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Evolution of the Cardiovascular Autonomic Nervous System in Vertebrates
Abstract
This chapter discusses the evolution of the cardiovascular autonomic nervous system (ANS) in vertebrates and comparative aspects of the autonomic regulation of the cardiovascular system. Many of the basic functions of the ANS were originally discovered in ectothermic vertebrates. Within hagfishes and lampreys the ANS is considered rudimentary, because either some organs are devoid of innervation or most organs lack dual innervation. Therefore, while the ANS of early vertebrates can be considered simple in comparison to subsequent groups of vertebrates, the basic foundations of the ANS were established by the time of fishes and evolved prior to the invasion of terrestrial habitats. The morphology and the function of the heart and cardiovascular systems of vertebrates have undergone large evolutionary changes associated with the transition between water and air-breathing and during the evolution of endothermy within mammals and birds. In tunicates and Amphioxus, the endothelium within the blood vessel is either very poorly developed or absent, and there is no endothelium in innervation amongst protostomes.
... All vertebrates maintain mean arterial blood pressure (MAP) within relatively narrow limits by fast autonomic reflexes that modulate heart rate ( f H ) and peripheral resistance (Bagshaw, 1985;Van Vliet and West, 1994;Wang, 2012). In reptiles, these autonomic reflexes are initiated by baroreceptors within the vessel wall of the truncus arteriosus and the systemic and pulmonary arteries, and the efferent regulation of f H is provided by a coordinated balance between parasympathetic inhibition and sympathetic stimulation (Bagshaw, 1985;Berger, 1987;Lillywhite and Donald, 1994;Van Vliet and West, 1994;Wang, 2012). ...
... All vertebrates maintain mean arterial blood pressure (MAP) within relatively narrow limits by fast autonomic reflexes that modulate heart rate ( f H ) and peripheral resistance (Bagshaw, 1985;Van Vliet and West, 1994;Wang, 2012). In reptiles, these autonomic reflexes are initiated by baroreceptors within the vessel wall of the truncus arteriosus and the systemic and pulmonary arteries, and the efferent regulation of f H is provided by a coordinated balance between parasympathetic inhibition and sympathetic stimulation (Bagshaw, 1985;Berger, 1987;Lillywhite and Donald, 1994;Van Vliet and West, 1994;Wang, 2012). ...
When snakes digest large meals, heart rate is accelerated by withdrawal of vagal tone and an increased non-adrenergic-non-cholinergic tone that seems to stem from circulating blood-borne factors exerting positive chronotropic effects. To investigate whether this tonic elevation of heart rate impairs the ability for autonomic regulation of heart during digestion, we characterised heart rate responses to pharmacological manipulation of blood pressure in the snake Boa constrictor through serial injections of sodium nitroprusside and phenylephrine. Both fasting and digesting snakes responded with a robust tachycardia to hypotension induced by sodium nitroprusside, with digesting snakes attaining higher maximal heart rates than fasting snakes. Both fasting and digesting snakes exhibited small reductions of the cardiac chronotropic response to hypertension, induced by injection of phenylephrine. All heart rate changes were abolished by autonomic blockade with the combination of atropine and propranolol. The digesting snakes retained the capacity for compensatory heart rate responses to hypotension, despite their higher resting values, and the upward shift of the barostatic response curve enable snakes to maintain the cardiac limb for blood pressure regulation.
... To stay alive, we require environmental support for basic functions such as temperature regulation, oxygen levels, the absence of threatening stimuli, as well as adequate physical shelter and protection. The autonomic nervous system could be placed at the core of the hierarchical structure of the human nervous system, suggesting an ancient origin (Wang, 2012). It handles functions such as homeostasis, blood pressure, heart rate, and respiration, that can be traced back at least to the very first mammals, if not longer (ibid.). ...
