volatile anesthetics is transmitted to the NTS via activation of the TRPA1 and is associated with a prolonged reflexively evoked expiratory apnea. The findings may help to explain local deleterious effects of irritant volatile general anesthetics on the airways during inhaled induction or bronchodilator therapy for status asthmatics.
Although simple in concept, reliable antidromic identification of efferent populations poses numerous technical challenges and is subject to a host of sampling biases, most of which select against the detection of the neurons with slowly conducting axons. This problem is particularly acute in studies of the neocortex. Many neocortical efferent systems have large sub-populations with very slowly conducting, nonmyelinated axons and these elements have been relatively neglected in antidromic studies of neocortical neurons. The present review attempts to redress this problem by analyzing the steps that must necessarily precede antidromic identification and the sampling biases associated with each of these steps. These steps include (1) initial recognition that the microelectrode is near a neuron; (2) activation of the efferent axon via the stimulating electrode; (3) conduction of the antidromic impulse from stimulation site to soma; (4) detection of the antidromic spike in the extracellular record and (5) discriminating antidromic from synaptic activation. Experimental strategies are suggested for minimizing the sampling biases associated with each of these steps; most of which can be reduced or eliminated by appropriate experimental procedures. Careful attention to such procedures will make it possible to better understand the nature and function of the information flow along the very slowly conducting axonal systems of the neocortex.
Coughing protects and clears the airways and lungs of inhaled irritants, particulates, pathogens, and accumulated secretions. An initial urge to cough, and an almost binary output suggests gating mechanisms that encode and modulate this defensive reflex. Whether this "gate" has a physical location for the physiological barrier it poses to cough is unknown. Here we describe a critical component to cough gating, the central terminations of the cough receptors. A novel microinjection strategy defined coordinates for microinjection of glutamate receptor antagonists that nearly abolished cough evoked from the trachea and larynx in anesthetized guinea pigs while having no effect on basal respiratory rate and little or no effect on reflexes attributed to activating other afferent nerve subtypes. Comparable microinjections in adjacent brainstem locations (0.5-2 mm distal) were without effect on coughing. Subsequent transganglionic and dual tracing studies confirmed that the central terminations of the cough receptors and their primary relay neurons are found bilaterally within nucleus tractus solitarius (nTS), lateral to the commissural subnucleus and perhaps in the medial subnuclei. These synapses possess the physiological characteristics of a cough gate. Their localization should facilitate more mechanistic studies of the encoding and gating of cough.
In many species including humans, antagonists of NMDA-type glutamate receptors such as dextromethorphan, when used at sufficient doses, have been found to be relatively safe and effective antitussives. Similarly, now in five different species (guinea pigs, rabbits, cats, dogs and pigs), neurokinin receptor antagonists have also proven to be safe and effective antitussive agents. Both of these classes of drugs act centrally to prevent cough. A brief review of what is known about the central encoding of cough is presented, as are the advantages of centrally acting antitussives. Also discussed are new insights into cough and NMDA receptor signaling that may lead to the development of more effective antitussive agents with limited side effects and broad application in treating cough associated with a variety of aetiologies.
The commissural subnucleus (COM) of the nucleus of the solitary tract (NTS) is known to receive primary afferents from the lungs and other viscera innervated by the vagus nerve, and thus to participate in central autonomic and respiratory control. The aim of the present study was to identify the areas of terminal arborizations of COM neurons in order to examine brainstem sites which may be involved in reflex responses mediated by these neurons. The projections were studied in cats, using biocytin as an anterograde tracer. Labeled fibers and terminal boutons were visualized by horseradish-peroxidase histochemistry, 2–3 days after microinjection of the tracers into the COM 1–2 mm caudal to the obex. Labeled axons were examined in the brainstem from the rostral pons to the caudal medulla and were found bilaterally, with an ipsilateral predominance, mainly in the following regions: (1) The dorsolateral rostral pons. Terminal boutons were observed in the lateral and medial parabrachial nuclei, Kölliker-Fuse nucleus, and around the mesencephalic trigeminal tract. This area corresponds to the pontine respiratory group also known as the “pneumotaxic center.” (2) The pontine area dorsolateral to the superior olivary nucleus. This region contains the A5 noradrenergic cell group; (3) Near the ventral surface, below the facial nucleus. This area overlaps with the ‘retrotrapezoid nucleus.’ (4) Respiration-related areas of the medulla, including the dorsal and ventral respiratory groups, and the Bötzinger complex. (5) The dorsal motor nucleus of the vagus.
Axonal projections of second order neurons activated by vagal afferent fibers originating from pulmonary rapidly adapting receptors (RARs) were studied electrophysiologically in Nembutal-anesthetized, paralyzed and artificially ventilated cats. Extracellular recordings from these neurons (referred to as 'RAR-cells') were made in the commissural subnucleus (COM) of the nucleus of the solitary tract (NTS). They were identified by a combination of stimuli including stimulation of the vagus nerve(s), hyperinflation and deflation of the lungs, and a brief period of ammonia vapor inhalation. A total of 80 RAR-cells were tested for axonal projections to a respiration-related area in the brain-stem, either the dorsolateral rostral pons or the dorsal respiratory group (DRG) or the ventral respiratory group (VRG) or the Bötzinger complex (BOT) and/or the spinal cord. Twenty-two of the 47 (47%) RAR-cells tested for ipsilateral pontine projection could be antidromically activated, and in 8 cases evidence for axonal arborization was obtained. Only 1 of the 11 RAR-cells tested for DRG projection, and 1 of the 10 RAR-cells tested for VRG projection, were antidromically activated. No RAR-cells were activated from the BOT (n = 8) or from the C3-C4 segments of the spinal cord (n = 11). Bilateral lesions of the COM abolished the reflex responses induced by ammonia inhalation or hyperinflation of the lungs, but not the Hering-Breuer reflex. These results indicate that a pathway from the COM to the rostral pons forms part of the reflex arc originating from RARs.
1. The pulmonary C fibre reflex, triggered by activating pulmonary C fibre endings in the lung, consists of rapid shallow breathing (which may be preceded by apnoea), bradycardia, and hypotension. The purpose of this work was to identify proximal synapses in this reflex. From pilot data, we hypothesized that neurones in a discrete region of the commissural nucleus in the nucleus tractus solitarii (NTS) are required for full expression of the pulmonary C fibre reflex. Studies were carried out in urethane-anaesthetized, unilaterally vagotomized, spontaneously breathing rats, in which diaphragm electromyogram, arterial pressure, and blood gases were measured. Phenyldiguanide (PDG) was injected in the right atrium to elicit the pulmonary C fibre reflex. Unilateral NTS injections were made through multibarrelled pipettes containing DL-homocysteic acid (DLH) to mimic the reflex, cobalt chloride to reversibly impair the reflex, and/or dye to mark the injection sites. 2. PDG (5-16 micrograms kg-1) injected in the right atrium of twenty-six rats produced the classic pulmonary C fibre reflex: a vagally mediated, rapid onset of rapid shallow breathing, bradycardia and hypotension. 3. Injection of DLH (3-12 nl of 20 mM for a total of 60-240 pmol) in the dorsomedial aspect of the commissural nucleus of the NTS in thirty rats mimicked the pulmonary C fibre reflex, producing rapid shallow breathing, hypotension, and a slight bradycardia. 4. Interruption of neuronal transmission by injecting cobalt chloride (15-30 nl of 100 mM) in the site where DLH produced rapid shallow breathing, reversibly impaired the rapid shallow breathing and bradycardia produced by right atrial injections of PDG in fifteen rats. The commissural region where DLH produced rapid shallow breathing and cobalt impaired the pulmonary C fibre reflex extended from 720-1100 microns caudal to the obex, 30-200 microns lateral to mid-line, and 200-600 microns ventral to the dorsal surface of the brain stem within the NTS. 5. Taken together, the results suggest that neurones within a discrete region in the dorsomedial commissural nucleus in caudal NTS are required for full expression of the pulmonary C fibre reflex.
