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The Physiology of Pain Mechanisms: From the Periphery to the Brain

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This article introduces the scientific basis for the understanding of pain mechanisms and highlights the importance of endogenous excitatory and inhibitory controls in the central nervous system. These innate control systems have an impact on the evolution of chronic pain and may be manipulated to alter the pain process, and therefore have implications regarding treatment. Understanding neurophysiologic mechanisms involved in the development and maintenance of pain can help the clinician to devise a more effective treatment plan guided by pathophysiologic dysfunction. Keeping in mind the heterogeneity of the pain response and the unique characteristics of an individual patient will lead to better patient care.
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The Physiology of Pain Mechanisms:
From the Periphery to the Brain
Serge Marchand, PhD
Department of Neurosurgery, University of Sherbrooke, Axe Douleur CRC-CHUS, 3001,
12e Avenue Nord, Sherbrooke, QC J1H 5N4, Canada
Pain is a dynamic phenomenon. From the periphery to the brain, the no-
ciceptive signal will be modulated at all levels of the central nervous system
(CNS). This plasticity speaks to the ability to adapt and change within the
nervous system. The current science regarding concepts of pain mechanisms
also takes into account genetic and environmental factors that will influence
the development of persistent pain. Moreover, the maintenance of chronic
pain is not only the result of continued and increased nociceptive activity
mostly arising at the peripheral site of pathology, but also depends on addi-
tional changes within the CNS, such as an increase of excitatory or reduc-
tion of inhibitory endogenous pain modulation mechanisms.
Multiple endogenous excitatory and inhibitory mechanisms have been
identified [1]. This article introduces the scientific basis for the understand-
ing of pain mechanisms and highlights the importance of endogenous excit-
atory and inhibitory controls within the CNS. These innate control systems
have an impact on the evolution of chronic pain and may be manipulated to
alter the pain process, and therefore have implications regarding treatment.
Rheumatic pain, particularly as seen in longstanding osteoarthritis, may be
considered the prototype of chronic pain. Additionally, inflammatory ar-
thritic diseases also account for important pain complaints and suffering
across all ages. Understanding neurophysiologic mechanisms involved in
the development and maintenance of pain will help the clinician to devise
a more effective treatment plan guided by pathophysiologic dysfunction.
From nociception to pain
A good way to understand the physiology of pain is to follow the noci-
ceptive signal pathways from the periphery to the brain, with emphasis on
E-mail address: serge.marchand@usherbrooke.ca
0889-857X/08/$ - see front matter Ó2008 Elsevier Inc. All rights reserved.
doi:10.1016/j.rdc.2008.04.003 rheumatic.theclinics.com
Rheum Dis Clin N Am
34 (2008) 285–309
the integration and modulation of the nociceptive signal at different steps in
the CNS (Fig. 1).
Mechanical, chemical, or thermal nociceptive stimulation will recruit pe-
ripheral nociceptors that conduct the nociceptive signal in the primary so-
matosensory neuron to the dorsal horn of the spinal cord. In the dorsal
horn, the primary neuron will make a synaptic contact with the secondary
or projection neuron. Secondary neurons form the spinothalamic (lateral)
and spinoreticular (medial) tracts will immediately cross in the spinal
cord and send afferent projections to higher centers. A large proportion
of afferents will make a second synapse in the lateral and medial nuclei
of the thalamus, which subsequently make synaptic contact with tertiary
neurons. It is important to emphasize that the secondary neurons may
also synapse with neurons in different nuclei of the brainstem, including
the periaqueductal gray (PAG) and nucleus raphe magnus (NRM), areas
involved in descending endogenous pain modulation. Tertiary neurons
Fig. 1. The nociceptive pathways from the periphery will conduct to the brain after two synap-
tic relays. The Adand C-fibers will make their first synapse with the projection neurons in the
dorsal horn of the spinal cord. The secondary neurons will decussate immediately in the cord
and conduct to the thalamic nuclei, where they will make the second synaptic contact. The third
neurons will finally project to the somatosensory cortices for the sensory-discriminative compo-
nent of pain, and to limbic structures (anterior cingulated cortex and insula) for the motiva-
tional component of pain. ACC, anterior cingulate cortex; NRM, nucleus raphe magnus; SI,
SII, somatosensory cortices; PAG, periaqueductal gray.
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from the thalamus send afferents to the primary and secondary somatosen-
sory cortices (SI, SII). The SI and SII are involved in the sensory quality of
pain, which includes location, duration, and intensity. Tertiary neurons also
project to limbic structures, including the anterior cingulate cortex (ACC)
and the insula, which are involved in the affective or emotional component
of pain.
The various synaptic contacts with excitatory and inhibitory neurons at
all levels of the CNS are important integration regions that are the target
of most pharmacologic approaches.
The periphery: the nociceptors
An injury that causes a potential risk for the organism will activate free
nerve endings that respond to nociceptive stimulation (Fig. 2). Most of these
fibers are polymodal and will respond to different modalities, including me-
chanical, thermal, and chemical stimulation [2].
A nociceptive stimulation will initiate a cascade of events. Pronociceptive
inflammatory molecules will be released into the periphery and will produce
Fig. 2. The nociceptive afferent fibers can be separated according to their physical characteris-
tics and conduction velocity. On this nerve section, where the myelin has been stained in black,
one can see the large myelinated Abfibers, the smaller and myelinated Adfibers, and the un-
myelinated C fibers. As seen in the table, the conduction velocity will increase with the diameter
and thickness of the myelin sheet.