Urbanisation and lifestyle-related illnesses increase globally. This highlights the need to shape modern human habitats to support basic recreational needs, promoting such things as physical activity and restoration of high stress levels and cognitive fatigue. Previous research suggests eight perceived qualities in the outdoor environment, described as eight perceived sensory dimensions, as universally meaningful to people in this regard. However quite extensively studied in relation to various health and wellbeing outcomes, human sensitivity and appreciation for these qualities has not yet been explicitly analysed from an evolutionary perspective. This paper investigates their possible evolutionary roots and suggests an order for their development. This is linked with empirical findings on their relative capacity to support restoration of stress and cognitive fatigue. Qualities of earlier origin are suggested to correspond to older, more fundamental adaptations. Each subsequently developed quality implies an increased complexity of our environmental relations, associated with higher demands on more recently developed capacities. The proposed model thus links the more restorative Serene, Sheltered, Natural, and Cohesive perceived sensory dimensions with earlier stages of our development while the more demanding Diverse, Open, Cultural, and Social qualities are associated with more recent transitions. It might be of relevance when shaping modern human habitats from a health- promoting perspective, and have applications in the planning and design of, e.g., health care settings, rehabilitation gardens, urban green areas, recreational forests or other similar outdoor environments.
... The heart rate of vertebrates is determined by the spontaneous rhythm generated by the sinoatrial pacemaker, which is primarily regulated by the dual innervation by the autonomic nervous system. [8][9][10][11] The sympathetic nervous system, acting via catecholamines (ie, noradrenaline and adrenaline, which may also be released as hormones), increases heart rate by stimulating ß-adrenergic receptors. Conversely, the parasympathetic nervous system, acting via the vagus nerve, decreases heart rate, ultimately by cholinergic stimulation of muscarinic receptors. ...
Acute exposure to low oxygen (hypoxia) places conflicting demands on the heart. Whilst an increase in heart rate (tachycardia) may compensate systemic oxygen delivery as arterial oxygenation falls, the heart itself is an energetically expensive organ that may benefit from slowing (bradycardia) to reduce work when oxygen is limited. Both strategies are apparent in vertebrates, with tetrapods (mammals, birds, reptiles and amphibians) classically exhibiting hypoxic tachycardia and fishes displaying a characteristic hypoxic bradycardia. With a richer understanding of the ontogeny and evolution of the responses, however, we see similarities in the underlying mechanisms between vertebrate groups. For example, in adult mammals, a primary bradycardia results from the hypoxic stimulation of carotid body chemoreceptors that is overwhelmed by mechano-sensory feedback from the lung associated with hyperpnoea. A fish-like bradycardia prevails in the mammalian foetus (which, at this stage, is incapable of pulmonary ventilation), and in fish and foetus alike, the bradycardia ensues despite an elevation of circulating catecholamines. In both cases, the reduced heart rate may primarily serve to protect the heart. Thus, the comparative perspective offers fundamental insight into how and why different vertebrates regulate heart rate in different ways during periods of hypoxia.
... The heart of tetrapods and teleost fish is dually innervated by excitatory adrenergic sympathetic nerves and inhibitory cholinergic parasympathetic nerves (Burnstock 1969;Sandblom and Axelsson 2011;Wang 2012;Taylor et al. 2014). Resting reptiles typically exhibit a high cholinergic tone and a low adrenergic tone, and the characteristic tachycardia during exercise is normally mediated by withdrawal of the parasympathetic tone in combination with an increased sympathetic tone (Wang et al. 2001;Joyce et al. 2018;Joyce and Wang 2020). ...
The vertebrate heart is regulated by excitatory adrenergic and inhibitory cholinergic innervations, as well as non-adrenergic non-cholinergic (NANC) factors that may be circulating in the blood or released from the autonomic nerves. As an example of NANC signaling, an increased histaminergic tone, acting through stimulation of H2 receptors, contributes markedly to the rise in heart rate during digestion in pythons. In addition to the direct effects of histamine, it is also known that histamine can reinforce the cholinergic and adrenergic signaling. Thus, to further our understanding of the histaminergic regulation of the cardiovascular response in pythons, we designed a series of in vivo experiments complemented by in vitro experiments on sinoatrial and vascular ring preparations. We demonstrate the tachycardic mechanism of histamine works partly through a direct binding of cardiac H2 receptors and in part through a myocardial histamine-induced catecholamine release, which strengthens the sympathetic adrenergic signaling pathway.