Experiments were performed in unanaesthetized rabbits and rats to investigate the distribution, within the medulla oblongata, of neurons activated during the Bezold-Jarisch reflex. Repeated intravenous injections of phenylbiguanide evoked depressor and bradycardic responses in both rabbits and rats. Fos-positive neurons were present in the nucleus tractus solitarius and in the caudal ventrolateral medulla oblongata. Double-label tyrosine hydroxylase (TH) immunohistochemical studies in the ventrolateral medulla showed that most Fos-positive neurons in the caudal ventrolateral medulla were TH-negative neurons scattered between A1 noradrenaline cells, in the rabbit and in the rat. Approximately 20% of neurons in the caudal ventrolateral medulla in rabbits, and 50% in rats, were immunoreactive for both Fos and TH. Some Fos-positive, TH-negative neurons in the caudal ventrolateral medulla were retrogradely labelled with cholera toxin B-Gold after injection of this tracer into the sympathoexcitatory region of the rostral ventrolateral medulla. Our data suggests that neurons in the nucleus tractus solitarius, and rostrally projecting TH-negative neurons in the caudal ventrolateral medulla, are part of the pathway by which stimulation of cardiopulmonary receptors inhibits sympathetic vasomotor tone to decrease blood pressure during the Bezold-Jarisch reflex.
There is an abundance of literature concerning the possible transduction mechanisms and neurotransmitters that act within the carotid body, the activity of carotid chemoreceptor afferents to various stimulants, and the reflex effects produced by altering chemoreceptor afferent discharge. However, relatively little information is available regarding the mechanisms which intervene between the activity in the afferent nerve fibres and the alterations evoked in motor outflow. In this short review I will summarise what is known regarding the sites of termination of arterial chemoreceptor afferents and the integration of these afferents with other afferent inputs, respiration, and higher centres at the second and higher order neurones. In addition, these will be compared to the pathways underlying the responses evoked by a second set of chemoreceptor afferents, those arising from the pulmonary chemoreceptor afferents variously termed “J-receptors”, “pulmonary C-fibre afferents” or “pulmonary chemoreceptor afferents”. When activated these afferents evoke a marked vagal bradycardia, a depressor response and either rapid shallow breathing or apnoea. These afferents are of interest since the bradycardia they evoke is unaffected by respiratory gating (Daly & Kirkman, 1989; Daly, 1991) whereas the bradycardia evoked by arterial chemoreceptor or baroreceptor afferents is powerfully gated both by the central respiratory cycle and lung inflation (Daly, 1986; Davidson et al., 1976). This may suggest that the central mechanisms underlying the two chemoreceptor reflexes are fundamentally different. Hence, it is worth comparing the pathways responsible for both reflexes. The effects of these two types of afferent on the relevant motor neuronal pools has been discussed elsewhere in this volume (Jones et al., 1994) and will not be discussed in detail here.
The CNS cell groups that innervate the tracheal parasympathetic preganglionic neurons were identified by the viral retrograde transneuronal labeling method. Pseudorabies virus (PRV) was injected into the tracheal wall of C8 spinal rats and after 4 days survival, brain tissue sections from these animals were processed for immunohistochemical detection of PRV. Retrogradely labeled parasympathetic preganglionic neurons were seen in three sites in the medulla: the compact portion of the nucleus ambiguus, the area ventral to the nucleus ambiguus, and the rostralmost portion of the medial nucleus tractus solitarius (NTS); this labeling pattern correlated well with the retrograde cell body labeling seen following cholera toxin beta-subunit injections in the tracheal wall. PRV transneuronally labeled neurons were found throughout the CNS with the most abundant labeling concentrated in the ventral medulla oblongata. Labeled neurons were identified along the ventral medullary surface, and in nearby areas including the parapyramidal, retrotrapezoid, gigantocellular and lateral paragigantocellular reticular nuclei as well as the caudal raphe nuclei (raphe pallidus, obscurus, and magnus). Serotonin (5-HT) neurons of the caudal raphe complex (B1-B3 cell groups) and ventromedial medulla were labeled as well as a few C1 adrenergic neurons. The A5 cell group was the major noradrenergic area labeled although a small number of locus coeruleus neurons were also labeled. Several NTS regions contained labeled cells including the commissural, intermediate, medial, central, ventral, and ventrolateral subnuclei. PRV infected neurons were present in the Kölliker-Fuse and Barrington's nuclei. In the rostral mesencephalon, the precommissural nucleus of the dorsal periventricular gray matter was labeled. Labeling was present in the dorsal, lateral and paraventricular hypothalamic nuclei. In summary, the airway parasympathetic preganglionic neurons are innervated predominantly by a network of lower brainstem neurons that lie in the same regions known to be involved in respiratory and cardiovascular regulation. These findings are discussed in relationship to some of the potential CNS mechanisms that may be operative in airway disorders as well as potentially involved in certain fatal respiratory conditions such as Ondine's curse and sudden infant death syndrome (SIDS).
Efferent projections of the commissural subnucleus of the solitary tract (COM) to the ventrolateral medulla were studied in the cat using anterograde labeling with biocytin combined with dopamine beta-hydroxylase immunohistochemistry. COM neurons were observed to send their axons densely to the areas of distribution of respiration-related neurons in the ventrolateral medulla, e.g. ventral respiratory group, Bötzinger complex. Axon terminals from COM neurons were further found in the areas of distribution of catecholamine neurons (C1 and A1 cell groups), that were distributed in the close vicinity of the reported respiration-related areas in the ventrolateral medulla. Putative synaptic contacts of axon terminals from COM neurons with catecholamine neurons were often observed in the C1 area.
1. The aim of the present experiments was to determine whether 5-HT1A receptors play a role in the control of the reflex activation of pulmonary vagal motoneurones. This was carried out by investigating the effects of intracisternal injections (i.c.) of the 5-HT1A receptor ligands, 8-OH-DPAT (50 micrograms kg-1), buspirone (200 micrograms kg-1), WAY-100635 (100 micrograms kg-1), methiothepin (200 micrograms kg-1) and (-)-pindolol (100 micrograms kg-1) and the 5-HT2 receptor antagonist, cinanserin (200 micrograms kg-1), on the reflex bronchoconstriction evoked by inhaled capsaicin aerosol in alpha-chloralose anaesthetized, neuromuscularly blocked and artificially ventilated cats. Recordings were made of heart rate, blood pressure and upper tracheal pressure. 2. Central application of all the 5-HT1A receptor antagonists (methiothepin, WAY-100635 and (-)-pindolol) attenuated the reflex bronchoconstriction in the upper trachea. However, the same dose of WAY-100635 given i.v. had no effect on this reflex bronchoconstriction. The 5-HT1A receptor agonist, 8-OH-DPAT (50 micrograms kg-1) given i.c., potentiated the capsaicin-evoked reflex bronchoconstriction, whereas buspirone (200 micrograms kg-1) i.c. had no effect. The 5-HT2 receptor antagonist, cinanserin (200 micrograms kg-1) also had no effect. 3. It is concluded that the reflex excitation of pulmonary vagal motoneurones by inhaled capsaicin in alpha-chloralose anaesthetized cats involves the activation of central 5-HT1A receptors.
1. The activity of most brainstem serotonergic cells is suppressed during sleep, particularly the rapid eye movement (REM) phase. Thus, they may play a major role in state-dependent changes in CNS functioning. Our main goal was to search for medullary raphe cells having axonal branches in the region of the hypoglossal (XII) motor nucleus and assess their behaviour during the atonia produced by microinjections of a cholinergic agonist, carbachol, into the dorsal pontine tegmentum. In chronic animals, such microinjections evoke a desynchronized sleep-like state similar to natural REM sleep; in decerebrate animals, they produce eye movements and a motor suppression similar to the postural atonia of REM sleep. 2. In decerebrate, paralysed, vagotomized and artificially ventilated cats, we recorded extracellularly from medullary raphe cells antidromically activated from the XII nucleus region. Forty-five cells recorded in the raphe obscurus and pallidus nuclei were antidromically activated with latencies characteristic of non-myelinated fibres (4.4-42.0 ms). For thirty-three of the forty-five cells, we found one or more axonal branches within or just below the XII nucleus. The remaining twelve cells, in addition to the XII nucleus, had axonal ramifications in the medial nucleus of the solitary tract (NTS) and/or the dorsal motor nucleus of the vagus (DMV). 3. A subset of fourteen spontaneously active cells with identified axonal projections were held long enough to be recorded during the carbachol-induced atonia, and eight of these also during the subsequent recovery and a systemic administration of the serotonergic 1A receptor agonist (+/-)8-hydroxy-2-(di-N-propylamino)tetrealin hydrobromide (8-OH-DPAT). All but one were suppressed during the atonia in parallel to the suppression of XII, phrenic and postural nerve activities (firing rate, 1.3 +/- 0.7 Hz before and 0.1 +/- 0.2 Hz after carbachol (means +/- S.D.)). Following the recovery from the atonia, the firing rates of the eight cells increased to the pre-carbachol level (1.6 +/- 1.0 Hz). Subsequently, all were silenced by 8-OH-DPAT. 4. These cells fulfil most physiological criteria for serotonergic cells and have the potential to modulate, in a state-dependent manner, activities in the motor XII nucleus, visceral sensory NTS, and DMV. The decrements in serotonergic neuronal activity that occur during the carbachol-induced atonia suggest that a similar withdrawal of serotonergic input may occur during REM sleep and contribute to the characteristic reductions in upper airway motor tone.