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THE PHYSIOLOGY OF PAIN MECHANISMS
peripheral hyperalgesia. These pronociceptive inflammatory molecules
originate in various blood cells (mastocytes, polymorphonuclear cells, and
platelets) and include bradykinins, prostaglandins, histamine, serotonin,
adenosine triphosphate, and also from immune cells which produce interleu-
kins, interferon, and tumor necrosis factors [3–6]. Substance P and calcito-
nin gene related protein (CGRP), which act as neurotransmitters in the
CNS, are also released into the periphery and act as proinflammatory fac-
tors in the periphery, favoring neurogenic inflammation [3].
Primary hyperalgesia: peripheral nociceptors hyperactivity
Injured tissues will release various substances, such as potassium, prosta-
glandins, histamine, or bradykinins that are pronociceptive, and will also in-
voke an immune response. These inflammatory and immune factors will
sensitize the nociceptive receptors directly within the lesion and in the sur-
rounding neurons [6]. Primary hyperalgesia, which follows the release of
these factors, may be measured as a lowered pain threshold in and around
the lesion. This has been demonstrated to occur in the area of arthritic joints
both in animal studies and in human beings.
Sensory afference from periphery to spinal cord
Afferent fibers originating in the periphery fall into three groups: Ab fibers,
C fibers, and Adfibers. The Ab fibers are large myelinated fibers that conduct
at high speed and usually transmit non-nociceptive signals. They do, however,
also participate in pain modulation, as will be explained later in this article.
Nociceptor messages are mainly transmitted by the other two classes of fibers,
the larger myelinated Adfibers and the thin unmyelinated C fibers. The noci-
ceptors are frequently refer to by the characteristics of their fibers.
Myelination and increasing size of a nerve fiber facilitate the speed of
conduction of the stimulus. The Adfibers conduct the signal relatively rap-
idly from the periphery to the spinal cord. Because of this rapid conduction
velocity, they are responsible for the sharp localization of pain and for the
rapid spinal response, which can be measured in the laboratory as the noci-
ceptive reflex. In contrast, the C fibers, which have a slow conducting veloc-
ity, will mediate a second or dull aching pain.
Ab fibers
The Ab (or Ab) fibers are principally involved in the conduction of non-
nociceptive input, such as vibration, movement, or light touch. The Ab fi-
bers are large myelinated fibers with a rapid conduction velocity (35 m–75 m
per second). Besides conducting the non-nociceptive signal, the stimulation
of Ab fibers will recruit inhibitory interneurones in the substantia gelatinosa
of the dorsal horn of the spinal cord, which will inhibit nociceptive input at
the same spinal segment. This mechanism is one of the fundamental compo-
nents of the gate control theory, whereby an innocuous stimulus will reduce
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the nociceptive input from the same region [7]. Besides playing a dynamic
inhibitory role when recruited, the Ab fibers seem also to play a tonic inhib-
itory role on the nociceptive input. Blocking the input from these large fibers
will result in an increased response to nociceptive stimuli [8].
Adfibers
The Adfibers are relatively large myelinated fibers with slower conduc-
tion velocity than the Ab fibers, but faster (5 m–30 m per second) than
the C fibers. They represent the majority of the myelinated fibers. Two types
of Adfibers exist depending on the specificity of their responses to different
stimulation [9]: the mechanonociceptors respond preferentially to intense
and potentially harmful mechanical stimulation; the polymodal Adfibers re-
spond to mechanical, thermal, and chemical stimulation. However, the me-
chanonociceptor Adfibers will also increase their discharge after intense
thermal stimulation, a phenomenon known as hyperalgesia. Because of
the rapid conduction velocity, the Adfibers are responsible for the first
pain sensation, a rapid pinprick, sharp, and transient sensation.
C fibers
Because of their small caliber and lack of myelin, the conduction of the
C fibers is relatively slow (0.5m–2 m per second). They represent three quar-
ters of the sensory afferent input and are mostly recruited by nociceptive
stimulation. However, they are also involved in non-nociceptive somatosen-
sory information, such as in the sensation of pruritus [10], and paradoxically,
in the perception of pleasant touch, as documented in a patient with a rare
disease linked to a deafferentation of the myelinated sensory fibers [11].
First and second pain
The conduction velocity differences between the Adand C fibers can be
appreciated when isolating the sensation of first and second pain (Fig. 3).
Following a brief nociceptive stimulation, the Adfibers will rapidly transmit
a brief and acute pinprick-like sensation, perceived to be precisely located at
the point of stimulation. It is this precision and fast conduction that will re-
sult in the nociceptive withdrawal reflex. Following this activity, C fibers will
transmit their information with a relatively long delay (100 milliseconds to
a second, depending on the stimulus location). This second sensory input re-
sults in a more diffuse deep pain sensation.
It is possible to isolate first and second pain in the laboratory. Using
a blood pressure cuff, one can temporarily block trophic factors present
in the blood from localizing to the nerves. The first fibers that will show re-
duced activity are those with largest diameter, including the Adfibers. This
allows the activity of C fibers to be isolated and independently studied. Fol-
lowing this procedure, a nociceptive stimulation, independent of the nature
of the stimulation, hot, cold or mechanical, will be perceived with a certain
delay as a deeper pain sensation.
289THE PHYSIOLOGY OF PAIN MECHANISMS
The application of capsaicin, the hot pepper extract, will produce a burn-
ing sensation because of the activation of the vallinoid receptors on the C
fibers. However, at higher doses, the C fibers will be blocked as a result
of a specific action on ionic calcium channels, with resulting isolation of
the Adfibers at the skin surface. This time, the same nociceptive sensation
will be perceived as a sharp pinprick-like sensation without the second burn-
ing pain sensation.