... Heart rate ( f H ) is predominantly regulated by the autonomic nervous system. This includes the parasympathetic (cholinergic) and sympathetic (adrenergic) limbs, which are inhibitory and stimulatory, respectively (Burnstock, 1969;Wang, 2012). The mechanisms underlying f H responses can be inferred using muscarinic cholinergeric (e.g. ...
In the 1950s, Arthur C. Guyton removed the heart from its pedestal in cardiovascular physiology by arguing that cardiac output is primarily regulated by the peripheral vasculature. This is counterintuitive, as modulating heart rate would appear to be the most obvious means of regulating cardiac output. In this Review, we visit recent and classic advances in comparative physiology in light of this concept. Although most vertebrates increase heart rate when oxygen demands rise (e.g. during activity or warming), experimental evidence suggests that this tachycardia is neither necessary nor sufficient to drive a change in cardiac output (i.e. systemic blood flow, Q̇sys) under most circumstances. Instead, Q̇sys is determined by the interplay between vascular conductance (resistance) and capacitance (which is mainly determined by the venous circulation), with a limited and variable contribution from heart function (myocardial inotropy). This pattern prevails across vertebrates; however, we also highlight the unique adaptations that have evolved in certain vertebrate groups to regulate venous return during diving bradycardia (i.e. inferior caval sphincters in diving mammals and atrial smooth muscle in turtles). Going forward, future investigation of cardiovascular responses to altered metabolic rate should pay equal consideration to the factors influencing venous return and cardiac filling as to the factors dictating cardiac function and heart rate.
... The tachycardia is achieved by withdrawal of vagal tone and elevated sympathetic tone [43,44], and the rise in V s stems from elevated venous return. At steady-state, cardiac output must equal venous return, meaning that V s is largely determined by the peripheral vasculature [45,46]. ...
The anatomy of the heart and the respiratory organs differs enormously amongst vertebrates, and the absolute rates of oxygen uptake – both at rest and exercise — are several-fold higher in the endothermic birds and mammals when compared to fish, amphibians and reptiles (all ectothermic). Despite these large differences, all vertebrates can elevate the rate of oxygen consumption by 5–10 times when engaging in physical exercise. The increased oxygen delivery is attained by increasing the convective flows (i.e. increased ventilation and cardiac output) as well as increased extraction of oxygen from the blood leading that widens the arterial-venous oxygen concentration difference with an extraction of approximately 90%. All members of all vertebrate classes appear to exhibit some diffusive limitation for oxygen in the gills or lungs, whereas arterial PCO2 tends to decrease due to hyperventilation.
... The teleost fish heart operates under dual (i.e., cholinergic and adrenergic) autonomic control (Burnstock, 1969;Farrell and Smith, 2017;Vornanen, 2017;Wang, 2012). Cholinergic tone is especially pronounced in red-blooded Antarctic fishes, which, in conjunction with the near-freezing temperatures, results in low resting f H (< 15 beats min −1 (Axelsson et al., 1992;Campbell et al., 2009;Lowe et al., 2005;Sandblom and Axelsson, 2011)). ...
Icefishes characteristically lack the oxygen-binding protein haemoglobin and therefore are especially reliant on cardiovascular regulation to augment oxygen transport when oxygen demand increases, such as during activity and warming. Using both in vivo and in vitro experiments, we evaluated the roles for adrenaline and adenosine, two well-established cardio- and vasoactive molecules, in regulating the cardiovascular system of the blackfin icefish, Chaenocephalus aceratus. Despite increasing cardiac contractility (increasing twitch force and contraction kinetics in isometric myocardial strip preparations) and accelerating heart rate (ƒH), adrenaline (5 nmol kg⁻¹ bolus intra-arterial injection) did not significantly increase cardiac output (Q̇) in vivo because it elicited a large decrease in vascular conductance (Gsys). In contrast, and despite preliminary data suggesting a direct negative inotropic effect of adenosine on isolated atria and little effect on isolated ventricle strips, adenosine (500 nmol kg⁻¹) generated a large increase in Q̇ by increasing Gsys, a change reminiscent of that previously reported during both acute warming and invoked activity. Our data thus illustrate how Q̇ in C. aceratus may be much more dependent on peripheral control of vasomotor tone than direct regulation of the heart.