The central control of cardiovascular function has been keenly studied for a number of decades. Of particular interest are the homeostatic control mechanisms, such as the baroreceptor heart-rate reflex, the chemoreceptor reflex, the Bezold-Jarisch reflex and the Breuer-Hering reflex. These neurally-mediated reflexes share a common termination point for their respective centrally-projecting sensory afferents, namely the nucleus tractus solitarius (NTS). Thus, the NTS clearly plays a critical role in the integration of peripherally initiated sensory information regarding the status of blood pressure, heart rate and respiratory function. Many endogenous neurochemicals, from simple amino acids through biogenic amines to complex peptides have the ability to modulate blood pressure and heart rate at the level of the NTS. This review will attempt to collate the current knowledge regarding the roles of neuromodulators in the NTS, the receptor types involved in mediating observed responses and the degree of importance of such neurochemicals in the tonic regulation of the cardiovascular system. The neural pathway that controls the baroreceptor heart-rate reflex will be the main focus of attention, including discussion of the identity of the neurotransmitter(s) thought to act at baroafferent terminals within the NTS. In addition, this review will provide a timely update on the use of recently developed molecular biological techniques that have been employed in the study of the NTS, complementing more classical research.
Cholera toxin beta-subunit was used as both a transganglionic and retrograde cell body tracer to determine respectively the central sensory and motor systems innervating the trachea in three mammalian species, dog, ferret and rat. A basic pattern was found in all three animals. Sensory fibers terminated in three subnuclei of the nucleus tractus solitarius (NTS) with the densest concentration localized in a restricted part of the medial part of the rostral NTS. Weaker projections were identified in the ventrolateral NTS subnucleus and sparse labeling was seen in the commissural NTS subnucleus. No labeling was identified in the area postrema. The pattern of retrograde cell-body labeling was also similar in all three species. Two main sites were labeled: the rostralmost part of the dorsal vagal nucleus and the rostral nucleus ambiguus (NA). In the NA, cell labeling was found in mainly in the ventral (or external) portion of the nucleus, but some labeled neurons were consistently found in the compact NA as well. In addition, labeled neurons were also seen in the dorsomedial part of the C1-C2 ventral horn. In summary, the central sites of termination of the sensory fibers and cells of origin innervating the trachea were similar in all three species.
The effects of intracisternal (i.c.) application of the 5-HT3 receptor antagonist granisetron (0.016-0.16 microg kg-1) and the agonist phenylbiguanide (0.3-3 microg kg-1) on reflex bradycardia evoked by injection of phenylbiguanide (i.v.; 10 microg kg-1) were investigated in urethane anesthetized atenolol-pretreated rats. The effect of bilateral microinjection of granisetron (10 nmol per side, 100 nl) into the nucleus tractus solitarius (NTS) on the reflex was also investigated. Intracisternal administration of granisetron dose-dependently (0.016-0.16 microg kg-1) and significantly attenuated the reflex bradycardia whilst the highest dose given i.v. had no significant effect on the reflex bradycardia. Phenylbiguanide given i.c. only caused significant potentiation at the middle dose (1 microg kg-1), having no significant effects at the other doses. Neither granisetron nor phenylbiguanide given i.c. affected resting heart rate or blood pressure. Granisetron microinjected bilaterally into the NTS also significantly attenuated both reflex bradycardia and hypotension. It is concluded that excitation of cardiac vagal motoneurones evoked by cardiopulmonary afferents involves activation of 5-HT3 receptors located in the nucleus tractus solitarius and other brainstem areas.
Wheat germ agglutinin-conjugated horseradish peroxidase (WGA-HRP) was injected into the rat lung parenchyma, just beneath the lateral surface of the left upper lobe, in order to demonstrate the pulmonary afferents. This injection resulted in heavy accumulation of labeled fibers in the medial nucleus tractus solitarius (NTS). The labeling in the medial NTS was divided into the ventral and dorsal parts at the level around the obex. Some labeling was found in the commissural and ventrolateral NTS. Further confirmation of the central distribution of these pulmonary afferent fibers was made by the expression of fos-like immunoreactivity (FOS-LI) induced by injection of formalin into the lung. It is concluded that afferents of lung parenchyma terminating predominantly in the medial NTS might come from alveoli and terminal bronchioles, because WGA-HRP and formalin injected into the lung are considered to be confined to the terminal areas of the respiratory tract.
A technique is described which details simple and rapid surgical procedures for an acute midcollicular decerebration in the adult cat. This decerebration procedure includes a bilateral ligation of the external carotid arteries and is performed under halothane anesthesia. Mortality resulting from this procedure is less than 5%. Thus, a long-term, homeostatically stable decerebrate preparation is obtained in most experiments. Within one hour postdecerebration, the experimental animal exhibits values of respiratory tidal volume, frequency, end-expired CO2 partial pressure and mean arterial blood pressure which approximate those of the unanesthetized, intact cat.
The present series of experiments was designed to study the organization of preterminai processes and synaptic boutons of single physiologically identified slowly adapting receptor (SAR) pulmonary stretch afferent fibers. Intra‐axonally injected horseradish peroxidase‐wheat germ agglutinin (HRP‐WGA) conjugate was used as the label. In the first paper, we describe the pattern of arborization of axon collaterals from single physiologically identified SAR afferent fibers evident in the various subnuclei of the nucleus of the tractus solitarius (nTS). In the second paper, details are presented regarding the ultrastructure of these synaptic boutons and axon collaterals.
A number of significant findings resulted from this study: (1) A single lung stretch SAR afferent fiber arborized over a considerable distance rostrocaudally in the brain stem (1,700–2,100 μm). (2) A single lung stretch SAR afferent fiber terminated as hundreds of bouton terminals (650–1,180). (3) There was a remarkable consistency in the subnuclei of the nTS that received these terminal arborizations of SAR afferents. (4) The ventral (vnTS), intermediate (nI), ventrolateral (vInTS), and interstitial (ni) subnuclei of the nTS were the only regions of the nTS receiving bouton terminals of SAR afferent fibers. (5) Under the light microscope the pattern of termination of SAR afferents was similar in all the axons studied in this series. (6) The injected parent axon in each case could be followed in the TS at all levels and remained consistent with regard to position and orientation and could be traced rostrally to levels as far as 3.5 mm rostral to the obex whereas the region of terminal arborization was located around 1.7–2.1 mm rostral to the obex. This pattern indicates that a single lung stretch SAR afferent fiber descends caudally upon entering the nTS. In the cat vagal afferent fibers are known to enter the medulla at levels between 0.5 mm and 3.2 mm rostral to the obex (Kalia and Mesulam, '80a).
The results of the light microscopic analysis presented in this article indicate that lung stretch (SAR) afferents from the lungs and tracheobronchial tree have distinctly localized patterns of distribution in the nTS. In addition, these findings support the concept that representation of pulmonary afferents in the medulla is constituted by a differentiated distribution of nerve terminals in the various subnuclei of the nTS. Modality‐specific localization (SAR afferents in this case) appears to be predominant in the n'1'S. The widespread rostrocaudal distribution of the terminal field of a single lung stretch SAR afferent is remarkable. In addition, the finding that the terminals of different SAR afferents are localized in only a few subnuclei of the nTS suggests that specificity exists in this afferent system which could provide the morphological substrate for focussed physiological effects.