Secondary neurons in the spinal cord
When recruited, the Adand C fibers transport their signal to the spinal
cord, where they will have a first synaptic contact with secondary neurons
that are principally located in the superficial zones of the dorsal horn (I, II)
and lamina V [12]. Both nociceptive and non-nociceptive afferents to the
spinal cord will also have synaptic contact with an important network of
Fig. 3. Because of the differences in conduction velocity between the relatively rapid and slow
C-fibers, a nociceptive stimulation will induce a first pain having the characteristics of a localized
and sharp pinprick sensation related to the fast action of the fibers, and a second slower and
more diffuse burning-like perception related to the slower activity of the C-fibers (A). Using
a blood pressure cuff, one can temporarily block the activity of the fibers with largest diameter,
including the Adfibers (B). This allows the activity of C-fibers to be isolated and independently
studied. A nociceptive stimulus will only conduct the C-fiber activity, perceived as a diffuse
burning sensation independently of the stimulation type (B). If the activity of the small C-fibers
is blocked by using capsaicin, only the sharp pinprick perception will persist (C).
290 MARCHAND
inhibitory and excitatory interneurones that modulate the nociceptive signal
before the secondary neuron projects to superior centers. The secondary
neurons can be divided into two classes: the nociceptive specific neurons
and the wide dynamic range (WDR) neurons [2,3,13].
Nociceptive specifics neurons
As indicated by their name, the nociceptive specific neurons respond only to
nociceptive stimulation. They can be divided in two subclasses depending on
their recruitment by Ador the combination of Ad(or Ad) and C fibers.
Wide dynamic range neurons
WDR neurons respond gradually to stimuli ranging from innocuous to
nociceptive. Their capacity to respond to both innocuous and nociceptive
stimuli is related to the fact that they have received input from Adfibers,
C fibers, and also Ab fibers (Fig. 4). Interestingly, the receptive field of
WDR neurons is dynamic, as the name implies, and changes in conditions
of persistent pain. Animal studies have shown that inflammatory pain will
have multiple effects on the function of WDR neurons. Changes in the re-
ceptive field, the membrane permeability to ion exchange, and the discharge
frequency of these neurons all suggest that they play a substantial role in the
chronicity of pain [14].
Excitatory mechanisms: secondary hyperalgesia
Secondary hyperalgesia is a phenomenon that refers to sensitization that
occurs within the CNS [13]. Repeated recruitment of C-fibers following an
injury will produce central sensitization by changing the response properties
of the membranes of secondary neurons. This will result in an increase of the
firing rate, a phenomenon known as windup [15]. The high frequency re-
cruitment of C fibers, either by increased repetitive stimuli or by a tonic
stimulation [16], will then induce an increase of the perceived pain, even if
the intensity of the stimulation remains constant. This spinal sensitization
can persist for minutes, but can also be present for hours and even days
[17]. The prolonged activation of the NMDA receptors will induce the tran-
scription of rapidly expressed genes (c-fos, c-jun), resulting in sensitization
of nociceptors. This neuronal plasticity of the secondary neuron will result
in reduced recruitment threshold of secondary neurons in the spinal cord
and produce hyperalgesic and allodynic responses that may persist even af-
ter the healing of the injury. Taking note of the impact of sensitization, an
aggressive and early treatment plan to reduce pain will help in preventing
ongoing chronic pain (see Fig. 4).
Excitatory mechanisms: temporal summation
The temporal summation paradigm is a good illustration of the impor-
tance of signal conduction in Adand C fibers [18]. In this paradigm, pain
perception is compared with repeated stimulations at the same intensity
291THE PHYSIOLOGY OF PAIN MECHANISMS
Na+
K+
NMDA AMPA
Mg2+
Ad and C fibers
Substance P
Ab fiber
Glutamate
NK-1
Spinothalamic projection
secondary neuron
Ab fiber
Na+
K+
NMDA AMPA
Mg2+
Ad and C fibers
Ca2+
NO
C-Fos
Allodynia
Hyperalgesia
Sensitization
NK-1
Substance P
Glutamate
Spinothalamic projection
secondary neuron
A
B
292 MARCHAND
but at different rates. The rationale is that high frequency of stimulation will
produce a temporal summation of the C-fiber activity, as a result of the rel-
atively slow conduction of these fibers. This temporal summation results in
an increase in the perceived intensity of the second pain, which is related to
C-fiber activity, without changing the perception of the first pain, related to
Adfibers [19]. The accumulation of nociceptive activity will produce a tran-
sient change in the excitability of the spinal cord second neuron, or windup,
that may lead to central sensitization [20]. However, windup, a transient ef-
fect related to the frequency of discharge from the primary neuron, is differ-
ent from central sensitization [17].
Central sensitization refers to a phenomenon whereby the second neuron
membrane permeability changes and responds at higher frequency when re-
cruited by nociceptive (hyperalgesia) and non-nociceptive primary input (al-
lodynia). Central sensitization is defined as an increase of excitability and
spontaneous discharge at the dorsal horn neurons with an associated increase
in the receptive field for these neurons. This phenomenon will principally af-
fect the WDR neurons from the dorsal horn (see next section on the spinal
cord neurons) and is dependent on the activity of the NMDA receptors
[21,22]. Central sensitization may persist for prolonged periods after termina-
tion of stimulation and has important effect on the persistence of pain.