... The structure and exact location of the pacemaker region differ amongst species (Jensen et al., 2017), but it is always innervated by the autonomic nervous system. This allows the body to increase or decrease the heart rate in response to metabolic demands (Wang, 2012). ...
Why is the alligator heart so similar to the hearts of birds and mammals?
... This may be caused by blood deposition in cutaneous blood vessels due to the venous return being obstructed [40,41]. We hypothesized that when the cuff blocked the single-arm blood flow, the blood pressure of systemic circulation increased, which further triggered sinusoidal reflex by the baroreceptor and peripheral vasculature dilated reflexively [42]. Since 200mmHg pressure was enforced by the cuff, no arterial blood flew into the forearm area while the tissue was in acute hypoxia. ...
We present the real-time single snapshot multiple frequency demodulation - spatial frequency domain imaging (SSMD-SFDI) platform implemented with a visible digital mirror device that is capable of imaging and monitoring dynamic turbid medium and processes over a large field of view. One challenge in quantitative imaging of biological tissue such as the skin is the complex structure rendering techniques based on homogeneous medium models to fail. To address this difficulty we have also developed a novel method that maps the layered structure to a homogeneous medium for spatial frequency domain imaging. The varying penetration depth of spatially modulated light on its wavelength and modulation frequency is used to resolve the layered structure. The efficacy of the real-time SSMD-SFDI platform and this two-layer model is demonstrated by imaging forearms of 6 healthy subjects under the reactive hyperemia protocol. The results show that our approach not only successfully decouples light absorption by melanin from that by hemoglobin and yields accurate determination of cutaneous hemoglobin concentration and oxygen saturation, but also provides reliable estimation of the scattering properties, the melanin content and the epidermal thickness in real time. Potential applications of our system in imaging skin physiological and functional states, cancer screening, and microcirculation monitoring are discussed at the end.
... This innervation, however, differs from the rest of the vertebrates by being excitatory rather than inhibitory. The inhibitory action of the vagus nerve appeared with cartilagenous fishes, and the opposing excitatory sympathetic innervation evolved later with the emergence of bony fishes (reviewed in Burnstock (1969) and Wang (2011)). Interestingly, unlike higher vertebrates, the vagal innervation of the heart in the lamprey is entirely placode derived (McCauley and Bronner-Fraser, 2003 ), suggesting that the contribution of the cardiac NC to the parasympathetic division of the autonomic nervous system is a derived character in vertebrate evolution, which has presumably evolved in parallel with the specialization of the heart. ...
To illustrate the impact developmental biology and genetics have already had on the clinical management of the million infants born worldwide each year with CHD, we have chosen three stories which have had particular relevance for pediatric cardiologists, cardiothoracic surgeons, cardiac anesthesiologists, and cardiac nurses. First, we show how Margaret Kirby's finding of the unexpected contribution of an ectodermal cell population - the cranial neural crest - to the aortic arch arteries and arterial pole of the embryonic avian heart provided a key impetus to the field of cardiovascular patterning. Recognition that a majority of patients affected by the neurocristopathy DiGeorge syndrome have a chromosome 22q11 deletion, have also spurred tremendous efforts to characterize the molecular mechanisms contributing to this pathology, assigning a major role to the transcription factor Tbx1. Second, synthesizing the work of the last two decades by many laboratories on a wide gamut of metazoans (invertebrates, tunicates, agnathans, teleosts, lungfish, amphibians, and amniotes), we review the >20 major modifications and additions to the ancient circulatory arrangement composed solely of a unicameral (one-chambered), contractile myocardial tube and a short proximal aorta. Two changes will be discussed in detail - the interposition of a second cardiac chamber in the circulation and the septation of the cardiac ventricle. By comparing the developmental genetic data of several model organisms, we can better understand the origin of the various components of the multicameral (multi-chambered) heart seen in humans. Third, Martina Brueckner's discovery that a faulty axonemal dynein was responsible for the phenotype of the iv/iv mouse (the first mammalian model of human heterotaxy) focused attention on the biology of cilia. We discuss how even the care of the complex cardiac and non-cardiac anomalies seen in heterotaxy syndrome, which have long seemed impervious to advancements in surgical and medical intensive care, may yet yield to strategies grounded in a better understanding of the cilium. The fact that all cardiac defects seen in patients with full-blown heterotaxy can also be seen in patients without obvious laterality defects hints at important roles for ciliary function not only in left-right axis specification but also in cardiovascular morphogenesis. These three developmental biology stories illustrate how the remaining unexplained mortality and morbidity of congenital heart disease can be solved.