Central distribution of efferent and afferent components of the cervical branches of the vagus nerve in the cat was studied by applying horseradish peroxidase (HRP) to the superior laryngeal nerve (SLN), pharyngeal branch (PhB), recurrent laryngeal nerve (RLN) or middle portion of the cervical trunk of the vagus nerve (CTV). After applying HRP to the SLN, PhB or RLN, labeled neurons were mainly seen ipsilaterally in the ambiguus nuclear complex (Amb) at levels of the rostral, middle, or caudal portions of the Amb, respectively. After application of HRP to the CTV, labeled neurons were distributed ipsilaterally throughout the Amb. Neurons in the lateral reticular formation medial to the spinal trigeminal nucleus were also labeled ipsilaterally with HRP from the SLN or CTV; these neurons appeared to be the caudal extension of the inferior salivatory nucleus. Neurons within the dorsal motor nucleus of the vagus nerve (DMV) were labeled ipsilaterally only after HRP application to the CTV. The axons of neurons in the rostral Amb and the lateral reticular formation, labeled retrogradely from the SLN or CTV, ran dorsomedially to form a genu in the dorsomedial tegmental region near the floor of the fourth ventricle, and then turned and left ventrolaterally from the medulla oblongata. On the other hand, axons of neurons in the caudal two-thirds of the Amb, labeled retrogradely from the PhB, RLN or CTV, ran dorsomedially to a region ventral to the DMV, where they turned ventrolaterally to form loops before leaving the brainstem. A few axons of neurons in the rostralmost regions of the DMV were also found to form a genu near the floor of the fourth ventricle before running ventrolaterally to leave the brianstem. Axon terminals in the solitary nucleus (Sn) were transganglionically labeled most densely in the medial and interstitial subnuclei from the SLN, in the medial and dorsolateral subnuclei from the RLN, and in the medial and gelatinous subnuclei from the CTV. Application of HRP to the SLN also revealed labeling of axon terminals in the principal sensory and spinal trigeminal nuclei, the medial cuneate nucleus and the dorsal horn of the C1 and C2 cord segments.
A reconstruction was made of the intramedullary trajectory of 23 physiologically identified Ia afferents from cat hind limb muscles (medial gastrocnemius, soleus, plantaris, flexor digitorum‐hallucis longus, and hamstring). The afferents were stained by intra‐axonally injected HRP. The axons of these afferents were traced over distances of 5.8 mm to 15.7 mm rostrocaudally. In the dorsal funiculus fibers from all the muscles showed a similar course and similarly bifurcated into an ascending and a descending branch. The mean diameters of stem axons, ascending branches, and descending branches were 6.6 μm, 5.8 μm, and 3.0 μm, respectively. Within the analyzed lengths of the spinal cord five to eleven collaterals were given off from the two branches. The distances between adjacent collaterals of the ascending and descending branches averaged 1200 μm and 790 μm, respectively.
The collaterals as a rule passed through the medial half of the dorsal horn before they entered the deeper parts of the gray matter. The terminal distribution areas common to all Ia collaterals were: (1) the medial half of the base of the dorsal horn, mainly lamina VI; (2) lamina VII; and (3) lamina IX. The numbers of terminals were largest in lamina IX and smallest in lamina VII. The density of terminals in lamina IX was highest in the homonymous motor cell column. The terminal distribution areas of adjacent collaterals showed no overlap in the sagittal plane. Terminal branches carried one bouton terminal and up to six boutons en passage with an average of 1.8 terminals per terminal branch. Apparent axosomatic and axodendritic contacts were seen on small‐sized and medium‐sized neurons in laminae V–VI, medium‐sized neurons in lamina VII, and large neurons in lamina IX. One motoneuron was contacted by an average of 3.3 terminals.
In addition to the common features, Ia collaterals of various muscles of origin showed some differences in their trajectories in the ventral horn, and in their terminations in the gray matter.
The efferent projections from the solitary complex to the lower brain stem and spinal cord were studied in the cat with the autoradiographic anterograde axonal transport and retrograde horseradish peroxidase (HRP) techniques. A revised cytoarchitectonic description of the caudal two-thirds of the complex is presented in which the complex was subdivided into six nuclei: lateral, ventrolateral, intermediate, medial, parvocellular, and commissural solitary tract nuclei. Following injections of ³H amino acids into electrophysiologically defined regions of the complex in which cardiac or respiratory units were recorded, labeled fibers could be traced to a number of sites in the caudal brain stem including the medial and lateral parabrachial nuclei, Kölliker-Fuse nucleus and the area ventral to this nucleus, lateral periaqueductal gray matter, ambiguus complex, which consists of the retrofacial, ambiguus and retroambiguus nuclei, ventrolateral reticular nucleus (in an area equivalent to the A1 cell group of Dahlström and Fuxe, '64), medial accessory olive, paramedian reticular formation, and lateral cuneate nucleus. Descending solitario-spinal projections have been traced bilaterally, but predominantly to the contralateral side, to the region of the phrenic motor neurons in the C4-C6 ventral horn, to the thoracic ventral horn, and intermediolateral cell column.
We used the wallerian degeneration of vagal afferents and the retrograde transport of WGA-HRP microinjected in the nucleus of the tractus solitarius (NTS) to study the central projections of myelinated and unmyelinated vagal axons. We concluded that the set of largest nodose cells projected to the dorsolateral, interstitial, ventral, ventrolateral and intermediate NTS subnuclei, while the smaller nodose cells terminated in the medial, dorsal, gelatinosus and commissural NTS subnuclei.
The neural connections of the carotid sinus were studied by administration of horseradish peroxidase or a lectin conjugate to the adventitia of the carotid sinus of cats. The carotid sinus afferents project exclusively to the nucleus of the tractus solitarius (NTS). Rostral to the obex the projection is mainly ipsilateral with a strong contralateral component caudal to the obex. The carotid sinus projects to several NTS territories that do not receive afferents from the carotid body chemoreceptors; they are: the dorsolateral, the lateral extension of the commissural, the caudal intermediate, the ventrolateral and the gelatinosus subnuclei. In addition the carotid sinus central representation includes territories occupied also by carotid body terminals: dorsal, interstitial, rostral intermediate, medial and the medial part of commissural subnuclei (see previous paper). Labeled cell bodies were found in the petrosal (216 +/- 37, mean +/- S.E.M.) nodose (825 +/- 434) and superior cervical ganglia (3583 +/- 1227) demonstrating the sympathetic efferent innervation of the carotid sinus and a dual sensory innervation via both the glossopharyngeal and vagus nerves.
Previous neuroanatomical studies described the central representation of the carotid sinus nerve, but did not differentiate the projections of the baroreceptors from the chemoreceptors present in the carotid bifurcation. In this research we investigated the individual territories occupied by the primary afferents from the carotid body in the brainstem of the cat. We also studied the distribution of afferent and efferent neurons to the carotid body. We injected into the carotid body lectin coupled to horseradish peroxidase. We found labeled axons only in the nucleus of the tractus solitarius; in particular, we found strong projections to the following ipsilateral subnuclei: dorsal, interstitial, and medial part of the commissural subnucleus. Moderate labeling was found in the ipsilateral medial and intermediate subnuclei and in the contralateral dorsal subnucleus and the medial region of the commissural subnucleus. We found a mean of 256 +/- 79 (S.E.M.) labeled afferent ganglion cells in the petrosal ganglia, and no evidence of efferent neurons in the brainstem that innervate the carotid body; conversely, about 4000 efferent neurons of the superior cervical ganglion send terminals to the ipsilateral carotid body.
The organization of axon collaterals, preterminal processes, and presumptive synaptic boutons of single physiologically identified rapidly adapting receptor (RAR) pulmonary afferent fibers was examined following the intraaxonal application of wheat germ agglutinin conjugated with horseradish peroxidase (WGA‐HRP). The RAR axons were injected 200–300 μm lateral to the nucleus of the tractus solitarius (nTS) at a number of different rostrocaudal levels in seven individual experiments. The trajectories of the stained axons were reconstructed from individual 50‐μm‐thick serial sections. The rostrocaudal extent, as well as the distribution of the trajectory of each RAR afferent, was reconstructed from every section by using a camera lucida attachment. In this first of two papers, we describe the pattern of organization of bouton terminals of RAR afferents related to cytoarchitectonically distinct subnuclei of the nTS. In the companion paper, morphological details of the fine structure of these synaptic boutons and axonal branches are described in different subnuclei in order to illustrate morphological differences in these functionally distinct regions.