Excitatory mechanisms: spatial summation
Another important phenomenon in the CNS is spatial summation. The
stimulation of a large territory will recruit more nociceptors than when
a smaller area is stimulated and will result in more intense pain perception.
It is worth noting that increasing the surface area that is stimulated recruits
both excitatory and inhibitory mechanisms [23].
Clinical implications of temporal and spatial summation. It is possible to
study the relative role of endogenous pain excitatory and inhibitory mecha-
nism dysfunction in certain chronic pain conditions using temporal [24] and
spatial summation [25]. The author and colleagues have examined this spa-
tial summation paradigm in patients with fibromyalgia (FM), and have
Fig. 4. Spinal sensitization occurs when the secondary neurons of the spinal cord change their
discharge frequency following a sustained recruitment from the primary nociceptive afferences.
In this schematic representation, one can see that an acute discharge from the nociceptive pri-
mary afferences (C-fibers) will induce the release of peptide (substance P, CGRP) and glutamate
that will produce the recruitment of the NK1 and AMPA receptors (A). A sustained discharge
(B) will recruit the NMDA receptors and produce a sensitization of the secondary neurons that
will now discharge at a higher frequency when recruited by nociceptive (hyperalgesia) and non-
nociceptive stimulation (allodynia). This phenomenon is generally transitory, but may persist
over a long time and participate in pain chronicization. AMPA, alpha-amino-3-hydroxy-
5-methyl-4-isoxazolepropionic acid; Ca, calcium; K, potassium; Mg, magnesium; Na, sodium;
NK1, neurokinin; NMDA, N-methyl-D-aspartate.
=
293THE PHYSIOLOGY OF PAIN MECHANISMS
observed no differences in pain perception between the increasing or de-
creasing nociceptive area, suggesting a deficit of endogenous pain modula-
tion. This deficit was not present in patients with low back pain,
suggesting that the deficit of endogenous pain modulation is not present
in all chronic pain conditions [25]. A chronic pain condition may therefore
be related to either hyperactivity of nociceptive activity or, conversely, to
hypoactivity of endogenous pain inhibitory mechanisms at different levels
of the CNS. Exploring the role of excitatory and inhibitory mechanism dys-
function in different patient populations will help to better characterize the
underlying deficit and facilitate treatment choices.
Pain pathways from the spine to the superior centers
The secondary neuron travels to superior centers by two main pathways:
the spinothalamic tract, which sends afferents to the lateral nuclei of the thal-
amus, and the spinoreticular tract, which send afferents to the medial thala-
mus and nuclei of the brainstem, including the NRM and the PAG, two
nuclei involved in descending pain modulation [26]. A third pathway, from
the medial dorsal cord (lemniscal) is mostly associated with non-nociceptive
afference, but also conducts nociceptive afference from the viscera [27,28].
Sensory spinothalamic tract
The spinothalamic tract comprises the lateral part of sensory input and
projects directly to the lateral nuclei of the ventrobasal thalamus (ventropos-
terolateral or VPL; ventroposteromedian or VPM). The projection neurons
from the spinothalamic tract are primarily from lamina I and IV to VI of the
spinal cord [29], and project to the contra lateral nuclei of the thalamus.
The fibers of the spinothalamic tract conduct rapidly and the projection
neurons have relatively small receptive fields, directed toward regions of the
thalamus and somatosensory cortex that have defined somatotopic repre-
sentation. The spinothalamic tract has all the characteristics for localization
of a sensory pathway [26].
Affective spinoreticular tract
Most afference from the spinoreticular tract is from the deep lamina VII
and VIII of the spinal cord and projects to the medial nuclei of the thala-
mus, as well as to structures of the brainstem involved in pain modulation,
including the PAG and NRM [26]. Unlike the spinothalamic tract, the spi-
noreticular tract projects to neurons having large receptive fields that may
cover wide areas of the body and play a role in the memory and affective
component of pain [26].
Thalamic organization
The secondary neurons from the spinal cord project to the thalamus. The
thalamic nociceptive neurons are localized in two groups of nuclei: the
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ventrobasal (VPL and VPM) and the centromedian nuclei. The ventrobasal
nuclei project their tertiary neurons to the primary and secondary somato-
sensory cortices (SI, SII). The centromedian neurons project to structures of
the limbic system. This simplified description of the thalamus projections
suggests that the sensorial and motivational components of pain are orga-
nized early in the CNS.
Moreover, the thalamus also plays a role in certain chronic pain condi-
tions. Upon stimulation of specific nuclei of the thalamus, patients have ex-
perienced memory recall of the sensory and affective component of
a previous pain that had long since disappeared. This suggests that some
thalamic neurons may conserve a past painful experience that can be
masked by a circuitry of inhibitory interneurones [30]. It is therefore plausi-
ble that a localized stroke in the thalamus may destroy tonic inhibitory cir-
cuitry and unmask nociceptive activity, leading to the well-recognized
painful clinical thalamic pain syndrome.
Brain and pain
Pain can only be experienced when nociceptive afference reaches the cortex.
It is for this reason that the term nociception is used to describe the signal fol-
lowing a lesion, whereas pain is a complex perception requiring CNS activity.
A complex network of cortical structures is activated during pain percep-
tion. Similar to the thalamic nuclei, the cortex can be represented in a simpli-
fied way by subdivision into structures involved in either the sensory or the
affective components of pain. Brain imaging has demonstrated four cortical
structures important for the perception of pain [31,32]. There are the so-
matosensory cortex (SI) in the postcentral circumvolution of the parietal
lobe, the secondary somatosensory cortex (SII) in the parietal operculum,
the ACC above the corpus callosum circumvolution, and the insular cortex
(IC) under the temporal and frontal lobes at the level of the Sylvian fissure.