Whole-genome duplications (WGDs) have been at the heart of the diversification of ß-adrenergic receptors (ß-ARs) in vertebrates. Non-teleost jawed vertebrates typically possess three ß-AR genes: adrb1 (ß1-AR), adrb2 (ß2-AR), and adrb3 (ß3-AR), originating from the ancient 2R (two rounds) WGDs. Teleost fishes, owing to the teleost-specific WGD, have five ancestral adrb paralogs (adrb1, adrb2a, adrb2b, adrb3a and adrb3b). Salmonids are particularly intriguing from an evolutionary perspective as they experienced an additional WGD after separating from other teleosts. Moreover, adrenergic regulation in salmonids, especially rainbow trout, has been intensively studied for decades. However, the repertoire of adrb genes in salmonids has not been yet characterized. An exhaustive genome survey of diverse salmonids, spanning five genera, complemented by phylogenetic sequence analysis, revealed each species has seven adrb paralogs: two adrb2a, two adrb2b, two adrb3a and one adrb3b. Surprisingly, salmonids emerge as the first known jawed vertebrate lineage to lack adrb1. adrb1 is nevertheless highly expressed in the hearts of non-salmonid teleosts, indicating that the wealth of data on adrenergic regulation in salmonids should be generalised to other teleost fishes with caution. It is hypothesised that the loss of adrb1 could have been viable because of the evolutionary radiation of adrb2 and adrb3 genes attributable to the salmonid WGD.
Adrenergic regulation, acting via the sympathetic nervous system, provides a major mechanism to control cardiac function. It has recently been shown that hypoxia inducible factor-1α (Hif-1α) is necessary for normal development of sympathetic innervation and control of cardiac function in the mouse. To investigate whether this may represent a fundamental trait shared across vertebrates, we assessed adrenergic regulation of the heart in wild-type and Hif-1α knockout (hif-1α -/- ) zebrafish (Danio rerio). Wild-type and hif-1α -/- zebrafish larvae (aged 4 and 7 days postfertilisation) exhibited similar routine heart rates within a given age group, and β-adrenergic receptor blockade with propranolol universally reduced heart rate to comparable levels, indicating similar adrenergic tone in both genotypes. In adult fish, in vivo heart rate measured during anaesthesia was identical between genotypes. Treatment of spontaneously beating hearts in vitro with adrenaline revealed a similar positive chronotropic effect and similar maximum heart rates in both genotypes. Tyrosine hydroxylase immunohistochemistry with confocal microscopy demonstrated that the bulbus arteriosus (outflow tract of the teleost heart) of adult fish was particularly well innervated by sympathetic nerves, and nerve density (as a percentage of bulbus arteriosus area) was similar between wild-types and hif-1α -/- mutants. In summary, we did not find any evidence that adrenergic cardiac control was perturbed in larval or adult zebrafish lacking Hif-1α. We conclude that Hif-1α is not essential for the normal development of cardiovascular control or adult sympathetic cardiac innervation in zebrafish, although it is possible that it plays a redundant or auxiliary role.