A number of significant findings have resulted from this light microscopic study. The central process of a single RAR afferent fiber arborized in the medulla oblongata over a considerable distance in the rostrocaudal plane (2.5 mm rostral to 1.4 mm caudal to the obex). A single RAR afferent fiber terminated in numerous bouton terminals (range 500–1,050), and these terminals arose from over 400 segments of branches of the parent injected axon. A small number of en passant bouton terminals were found. There appeared to be a remarkable degree of consistency in the subnuclei of the nTS where these terminals arborized. The dorsal and dorsolateral subnuclei of the nTS received 144–647 bouton terminals. The second‐largest concentration of bouton terminals of RAR afferents was found in the intermediate (nI) subnucleus of the nTS. No labeled bouton terminal was found in the ventral and ventrolateral subnuclei of the nTS. This finding is in sharp contrast to the terminations of SAR afferents which terminated predominantly in the ventral and ventrolateral nuclei of the nTS, the interstitial nucleus of the nTS, and the nI.
The parent RAR axon could be traced as far rostrally as 2.5 mm, even though the region of terminal arborization could not be followed beyond 0.8 mm. The destination of this rostrally projecting RAR afferent could not be determined in this study. This pattern indicates that a single RAR afferent fiber ascends rostrally in the tractus solitarius giving off branches at a number of different levels.
Since the rostrocaudal location of the randomly selected RAR afferent impaled in the medulla varied considerably in all seven cases examined, it is likely the location of their peripheral endings was different in each case. In spite of this viscerotopic difference between the injected axons, the patern of distribution of the axon collaterals and synaptic boutons in the nTS was very similar in all cases.
This illustrates that modality specificity, and not viscerotopic specificity, is an important feature of the central organization of these afferents. These findings are similar to the modality specificity observed with SAR afferents (Kalia and Richter: J. Comp. Neurol. 233 : 308‐332, '85a). In addition, these results further support the concept of functional organization of the nTS along cytoarchitectonic boundaries.
Substance P binding sites in the nucleus tractus solitarius were visualized with receptor autoradiography using Bolton-Hunter [125I]substance P. Substance P binding sites were found to have distinct patterns within the cat nucleus tractus solitarius. The majority of substance P binding sites were present in the medial, intermediate and the peripheral rim of the parvocellular subdivisions. Lower amounts of substance P binding sites were present in the commissural, ventrolateral, interstitial and dorsolateral subdivisions. No substance P binding sites were present in the central region of the parvocellular subdivision or the solitary tract. The localization of substance P binding sites in the nucleus tractus solitarius is very similar to the patterns of substance P immunoreactive fibers previously described for this region. Results of this study add further support for a functional role of substance P in synaptic circuits of the nucleus tractus solitarius.
The distribution of substance P in the vagal system of the cat was studied by immunohistochemistry. Substance P-immunoreactive cell bodies and fibres were observed in the nodose ganglion. Numerous substance P-immunoreactive terminals and fibres were localized in their bulbar projection area, i.e. throughout the caudo-rostral extent of the nucleus of the solitary tract. Four subnuclei, among the nine forming the nucleus of the solitary tract, were strongly labelled: interstitial, gelatinosus, dorsal and commissural. The dorsal motor nucleus of the vagus nerve also exhibited numerous substance P-immunoreactive terminals, sometimes closely apposed on the somata of preganglionic neurons. To determine the substance P component of the vagal afferent system a nodose ganglion was removed on one side. The ablation triggered ipsilaterally a large decrease of substance P immunoreactivity in the four subnuclei strongly labelled on normal cats. These results suggest the involvement of substance P-containing vagal fibres in integrative processes of the central regulation of cardiovascular, digestive and respiratory systems, viscerotopically organized throughout these four subnuclei. The nodose ablation also resulted in a decrease of substance P immunoreactivity in the ipsilateral dorsal motor nucleus of the vagus nerve, suggesting monosynaptic vago-vagal interactions.
The morphology of midlumbar interneurones with peripheral input from group II muscle afferents was analysed after intracellular injection of horseradish peroxidase (HRP). Twenty-three interneurones were stained intrasomatically and five others intra-axonally. The majority (10 of 13) of interneurones located in lamina VII (intermediate zone and ventral horn interneurones) were found to project ipsilaterally. They had medium-sized somata and dendrites projecting radially over a distance of more than 1 mm. All of these neurones had axons that projected caudally within the ventral part of the lateral funiculus or in the lateral part of the ventral funiculus, although four had in addition an ascending secondary axonal branch. Numerous axon collaterals were given off from these axons, both before and after they left the grey matter. The collaterals arborized within laminae VII, VIII, and IX, where they covered the area of several motor nuclei. Intra-axonal labelling of five neurones with similar input and axon trajectories revealed several axon collaterals given off between the cell body and the terminal projection areas in L7 or S1 segments. Only three of the labelled interneurones located in lamina VII and displaying the same kind of input had axons with different destinations; their axons crossed to the opposite side of the spinal cord and ascended within the contralateral ventral funiculus. These were large neurones with extensive dendritic trees, which had fairly thick axons with initial axon collaterals that branched primarily ipsilaterally (within laminae V-VIII). Interneurones located in lamina V and in the bordering parts of laminae IV and VI (dorsal horn interneurones; n = 10) constituted a very nonhomogenous population. They projected either ipsilaterally or contralaterally and had either ascending or descending axons running in either the lateral or ventral funiculi. Generally, dorsal horn interneurones had cell bodies smaller than those of intermediate zone and ventral horn interneurones, and their dendrites extended less extensively and less uniformly around the soma. Their initial axon collaterals branched primarily in the dorsal horn, or in lamina VII, but not in or close to the motor nuclei. Our results support the conclusions of previous physiological studies that the intermediate zone and ventral horn midlumbar interneurones with group II input and that project to motor nuclei have collateral actions on other interneurones in the L4-L6 segments, and that dorsal horn interneurones do not project to motoneurones, but have as their targets other interneurones or ascending cells.(ABSTRACT TRUNCATED AT 400 WORDS)
In cats anaesthetized with a mixture of chloralose and urethane, stimulation of pulmonary C fibres by right atrial injections of phenylbiguanide caused, after a latency of about 3 s, a reduction in pulmonary ventilation or apnoea, bradycardia and systemic hypotension, confirming previous work. A decrease in femoral artery perfusion pressure also occurred under conditions in which the hindlimb was vascularly isolated, the blood flow was maintained constant and the inferior vena caval pressure did not change. This indicates a reduction in vascular resistance due to vasodilatation. The response was unaffected by atropine and propranolol, but was reduced or abolished by guanethidine, hexamethonium and denervation of the limb, indicating that it is due to a reduction in activity in sympathetic vasoconstrictor fibres. Similar cardiovascular responses were observed when the arterial blood pressure was maintained constant, and also in artificially ventilated animals. Evidence is presented that the receptors responsible for the respiratory and cardiovascular responses to right atrial injections of phenylbiguanide lie in the pulmonary vascular bed. When the pulmonary C fibres were excited during a period of apnoea which was induced reflexly by electrical stimulation of the central end of a superior laryngeal nerve, there were no consistent differences in the size of the cardiac or vascular responses compared to the control responses in the absence of the laryngeal input. This result occurred irrespective of the size of the control ventilatory response to phenylbiguanide. By contrast in the same experiments, the cardio-inhibitory and vasoconstrictor responses to excitation of the carotid body chemoreceptors were invariably potentiated by electrical stimulation of a superior laryngeal nerve, as found previously. The possible central mechanisms responsible for the differential modulation of pulmonary C fibre and carotid chemoreceptor reflexes by respiration are discussed.
Baroreceptor afferent fibres and second order baroreceptor neurones were identified by their discharge pattern and were intracellularly injected with horseradish peroxidase. Three afferent fibres and three second order neurones were reconstructed by camera lucida drawings from serial sections of the brainstem. The afferent fibres were classified as A delta-fibres and had terminal arborizations with synaptic boutons in the dorsomedial region of the nuclei of the solitary tract (TS). The afferent fibres had additional collaterals with a medial projection to the commissural nucleus and in a direction lateral to the TS. The terminals of these collaterals could not be demonstrated. The second order neurones were located in the same dorsomedial region as the synaptic boutons of the afferent fibres. Neurones were small and spindle-shaped with two primary dendrites: one dendrite projected cranially along the medial border of the TS, and the second one projected caudally and medially into the commissural nucleus. The unmyalinated axons of these neurones could be traced over a distance of 1 mm. In only one neurone could an axon collateral be detected. The axons projected dorsally around the TS in a ventrolateral direction beyond the boundaries of the nuclei of the TS. The axon collateral projected in the medial direction into the commissural nucleus. In no case were axon terminals demonstrated.