The first two structures (SI and SII) are mainly involved in the sensory dis-
criminative aspect of pain, while the ACC and IC are associated with the
affective component of pain.
Most brain imaging studies report an activation of the sensory and affec-
tive brain structures following a nociceptive stimulus, demonstrating that
pain perception is a complex experience with emotion, cognitive factors,
and previous experience playing an important role in perceived pain. It
can therefore be understood why the clinician should address pain from
both the physical as well as the emotional aspect.
Endogenous pain modulation mechanisms
As pain is a dynamic phenomenon, the nociceptive signal will be modu-
lated at multiple levels of the CNS before pain is fully perceived. Because of
the dynamic and plastic characteristics of the nervous system, pain
295THE PHYSIOLOGY OF PAIN MECHANISMS
perception, especially in a chronic pain condition, will change over time, de-
pendent on different factors. Pain perception is the final outcome of complex
mechanisms that modulate the nociceptive afferent signal. The modulation
of the nociceptive signal starts at the periphery and involves several CNS
structures, including excitatory and inhibitory mechanisms from the brain-
stem, the autonomic nervous system, and the cortical structures responsible
for the emotional and cognitive aspect of pain perception.
Based on knowledge of the neurophysiology of pain, one can conclude
that the development and maintenance of chronic pain is dependent upon
several factors. Persistent pain can arise from the activity of nociceptive af-
ference, but can also be related to a reduction of endogenous inhibition or
augmentation of endogenous excitatory mechanisms. The literature on cen-
tral sensitization supports the importance of endogenous pain excitatory cir-
cuitry on the development and maintenance of pain. The excitatory and
inhibitory roles played by different structures of the brainstem have been
well documented [33–35].
Endogenous excitatory mechanisms
Primary afferents are normally activated by nociceptive stimuli that can
potentially induce an injury and by pronociceptive activation triggered by
the inflammatory response.
Spinal excitatory mechanisms
As previously described, excitatory mechanisms can induce a central sen-
sitization at the spinal level. Spinal sensitization is defined as an increase in
the excitability and spontaneous discharge of the nociceptive spinal neurons,
augmentation of the receptive field, and an accentuated response of spinal
neurons to nociceptive (hyperalgesia) and non-nociceptive (allodynia) input.
Spinal sensitization depends on the activation of the NMDA receptors of
the spinal neurons, which are activated by a sustained released of glutamate
[21,22,36]. These neurophysiologic and neurochemical mechanisms involved
in spinal sensitization are responsible for modification of the spinal nocicep-
tive circuitry and contribute to the maintenance of pain.
Descending excitatory mechanisms
It is now well documented that several supraspinal excitatory and inhib-
itory mechanisms play a major role in pain perception and, most probably,
in certain chronic pain conditions [1]. The work of Fields and colleagues [37]
describing activation of ‘‘ON’’ cells and inhibition of ‘‘OFF’’ cells in the
brainstem during nociceptive activity has demonstrated the importance of
excitatory mechanisms in amplifying the nociceptive response.
Recent studies have also demonstrated that certain physiologic condi-
tions, such as nociceptive hyperactivity, may change the usual neuronal
response to specific neurotransmitters. A particular example is the
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hyperalgesic effect that can be observed in some patients using opioid med-
ications, mostly at higher doses [38]. Therefore, drugs with opioid activity
could, under some circumstances, produce a completely opposite effect
and enhance pain by producing an hyperalgesic response [38,39]. The
same is also true for GABA, which has been clearly identified as an inhibi-
tory neurotransmitter, but in certain conditions may cause hyperpolariza-
tion of neurons [40]. These observations support the concept of pain as
a dynamic phenomenon. An understanding of these complex mechanisms
can help explain the clinical variability of response to treatments in patients
with chronic pain.
Endogenous inhibitory mechanisms
To better understand the role of endogenous pain inhibitory mechanisms
in the development and treatment of pain, one should appreciate three levels
of modulation in the CNS (Fig. 5): (1) spinal mechanisms producing local-
ized analgesia; (2) descending inhibitory mechanisms from the brainstem
producing diffuse inhibition and; (3) superior center effects that will either
modulate descending mechanisms or change the perception of pain by rein-
terpreting the nociceptive signal.
Spinal mechanisms
Since the proposal of the gate control theory by Melzack and Wall [7], the
modulation of nociceptive afference at entry into the spinal cord has been
well documented. This input may be increased or decreased at the level of
the spinal cord. The gate control theory hypothesizes that, among other
mechanisms, selective activation of non-nociceptive afferent Ab fibers will
recruit inhibitory interneurones in the substantia gelatinosa of the posterior
spinal cord, producing a localized analgesia and decreasing pain perception.
In contrast, in certain neuropathic pain conditions, the nociceptive second-
ary projection neurons will be recruited at high frequency to transmit a pain
signal following an innocuous stimulation, a phenomenon known as allody-
nia and increased pain perception. Certain pain conditions may also result
from a reduced efficacy of tonic inhibitory controls within the spinal cord
[41,42].