Our meta-analysis (Filogonio et al., 2017) on central vascular blood flows in a snake (Crotalus durissus) and a turtle (Trachemys scripta) was motivated by Hillman et al.’s (2014) analysis on amphibians to investigate whether cardiac shunt patterns depend on cardiac output and vascular distensibilities. In contrast to Hillman et al. (2014), we did not uncover a general trend that supports the notion that cardiac shunts in reptiles are dictated by vascular distensibilities. In addition to our response to the criticism raised by Hillman et al. (2017), we suggest that future experiments should consider 1) both compliance and distensibility of the major arteries; 2) differences in volume of the systemic and pulmonary circuits to account for the accommodation of stroke volume; and 3) an evaluation of the pulsatile pressures in both the ventricle and the major arteries to consider the timing of the ventricular ejection provided by opening of the ventricular valves. We hope these suggestions may help future clarification of the relative importance of passive arterial mechanical properties compared to autonomic regulation in determining intracardiac shunts in both amphibians and reptiles.
The multiple convergent evolution of high systemic blood pressure among terrestrial vertebrates has always been accompanied by lowered pulmonary pressure. In mammals, birds and crocodilians, this cardiac separation of pressures relies on the complete division of the right and left ventricles by a complete ventricular septum. However, the anatomy of the ventricle of most reptiles does not allow for complete anatomical division, but the hearts of pythons and varanid lizards can produce high systemic blood pressure while keeping the pulmonary blood pressure low. It is also known that these two groups of reptiles are characterised by low magnitudes of cardiac shunts. Little, however, is known about the mechanisms that allow for this pressure separation. Here we provide a description of cardiac structures and intracardiac events that have been revealed by ultrasonic measurements and angioscopy. Echocardiography revealed that the atrioventricular valves descend deep into the ventricle during ventricular filling and thereby greatly reduce the communication between the systemic (cavum arteriosum) and pulmonary (cavum pulmonale) ventricular chambers during diastole. Angioscopy and echocardiography showed how the two incomplete septa, the muscular ridge and the bulbuslamelle - ventricular structures common to all squamates - contract against each other in systole and provide functional division of the anatomically subdivided ventricle. Washout shunts are inevitable in the subdivided snake ventricle, but we show that the site of shunting, the cavum venosum, is very small throughout the cardiac cycle. It is concluded that the python ventricle is incapable of the pronounced and variable shunts of other snakes, because of its architecture and valvular mechanics.
Autonomic control of the cardiovascular system in reptiles includes sympathetic components but heart rate (f(H)), pulmonary blood flow (Q(pul)) and cardiac shunt patterns are primarily controlled by the parasympathetic nervous system. The vagus innervates both the heart and a sphincter on the pulmonary artery. The present study reveals that whereas both the left and right vagi influence f(H), it is only the left vagus that influences pulmonary vascular resistance. This is associated with the fact that rattlesnakes, in common with some other species of snakes, have a single functional lung, as the other lung regresses during development. Stimulation of the left cervical vagus in anaesthetised snakes slowed the heart and markedly reduced blood flow in the pulmonary artery whereas stimulation of the right cervical vagus slowed the heart and caused a small increase in stroke volume (V(S)) in both the systemic and pulmonary circulations. Central stimulation of either vagus caused small (5-10%) reductions in systemic blood pressure but did not affect blood flows or f(H). A bilateral differentiation between the vagi was confirmed by progressive vagotomy in recovered snakes. Transection of the left vagus caused a slight increase in f(H) (10%) but a 70% increase in Q(pul), largely due to an increase in pulmonary stroke volume (V(S,pul)). Subsequent complete vagotomy caused a 60% increase in f(H) accompanied by a slight rise in Q(pul), with no further change in V(S,pul). By contrast, transection of the right vagus elicited a slight tachycardia but no change in V(S,pul). Subsequent complete vagotomy was accompanied by marked increases in f(H), Q(pul) and V(S,pul). These data show that although the heart receives bilateral vagal innervation, the sphincter on the pulmonary artery is innervated solely by the left vagus. This paves the way for an investigation of the role of the cardiac shunt in regulating metabolic rate, as chronic left vagotomy will cause a pronounced left-right shunt in recovered animals, whilst leaving intact control of the heart, via the right vagus.