1. The activity of pump (p.) cells, second-order neurones in the pulmonary stretch receptor pathway, was recorded extracellularly in the nucleus of the tractus solitarius (n.t.s.) of the decerebrate cat. Their firing was proportional to changes in lung volume but unrelated to the centrally determined respiratory rhythm. A systematic search of the n.t.s. for the location of p. cells was made and an assessment of their efferent projection to the contralateral n.t.s. was determined electrophysiologically by the antidromic mapping technique. 2. P. cells were located around, and in close proximity to, the solitary tract. The two sites of greatest density were ventromedial and dorsolateral to the tract, with lower concentrations found laterally and ventrolaterally. 3. For twelve of the thirty p. cells tested, evidence of a projection to the contralateral n.t.s. was obtained; in seven of these cells, axonal arborizations within the projection area were identified. Almost all the cells that sent axons to the contralateral n.t.s. were located dorsolateral to the tract; there was no evidence that cells in the ventromedial region had contralateral projections. 4. No evidence that R beta neurones project to the contralateral commissural and ventrolateral subnuclei was found. 5. No p. cells projected to the contralateral ventrolateral n.t.s. The site of projection and branching was consistently localized just caudal to the obex and medial to the solitary tract, in the caudal medial, and commissural subnuclei of the n.t.s. This same region has been shown to receive a dense, direct projection from pulmonary rapidly adapting receptors.
The activity of pulmonary rapidly adapting receptor (r.a.r.) neurones was recorded extracellularly in the nodose ganglion of the decerebrate cat. The receptors were identified by their rapid adaptation to 'ramp and hold' hyperinflations of the lung. The antidromic mapping technique was used to determine the sites of projection and branching patterns within the nucleus of the tractus solitarius (n.t.s.) of eleven r.a.r.s. The medulla was explored with a stimulating electrode to activate the r.a.r.s. antidromically. In each penetration, depth-threshold measurements were made for each antidromic response characterized by a distinct latency. Using the anatomical sites of the minimum threshold points, the locations of central branches of individual r.a.r.s. were determined. The main axons of all of them coursed within the tractus solitarius (t.s.) at levels from 2 mm rostral to 0.5 mm caudal to the obex. The axonal conduction velocities within the t.s. were 6.2-9.7 m/s, where the peripheral conduction velocities were 11.2-20.4 m/s (28 degrees C). Different latencies of response evoked in a single penetration were considered to indicate branching. The densest branching was found in the ipsilateral commissural subnucleus of the n.t.s. at levels 0.3-1.3 mm caudal to the obex and, to a lesser degree, in the contralateral commissural subnucleus. All r.a.r.s. sent a few branches to the medial n.t.s. rostral to the obex. Four r.a.r.s. ramified in the ventrolateral n.t.s. where inspiratory cells are located. Depth-threshold graphs were interpolated by best fitting parabolic equations: Ith = Ad2 + Bd + C; where Ith is the threshold current, d the corresponding depth of stimulation, and A, B and C are coefficients. Coefficient A is a measure of steepness of the parabola. The A coefficients were inversely related to the conduction velocity (v) of the stimulated branch. An analysis of the data from the present study (v = 5.0-9.7 m/s) combined with data from the literature (v = 2.2-85 m/s) led to a simple relationship between the A coefficient and the conduction velocity of the stimulated fibre: A = 6500/v, where A is expressed in microA/mm2 and v is expressed in m/s. Within the range 3-35 m/s, the formula is useful in predicting the effective current spread when the conduction velocity is known, or to estimate the conduction velocity from the shape of a depth-threshold curve. Two slowly adapting pulmonary stretch receptors (p.s.r.s) were studied.(ABSTRACT TRUNCATED AT 400 WORDS)
Unmyelinated (C) fibers are the most numerous sensory elements of mammalian peripheral nerve and comprise many of those responsible for initiating pain and temperature reactions; however, direct evidence has been lacking as to where and how these fibers terminate in the central nervous system. A plant lectin (Phaseolus vulgaris leukoagglutinin) was applied intracellularly by iontophoresis as an immunocytochemical marker. This permitted visualization of the central terminations of cutaneous C sensory fibers that had been identified by the nature of stimuli that excited them. The central branch of C-fiber units arborized and terminated mainly in the superficial layers of the spinal dorsal horn in defined patterns that related to their functional attributes. Thus, the superficial dorsal horn seems to act as a processing station for signals from fine sensory fibers.
Extracellular recordings were made from 164 neurones in the nucleus tractus solitarius and dorsal motor vagal nucleus of the chloralose-anaesthetized cat. 139 neurones were excited synaptically and 25 non-synaptically by electrical stimulation of cardiac and pulmonary vagal branches. Synaptically excited neurones fall into two populations, one activated solely by myelinated afferent fibres and a second activated solely by non-myelinated afferent fibres. 94 neurones were synaptically excited by afferent fibres in a single vagal branch while 45 were excited by stimulation of two or three branches. Neurones responding to volleys in myelinated afferent fibres were located in both medial and lateral regions of the nucleus tractus solitarius whilst those excited by non-myelinated afferent fibres were restricted to the medial region. Consistent differences in the locations of neurones excited by stimulation of either cardiac or pulmonary or by single or several branches could not be distinguished.
Data obtained from neonatally treated rats are fairly consistent. However, there is disagreement as to whether mechanical and thermal nociceptive thresholds are elevated or unchanged in this group. There are at least two major areas of disagreement in adult animal capsaicin research. Behavioral data are extremely variable. The thermal nociceptive threshold after systemic capsaicin has been reported to be both raised and lowered. After intrathecal capsaicin injection, the thermal nociceptive threshold was reported raised, but onset and duration of responses varied and some animals exhibited no changes. Capsaicin application to peripheral nerve, however, drastically increased thermal threshold. Mechanical pain threshold has been reported both increased and unchanged after systemic capsaicin treatment and unchanged after intrathecal injection. Obviously, capsaicin's effects upon pain perception are not fully understood. Although lower on the phylogenetic scale than many mammals, rodents exhibit complex individualistic behavior. Lower vertebrates may eventually provide more simple behavioral models for pain tolerance.
With the aid of immunohistochemical techniques the distribution of substance P, met-enkephalin, serotonin, somatostatin, α-melanocyte stimulating hormone, neurotensin and neurophysin immunoreactivities were mapped throughout the rostro-caudal extent of the cat's nucleus tractus solitarii. Three of the putative neurotransmitters (substance P, enkephalin and serotonin) were found to be widely distributed as varicose fibers and punctate structures. The densities of their immunoreactivities were plotted in a range from very dense, dense, moderate, occasional, to none, at different levels of the nucleus of the solitary tract. Substance P immunoreactivity was the most varied and dense of all the neurotransmitters studied. Its accumulations ranged from very dense in the lateral, dense in portions of the parvocellular and lateral, moderate in medial and commissural and occasional in ventrolateral and portions of the parvocellular subdivisions. Both the enkephalin and serotonin immunoreactivities had patterns similar to that of substance P immunoreactivity, although their amounts were not as great. Following colchicine treatment neurons containing substance P and enkephalin immunoreactivity were found in many subdivisions of the nucleus of the solitary tract. Somatostatin, α-melanocyte stimulating hormone, neurotensin and neurophysin immunoreactivities were present in the nucleus of the solitary tract as isolated varicose fibers scattered throughout the nucleus. Immunoreactive neurons were not found for these putative neurotransmitters after colchicine treatment.