Diffuse noxious inhibitory controls
A few years after the gate control theory was proposed in 1965, Reynolds
[43] demonstrated that stimulation of the PAG produced a strong inhibi-
tion. The role of the rostroventral medulla in the modulation of pain has
since been well documented [33,44]. Regions, such as the PAG and the
NRM, have been identified as important serotoninergic and noradrenergic
descending inhibitory pathways. These inhibitory pathways then recruit en-
kephalinergic interneurones in the spinal cord to produce the analgesic
response.
297THE PHYSIOLOGY OF PAIN MECHANISMS
It was not until the end of the 1970s before a model, known as DNIC,
was proposed. This model is based on the observation that a localized noci-
ceptive stimulation can produce a diffuse analgesic effect over the rest of the
body, an analgesic approach known as counter-irritation. In the DNIC
model, Le Bars and colleagues [35,45] proposed that a nociceptive stimulus
will send input to superior centers, but will also send afference to the PAG
and NRM of the brainstem, recruiting inhibitory output at multiple levels of
the spinal cord.
Fig. 5. Endogenous pain modulation. This schematic representation of the main three levels of
endogenous pain modulation presents: (1) the spinal, (2) descending from the brainstem, and (3)
higher center inhibitory mechanisms. As described in this article, better understanding these
mechanisms helps in developing a mechanistic approach for the treatment of some chronic
pain conditions that are related to a deficit of these mechanisms. Serotonin and noradrenalin
are two neurotransmitters implicated in diffuse noxious pain inhibitory mechanims (DNIC).
However, other neurotransmitters, such as dopamine, are also implicated. The spinal interneu-
ron is proposed to be enkephalinergic, but other inhibitory interneurons, such as gamma-ami-
nobutyric acid (GABA), are also implicated. The secondary projection neurons have NMDA
receptors that are implicated in the persistancy of certain neurogenic chronic pain conditions.
The PAG in the mesencephalon and the NRM are two regions implicated in descending
inhibition.
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Animal studies demonstrate that a lesion of the dorsolateral funiculus,
the main descending inhibitory pathway, will produce hyperalgesia, sug-
gesting a role for tonic descending inhibition under normal conditions
[46–48]. Certain clinical conditions are related to reduced endogenous in-
hibition. For example, the low concentration of serotonin and noradren-
aline in the cerebrospinal fluid of patients with FM [49] suggests
a deficit of DNIC, with increasing evidence corroborated by other studies
[25,50–52].
Documenting the role of descending inhibitory mechanisms will help
to better understand certain chronic pain conditions, such as FM. It
will also help toward understanding the mechanism of action of pharma-
cologic approaches, such as the use of antidepressants in chronic pain
conditions. Therefore, two key neurotransmitters involved in the DNIC
response are those subserving the serotoninergic and noradrenergic
mechanisms.
Superior control centers
There has been an increased appreciation of the role of supra-spinal cen-
ters in pain and pain modulation. Several cortical regions receive input from
the spinothalamic tract and interact to produce the multidimensional expe-
rience of pain perception [53]. The use of brain imaging techniques has
shown robust activation of certain cortical regions, including the primary
and secondary somatosensory cortices, related to the sensory aspect of
pain, and the ACC and IC for the affective component of the pain experi-
ence [54].
There is no doubt that cognitive manipulations, such as distraction, hyp-
nosis, and expectation influence pain perception [54]. Hypnosis has been
demonstrated to change both the sensory and affective component of pain
perception. Subjects given the same nociceptive stimulus perceived both in-
tensity as well as unpleasantness of pain differently, depending upon the sug-
gestion given [55]. Using positron emission tomography to obtain brain
activity images, activity of the primary somatosensory cortex was propor-
tional to the perceived intensity of pain [56], whereas cingulate cortex activ-
ity reflected unpleasantness of pain [57]. These data confirm that a simple
suggestion can change the brain activity related to pain perception. These
concepts will be further described in the article by Keefe and colleagues
on psychologic mechanisms, found elsewhere in this issue.
In a recent study, the author and colleagues were able to demonstrate
that manipulating the expectation related to an analgesic procedure can to-
tally reverse the analgesic effect of endogenous pain modulation and the re-
lated pain experience. By suggesting that a procedure that is normally
analgesic would produce more pain, subjects indeed reported more pain.
Therefore, suggestion was able to totally block the administered analgesic
effect. Suggestion was also able to reverse the inhibition of the spinal noci-
ceptive reflex (RIII) and of the brain activity measured by somatosensory
299THE PHYSIOLOGY OF PAIN MECHANISMS
evoked potential [58]. These results support the idea that cognitive informa-
tion can modulate the efficacy of endogenous pain modulation and empha-
size the importance of the patient’s expectations regarding analgesia. This
will be further addressed in the article by Pollo and Benedetti on placebo
mechanisms found elsewhere in this issue.
Risk factors for developing chronic pain
Understanding factors other than the primary disease process that are in-
volved in the development and maintenance of pain will help toward preven-
tion of a chronic pain state. Three factors have been proposed to play a role
in the chronicity of pain: personal predisposition, environmental factors,
and psychologic factors. Paying attention to these elements will facilitate
the management of patients with chronic pain.
Individual predisposition to chronic pain
Individual predisposition refers to the characteristics of a person that will
influence their predisposition to pain and which are either acquired or in-
nate. Under this category, are the role of gender and biological sex, the
role of age, and the role of endogenous pain modulation responses.