Many reptiles, particularly diving species, display characteristic cardiovascular changes associated with lung ventilation (cardiorespiratory synchrony). Previous studies on freshwater turtles show that heart rate and pulmonary blood flow rate (Qpul) increase two- to fourfold during ventilation compared with breath-holding, and some studies report concomitant decreases in systemic blood flow rate (Qsys). The primary aim of this study was to provide a detailed description of cardiorespiratory synchrony in free-diving and fully recovered turtles (Trachemys scripta). During breath-holds lasting longer than 5 min, Qpul averaged 15 ml min-1 kg-1 and increased more than threefold to a maximum value of 50 ml min-1 kg-1 during ventilation. Qsys also increased during ventilation compared with during breath-holds lasting longer than 5 min (from 44 to 73 ml min-1 kg-1 during ventilation). Neither Qpul nor Qsys was affected by the number of breaths in the ventilatory periods. Changes in Qpul and Qsys were accomplished entirely through a significant increase in heart rate during ventilation, while total stroke volume (systemic+pulmonary) remained constant. Irrespective of the ventilatory state, Qsys exceeded Qpul by 20-30 ml min-1 kg-1. Nevertheless, because Qpul increased relatively more than Qsys during ventilation, Qpul/Qsys increased from 0.29 during apnoea to 0.80 during lung ventilation. This study confirms cardiorespiratory synchrony in the turtle Trachemys scripta but, in contrast to earlier studies, a net right-to-left cardiac shunt prevailed regardless of ventilatory state.
The autonomic nervous system of cyclostomes appears rudimentary compared to that of the gnathostomes, and autonomic neurones can sometimes be hard to distinguish from sensory neurones. Vagal pathways are present in both lampreys and hagfishes; the left and right vagus (X) unite to form a single ganglionated nerve along the dorsal side of the intestine. Fibres run to the gut of hagfishes, but the function of this innervation is unclear. In fact, the only organ in Myxine for which a distinct vagal innervation has been shown is the gallbladder.
Sympathetic chains are absent in the cyclostomes, although scattered neurone clusters occur along the cardinal veins in the abdominal cavity of lampreys. A unique feature is the subcutaneous ganglionated nerve plexus where spinal, possibly autonomie, fibres may be involved in the control of effectors in the skin. Another remarkable feature of the systemic and portal hearts of cyclostomes is the presence of specialized endocardial cells that store catecholamines. Contrary to the hearts of other fishes, including that of lampreys, the hagfish heart receives no extrinsic innervation, although intracardiac neurones have been described.
The hagfish autonomie nervous system is not an ‘early template’ of a control system, but possibly a rudiment of a more elaborate system in the earliest vertebrates. The structure of the autonomie nervous system of hagfishes is not well known, and there is a flagrant lack of knowledge about the autonomie nerve functions. Thus, further physiological experiments are essential to our understanding of the autonomie nerve function in the hagfishes.
The cardiovascular system is crucial by virtue of its role in transporting nutrients, respiratory gases, hormones, and waste products. This chapter focuses on circulatory form and function: the anatomy of the cardiovascular system, cardiac dynamics, and cardiovascular control. Studying circulatory control in any fish is particularly difficult because discrete circulations of specific organs are not easily accessible. Therefore, by necessity, most information on cardiovascular control in primitive fishes is limited largely to the control of cardiac output (Q), as well as control of blood flow through the gills, to air‐breathing organs, and the gastrointestinal tract. Unusual adaptations of primitive fishes that deviate from those piscine features common to elasmobranchs and teleosts are highlighted. The chapter starts with the most primitive fishes, the cyclostomes, and moves through the cardiovascular anatomy of the coelacanth to the cardiovascular anatomy and physiology of dipnoans, the forerunners to tetrapods. It then closes by covering the limited physiological information for Polypterids, gars, bowfins, and sturgeons. By comparing cardiovascular adaptations among these primitive fishes, this chapter examines the evolutionary roots and the evolutionary divergence of the piscine cardiovascular system.
The circulatory system
- Morris