1. Central projections of single slowly adapting pulmonary stretch receptors were mapped in the medulla by the technique of spike-triggered averaging of extracellular field potentials. Discharge of pulmonary stretch receptors was recorded in continuity from the nodose ganglion; this activity provided the trigger for an averaging computer. 2. These pulmonary stretch receptors were characterized by a linear increase in firing rate in response to increases in transpulmonary pressure, an adaptation index, and peripheral axonal and intramedullary conduction velocities. 3. In accordance with the terminology used by Munson and Sypert (21), three types of electrical potentials were observed for the projection of a pulmonary stretch receptor in the medulla. Axonal potentials were recorded when the brain stem electrode was in the vicinity of the afferent axon. Terminal potentials were recorded when the electrode was adjacent to terminations of the afferent axon. Focal synaptic potentials were recorded when the electrode was near postsynaptic units receiving input from the pulmonary stretch receptor. Maxima of terminal potentials were recorded in a region 1 mm rostral to the obex in the medial nucleus of the tractus solitarius (six cases), in the ventrolateral nucleus of the tractus solitarius (three cases), and in an area just dorsolateral to the tractus solitarius (two cases). Focal synaptic potentials for five pulmonary stretch receptors were observed in a region 1 mm rostral to obex. Maxima of these potentials were recorded in the medial nucleus of tractus solitarius (two cases), in the ventrolateral nucleus of tractus solitarius (two cases), and in an area just dorsolateral to the tractus solitarius (one case). 4. Occasionally both terminal and focal synaptic potentials were observed for the same pulmonary afferent. The difference in the latencies of these potentials fell within the range previously reported for monosynaptic connections of muscle spindle Ia and group II afferents for alpha-motoneurons. This suggests that the afferents of pulmonary stretch receptors have monosynaptic connections with neurons in the medial nucleus of the tractus solitarius, in the ventrolateral nucleus of the tractus solitarius, and in an area dorsolateral to the tractus solitarius.
We have attempted in this review to give an account of an afferent vagal input from the lower respiratory tract that has still to be explored fully, and to present experimental evidence that this fine fibre afferent system plays a significant role in the neural control of respiratory function in both normal and pathological circumstances. We have made a distinction between the afferent C fibres that innervate the lung parenchyma adjacent to the pulmonary capillary bed and those that innervate the conducting airways, even though the afferent C fibres in the two locations appear to have reflex properties that are at least qualitatively similar. We believe that the functional significance of these lower respiratory tract C fibres is determined not simply by their location but also by certain differences in afferent properties that should not be overlooked. Douglas and Ritchie (1962) suggested, in their review of mammalian non-myelinated nerve fibres, that the teleological advantage of the finefibre afferent system, especially in the case of a visceral input where speed of impulse transmission was not of primary importance, was that it allows fibres of a variety of sensory modalities to be accommodated in a small cross-sectional area of nerve trunk. There is no reason to think that the full range of sensory modalities of the afferent C fibres in the lungs and airways has yet been explored. The custom of injecting certain chemicals to identify the presence of lower respiratory tract C fibres when recording the activity in vagal strands is highly selective, so that even now our view of this afferent fibre system may be unnecessarily narrow. Some of the conclusions arrived at in these pages are either purely speculative or derived from experimental evidence that is at best indirect. Whether they prove to be correct or incorrect — and some are sure to fall into the latter category — their purpose will have been served if they are put to the test of experiment.
The medullary projections of afferent neurons with cell bodies in the petrosal ganglion have been investigated using an antidromic mapping technique. Of the ninety-three units studied, fifty-eight were shown to have patterns of discharge indicating that they were baroreceptors and thirty-five showed responses to stimuli indicating that they were arterial chemoreceptors. Twelve baroreceptor and thirteen chemoreceptor afferents had sufficiently stable unitary discharges to permit a detailed estimation of some of their central projections using stimulation through monopolar tungsten micro-electrodes to evoke antidromic spikes. In order to estimate their pattern of projection, depth-threshold contours for each penetration through the dorsomedial medulla and the values of antidromic latency were considered. Baroreceptor afferent fibres with myelinated (six units) and non-myelinated (six units) axons showed similar patterns of central projection. All could be activated from the ipsilateral nucleus of the tractus solitarius (n.t.s.), most often from its lateral divisions rostral to the obex. The dorsolateral and dorsomedial portions of the n.t.s. were most often innervated, with the commissural subnucleus receiving an innervation in seven of the twelve neurones studied. Stimulation of the ventrolateral subnucleus was effective in activating two afferent fibres whilst stimulation of the ventral subnucleus was effective in only one case. All chemoreceptor afferent fibres had calculated conduction velocities less than 4 m/s and all were activated from the dorsomedial and medial subnuclei of the ipsilateral n.t.s. In twelve of the thirteen neurones investigated in detail there was evidence of an innervation of the commissural nucleus both at the level of the obex and behind it. In three cases this extended into the contralateral portion of the commissural nucleus. In four cases a sparse innervation of the lateral subnucleus, comprising its dorsolateral aspects, was seen. The potential significance of these distinctive patterns of projection of arterial baroreceptors and chemoreceptors is discussed in relation to cardiovascular and respiratory control.
1. In eighteen anaesthetized cats, action potentials in non-myelinated vagal afferent neurones were recorded in the nodose ganglion by means of extracellular micro-electrodes. 2. The pulmonary or bronchial origin of these C fibres was assessed in closed chest preparations by injecting phenyl diguanide into either the right atrium or the ascending aorta (bronchial circulation). This was confirmed in two animals by local mechanical stimulation. 3. Eighty per cent of bronchopulmonary C fibres increased their discharge frequency when the end-tidal CO2 concentration (FA,CO2) increased from 0.02 to 0.10. Most of these C endings showed a maximal response when FA,CO2 reached 0.04. For the others a further increase in discharge occurred when CO2 concentration reached 0.08-0.10. Continuous measurement of C fibre discharge frequency indicated that they detected preferentially changes in the inspired CO2 content, but adapted when a high CO2 level was maintained. Their activation by hypercapnia was followed by an increase in lung resistance. 4. Lowering the O2 content of the inspired gas had no effect on the spontaneous activity of bronchopulmonary C endings. 5. When the stroke volume of the pump was doubled, the spontaneous activity of bronchopulmonary C fibres decreased in intact chest preparations. Inflation of the lungs had the opposite effect after the chest was opened. In both cases hyperdeflation was a potent stimulus to these receptors. 6. In tracheotomized cats, the tracheal temperature was 28-29 degrees C. When normal thermal conditions were restored in the tracheal lumen (33-34 degrees C) the spontaneous discharge frequency of some bronchial C fibres was greatly increased. 7. It is concluded that the spontaneous activity of most of the bronchial or pulmonary C fibres was maximal when chemical and physical physiological conditions were restored in the lungs. It appears that changes in alveolar CO2 concentration constitute the usual stimulus for these fibres.
1. Micro-electrode recordings were made from slowly adapting pulmonary stretch afferents within the nodose ganglia of cats and rabbits. Recordings sites were distributed throughout the ganglia. 2. The projections of these afferents to the medulla oblongata were studied by antidromic stimulation. 'Point' and 'Field' type depth--threshold curves were interpreted as corresponding to stimulation of the main afferent axons and its branches, respectively. Increases in antidromic latency in conjunction with 'field' contours was additional evidence in support of this interpretation. 3. In cats, most (six out of seven) afferents had extensive branches, and probably also terminations, within the medial subnucleus of the ipsilateral nucleus tractus solitarius (n.t.s.). Two of these, plus one other afferent, also had projections to the lateral and ventrolateral subnuclei. 4. In rabbits the projections of such afferents were similar, i.e. mainly to the medial subnucleus of the n.t.s. (eight out of eleven) but also extending into the nucleus alaris, and occasionally to lateral and ventrolateral subnuclei (two out of eleven) or to both regions (one out of eleven). 5. Branching of single afferents was seen to occur over up to 3 mm of the rostro-caudal extent of the intermediate region of the n.t.s. The significance of the observations is discussed.
The composition of the bronchial branches of the vagus nerves was studied in cats using light and electron microscopy. In order to determine the number and the diameter of fibers in the afferent and the efferent components, a unilateral efferent vagotomy was performed. The myelinated and the non-myelinated fibers were counted from the total nerve area and the endoneural area of each one was measured by means of a computer. Composition of "afferent" bronchial nerves (after degeneration following the efferent vagotomy) was compared to that of "entire" nerves. The main results are: (1) efferent fibers represent about 40% of the "entire" bronchial nerve; (2) non-myelinated fibers constitute more than 90% of the total population of the "entire" nerves as well as of the efferent component; (3) the density of myelinated and non-myelinated fibers (i.e. their number per surface unit) was similar in all nerves. However, there were discrepancies between diameter histograms established from different areas of a section. This feature seems to be due to preferential compartmentalization by Schwann cell envelopment of fibers having comparable diameter. This "packing effect" was observed in both efferent and afferent components.