Gender and sex
Women are more frequently affected by chronic pain syndromes than are
men. The reason for this predisposition is probably multifactorial, with sex
hormones likely playing an important role. Animal research supports differ-
ential responses between the sexes and supports the effect of sex hormones
on pain experience. For instance, females show greater nociceptive re-
sponses than do males for the same stimulus, but this difference disappears
after gonadectomy [59]. Moreover, if gonadectomized rats receive replace-
ment hormones of the opposite sex, females receiving testosterone and males
receiving estrogen and progesterone, they demonstrate the same nociceptive
behavior attributable to the sex hormone status [60]. Interestingly, this influ-
ence of sex hormones seems also to be true in human beings, as differences in
response to pain between men and women appears only after puberty and
disappears after menopause or andropause [61,62].
Age
Even if we know that an increase in the prevalence of chronic pain among
older individuals is partly because of progressive musculoskeletal degenera-
tion that accompanies aging, decline in the efficacy of endogenous pain con-
trol systems may contribute to the high prevalence of pain in the elderly.
Studies have shown a deficit of endogenous mechanisms with aging
[63,64] and also a significant decrease of DNIC, which can occur as early
as 50 years of age [65]. This reduction in endogenous pain control with
300 MARCHAND
age probably contributes to the higher prevalence of chronic pain in the
older population.
Endogenous pain modulation
The efficiency of the endogenous inhibitory system, which can be mea-
sured by DNIC, has been shown to be a good predictor for the development
of chronic pain. More effective inhibitory control was correlated with less
clinical pain [66]. The deficit of DNIC in FM but not in low back pain
[25] also supports the role of DNIC in chronic pain, but may also be specific
to certain pathologies.
Genetic predisposition
There is increasing evidence that some individuals are more prone to the
development of chronic pain than others. Genetic predisposition to the de-
velopment of pain and to the response to pain treatments is now well docu-
mented in the literature [67,68]. This genetic predisposition helps toward
understanding the differential response between individuals to the develop-
ment of chronic pain following an injury. The persistence of pain following
an injury, which at times may be without objective pathology, such as occurs
in whiplash injury, may be influenced by genetic factors. The same applies
for certain treatments. It is well recognized by clinicians that different pa-
tients respond differently to individual analgesic medications with regard
to both efficacy as well as side-effect profile.
Environmental factors
External stressors, history of previous pain [69], or abuse [70] are also
good predictors of the development of chronic pain. For instance, it has
been demonstrated that children born prematurely, receiving painful clinical
interventions, will be more sensitive to pain later in life [71–73]. The mech-
anism by which these children may be sensitized to pain can be partly ex-
plained by deficits in pain inhibitory mechanisms. The author and
colleagues have recently reported that children born prematurely and ex-
posed to repeated painful clinical procedures will demonstrate a deficit of
DNIC when tested in later childhood [71].
Psychologic factors
Finally, psychologic factors, such as anxiety, depression, and catastroph-
izing are also important predictors of pain chronicity [74–77]. Psychologic
factors will not only predict the reactions to a pain experience or the ability
to cope with the pain, but will also affect the evolution of the chronic pain
symptoms. The treatment of pain should always take into consideration the
role of psychologic factors as an important predictor for a risk of pain chro-
nicity (discussed in the article by Keefe and colleagues in this issue).
301THE PHYSIOLOGY OF PAIN MECHANISMS
Mechanistic approach to pain treatment
Based on the understanding of pain neurophysiology, treatment plans for
pain management in the clinical setting may be devised. Treatments could be
aimed toward either reducing excitatory mechanisms or enhancing inhibi-
tory activity. The first goal is to identify as best as possible the mechanism’s
operative. For a nociceptive acute pain, depending on the nature of the in-
jury, topical or systemic anti-inflammatory (NSAIDs) or analgesic treat-
ments would be primarily indicated. However, even if the nociceptive
activity is clearly identified to be peripheral, central sensitization may also
have occurred.
Chronic pain is even more difficult to manage because of the complexity
of pain mechanisms and the evolution of the pathology over time. As a cen-
tral component to the pain will almost always be present, the use of opioid,
anticonvulsant, or antidepressant drugs could be included in pharmacologic
treatment.
Types of pain
The different types of pain may be divided into two categories and five
subcategories: nociceptive (somatic, visceral, and inflammatory) and neuro-
genic (causalgia and functional) [78] (Table 1). A purely psychogenic pain
may exist, but this is both rare and controversial because of multiple factors
influencing pain perception [79]. Caution is needed before diagnosing iso-
lated psychogenic pain.
Nociceptive pain
Nociceptive pain is generally transitory in response to nociceptive stimuli
that could be mechanical, thermal, or chemical. Nociceptive pain plays an
important protective role and is normally present for as long as the protec-
tion of the organism is necessary. However, in certain situations the pain
will persist even after healing of the initial injury. Clinicians are currently
unable to predict which patients will develop a persistent pain following
an acute pain, such as in the case of postsurgical pain, and several factors
are implicated. It is for this reason that early treatment of acute pain is so
important in the prevention of chronic pain.
The recommended treatments for nociceptive pain are analgesics,
NSAIDs, and sodium channels blockers. Opioids also have an important
place in the treatment of acute pain, as peripheral opioid receptors are
up-regulated following an inflammatory response [80,81]. Persistent acute
pain that may have a nociceptive origin may require treatment strategies
similar to those used for neuropathic pain to prevent central sensitization.
It is important to differentiate somatogenic versus viscerogenic nocicep-
tive pain that can, in certain cases, present a comparable clinical picture.