The number and caliber of myelinated and non-myelinated fibers of entire and sensory vagal nerves of cats were studied by means of light and electron miscroscopy. The results obtained with electron microscopy show that the non-myelinated component is particularly rich (about 40,000 elements at the cervical level), with clearly higher numbers of fibers than demonstrated thus far with light microscopy. The ratio of myelinated to non-myelinated fibers is on the average 1:4 for the total vagi and only 1:8 for the sensory vaga component. The comparison of the nerve above and below the level of the nodose ganglion shows that (1) mean fiber diameter is usually greater at the infranodose than at the supranodose level, and (2) some myelinated fibers of small diameter occurring below the nodose ganglion become non-myelinated above it. Additionally, the number of non-myelinated fibers per Schwann cell is greater at the supranodose than at the infranodose level; this speaks in favor of a reorganization of the C-fiber population from one level to the other.
The central sensory and motor connections of various respiratory, cardiovascular, and gastrointestinal viscera were analyzed using the transganglionic and retrograde transport of horseradish peroxidase (HRP).
In 42 adult cats, we examined the brain stem and peripheral ganglia following microinjections of HRP (10 μl) into individual visceral organs—larynx, extrathoracic trachea, intrathoracic trachea, right main bronchus, right lung (upper lobe), heart, and stomach. Comparison of individual cases led to the conclusion that distinct patterns of sensory and motor projections to the medulla exist for each visceral organ studied.
The nucleus of the tractus solitarius (nTS) receives the sensory projections from all the viscera listed above, with two exceptions: (1) a few sensory fibers from the larynx terminate in the ipsilateral spinal tract of the trigeminal nerve (spV), and (2) some sensory fibers from the bronchus, lung, and stomach terminate in the area postrema (ap). Within the nTS, the sensory fibers from each visceral organ terminate in a number of subnuclei. The dnTS, mnTS, and ncom receive sensory projections from all the viscera studied. The remaining five subnuclei (dlnTS, ni, nI, vlnTS, vnTS) of the nTS are not connected to all viscera, and the density of projections to these regions varies for different viscera. However, there does not seem to be any specific region of the medulla which is devoted entirely to receiving the sensory fibers from a particular visceral organ. Rather, the rostrocaudal extent of sensory fibers, from most of the viscera studied, spans the entire length of the medulla. Differences in the central representation of viscera were found to be subtle and to lie within the organization of the nuclear subgroups of the nTS.
The central representation of unpaired or midline viscera (e.g., trachea and heart) is bilateral for both sensory and motor innervation. However, for unilateral, paired viscera (e.g., bronchi and lungs), it was consistently found that over one third of the sensory and motor representation is contralateral. Control experiments involving vascular injections of HRP excluded the possibility that this contralateral labeling could have been due to vascular uptake of the enzyme. The localization of sensory perikarya of visceral afferents in the “principal visceral ganglion” of the vagus—the nodose ganglion—was overlapping, and no well‐demarcated regions in the nodose ganglion could be identified that received projections primarily from a given visceral organ.
The motor nuclei providing parasympathetic (preganglionic) and somatic motor innervation to the viscera were primarily the dmnX, nA, and nRA. The entire dmnX (extending over 10–15 mm rostrocaudally), contributed fibers to each area injected with HRP, with the exception of the extrathoracic trachea. No region in the dmnX was found where preference was given to a specific viscus. The nA contributed efferents to all the viscera studied, and this contribution came from the entire 6 mm of nA contributing vagal efferents. Visceral containing smooth muscle as well as skeletal muscle were innervated by the nA. The caudal nRA provided motor fibers to the larynx, trachea, and stomach, and again no preferred rostrocaudal representation of motoneurons to a given viscus was found. Postganglionic sympathetic innervation to the viscera studied was found to be localized to the stellate and superior cervical ganglia. Within these sympathetic ganglia, some regional preference for different viscera was detected. In the case of motor innervation, unilateral paired viscera received motor fibers from both ipsilateral and contralateral sides of the medulla via both vagus nerves.
The motor and sensory connections of the cervical vagus nerve and of its inferior ganglion (nodose ganglion) have been traced in the medulla oblongata of 32 adult cats with the neuroanatomical methods of horseradish peroxidase (HRP) histochemistry and amino acid autoradiography (ARG). In 14 of these subjects, an aqueous solution of HRP was applied unilaterally to the central end of the severed cervical vagus nerve. In 13 other cases, HRP was injected directly into the nodose ganglion. Three of these 13 subjects had undergone infranodose vagotomy 6 weeks prior to the HRP injection. A mixture of tritiated amino acid was injected into the nodose ganglion in five additional cats. The retrograde transport of HRP yielded reaction product in nerve fibers and perikarya of parasympathetic and somatic motoneurons in the medulla oblongata. Furthermore, a tetramethyl benzidine (TMB) method for visualizing HRP enabled the demonstration of anterograde and transganglionic transport, so that central sensory connections of the nodose ganglion and of the vagus nerve could also be traced. The central distribution of silver grain following injections of tritiated amino acids in the nodose ganglion corresponded closely with the distribution of sensory projections demonstrated with HRP, thus confirming the validity of HRP histochemistry as a method for tracing these projections. The histochemical and autoradiographic experiments showed that the vagus nerve enters the medulla from its lateral aspect in multiple fascicles and that it contains three major components--axons of preganglionic parasympathetic neurones, axons of skeletal motoneurons, and central processes of the sensory neurons in the nodose ganglion. Retrogradely labeled neurons were seen in the dorsal motor nucleus of X(dmnX), the nucleus ambiguus (nA), the nucleus retroambigualis (nRA), the nucleus dorsomedialis (ndm) and the spinal nucleus of the accessory nerve (nspA). The axons arising from motoneurons in the nA did not traverse the medulla directly laterally; rather, all of these axons were initially directed dorsomedially toward the dmnX, where they formed a hairpin loop and then accompanied the axons of dmnX neurons to their points of exit. Afferent fibers in the vagus nerve reached most of the subnuclei of the nTS bilaterally, with the more intense labeling being found on the ipsilateral side. Labeling of sensory vagal projections was also found in the area postrema of both sides and around neurons of the dmnX. These direct sensory projections terminating within the dmnX may provide an anatomical substrate for vagally mediated monosynpatic reflexes. Following deefferentiation by infranodose vagotomy 6 weeks prior to HRP injections into the nodose ganglion, a number of neurons in the dmnX were still intensely labeled with the HRP reaction product. The axons of these HRP-labeled perikarya may constitute the bulbar component of the accessory nerve.
The observations contained in this report stem from a study of pain afferent nerve fibers from craniofacial and cervical regions, particularly in relation to referred-pain syndromes and atypical neuralgias.Early studies by Cajal and van Gehuchten, among others, provided much of the basic information on the organization of the visceral afferent fibers of the facial (VII), glossopharyngeal (IX), and vagal (X) nerves. Subsequent investigators* have traced these afferent fibers in various species, including man.Despite these detailed and careful reports, numerous aspects of the connections of the afferent fibers remain undetermined. In particular, the site of termination of fibers mediating pain has been inferred mainly from surgical and clinical studies, but never established anatomically; tactile connections have been suggested on rather vague grounds, and deep sensibility connections have remained entirely unknown.Material and Methods
Twenty-three adult cats weighing between 4 and 9 lb. each were used; the results in 17
Pulmonary C-fibers project to the caudal, medial nucleus tractus solitarius of the cat
- L Davies
KUBIN, L. & DAVIES, R. 0. (1987). Pulmonary C-fibers project to the caudal, medial nucleus
tractus solitarius of the cat. Federation Proceedings 46, 826.
Distribution of broncho-pulmonary afferent C-fibers within the nucleus tractus solitarius (NTS) of the cat
- L Davies
KUBIN, L. & DAVIES, R. 0. (1989). Distribution of broncho-pulmonary afferent C-fibers within the
nucleus tractus solitarius (NTS) of the cat. Proceedings of the International Union of Physiological
Sciences XVII, 74-75.