It is known that referred pain from the internal organs, such as the gut, liver,
302 MARCHAND
Table 1
Mechanistic classification of pain and overview of categories and characteristics
Type of pain Characteristics Mechanisms
Example of pharmacologic
treatments
Nociceptive Somatic
(tissue injury)
Superficial (skin) or deep pain
(muscle, fascia, tendon)
Mechanical, thermal or chemical
stimuli
Acetaminophen
Naþblockers
NSAID, steroids,
opioids
Visceral
(irritable bowel, cystitis)
Constant or cramping, poor
localization. Autonomic
responses
Visceral distension NSAID,
antispasmodics
Inflammatory
(musculoskeletal)
Localized or diffuse pain
hyperalgesia, allodynia.
Associated with localized
inflammation
NSAID, steroids
Neurogenic Causalgia
(neuralgia, radiculopathy,
CNS lesions)
Spontaneous, paroxysmal pain.
allodynia, hyperalgesia.
Peripheral or CNS lesions Anticonvulsivants, opioids,
antidepressants
Functional
(FM, thalamic syndromes,
irritable bowel syndrome)
Diffuse deep pain,
hyperalgesia, allodynia
Dysregulation of excitatory or
inhibitory mechanisms in CNS
Antidepressants, anticonvulsivants,
opioids, cannabinoids.
303THE PHYSIOLOGY OF PAIN MECHANISMS
ovaries, or bladder will induce a pain perception localizing in somatic terri-
tories, giving the impression of a purely somatic pain. For example, pain
arising in the abdominal cavity can mimic low back pain. Appropriate diag-
nosis and management is dependent upon understanding the concept of re-
ferred pain.
Inflammatory pain
Inflammatory pain is associated with the healing process following a le-
sion. Inflammation is a natural protective reaction of the organism follow-
ing an injury. Inflammatory substances are released into the periphery by
cells in the area of damaged tissue but can also arise from hyperactivity
of the nociceptive neurons in the CNS, a phenomenon know as neurogenic
inflammation [82]. In that the molecules released during inflammation are
pronociceptive, the use of NSAIDs will reduce this nociceptive activity.
NSAIDs have well-known peripheral effects, but inhibition of cyclooxyge-
nase can also occur centrally in the spinal cord and the brain, and may
thereby participate in reducing neurogenic inflammation [83].
Neurogenic pain
Neurogenic pain is defined by the International Association for the Study
of Pain as pain arising as a direct consequence of diseases affecting the
somatosensory system [84]. The mechanisms involved and treatments are
different depending on whether the pain originates peripherally (vascular,
mechanical, or chemical lesion affecting the nerve) or centrally (spinal or
supraspinal hyperactivity). Neurogenic pain, even of a peripheral origin, is
frequently associated with sensitization of the CNS. Pharmacologic ap-
proaches to treatment of neurogenic pain aim to reverse or reduce the hyper-
activity of the nociceptive neurons. Commonly used agents include opioids,
anticonvulsivants, antiarrhythmics, antidepressants, and even NMDA re-
ceptors antagonists (eg, ketamine).
Functional neurogenic pain
Functional neurogenic pain is a subcategory of neurogenic pain occurring
in the absence of a defined anatomic lesion within the nervous system, but
rather representing a dysfunction of pain modulation mechanisms. This
may occur as a result of central activation of endogenous excitatory systems
that will amplify the nociceptive signal or by a dysfunction of endogenous
inhibitory mechanisms.
An example of central hyperactivity is the thalamic syndrome following
a lesion of thalamic nuclei as a result of a cerebral event. This lesion will pro-
duce hyperactivity of thalamic neurons that are normally inhibited by a com-
plex interneuron network. A small lesion within the thalamus may result in
intense pain over a large body area, frequently involving almost half of the
body. In contrast, another pain syndrome, namely FM, may be at least
partly explained by a deficit of descending endogenous pain inhibitory
304 MARCHAND
mechanisms [25]. In this condition the hyperalgesia is related to lack of
inhibition, rather than just hyperactivity of the nociceptive neurons.
Therefore, strategies for the treatment of functional neurogenic pain will
be either focused toward reduction of nociceptive hyperactivity or activation
of endogenous inhibition. Anticonvulsants will reduce sensory input by ef-
fect on ion channels, whereas antidepressant medications will augment inhi-
bition by effect on serotonin and noradrenergic systems.
Summary of the mechanistic approaches to the treatment of pain
Based on one’s knowledge of the neurophysiology of pain and the specific
mechanisms involved, therapeutic strategies may be selected.
Nociceptive pain arising in the periphery may be treated by reduction of
inflammation (NSAID), blocking the activity of the nerve fibers (ion
channels blockers), or by acting on the C-fiber receptors by using an
agent such as capsaicin.
If there is reason to suspect hyperactivity of spinal neurons following
a sensitization of the CNS (allodynia, hyperalgesia), then anticonvulsant
or antiarrhythmic agents are used to reduce neuronal hyperactivity.
NMDA antagonists could also be used to reverse this hyperactivity. It
is also possible that the prophylactic use of these agents, such as during
surgery, may prevent chronic pain.
If the descending inhibitory mechanisms are implicated, the use of sero-
toninergic and noradrenergic agonists, as in the antidepressants, may
help to recruit and modulate these systems.
Finally, antidepressant medications, opioids, and the anticonvulsant
drugs would also have an effect on the higher centers influencing the mo-
tivational aspect of pain.
Cognitive manipulation can also play a role in pain modulation. Har-
nessing these mechanisms from higher cerebral centers by use of distrac-
tion, relaxation, suggestion, and positive support will facilitate pain
management.
These pharmacologic and nonpharmacologic examples, based on the
neurophysiologic characteristics of the pain, demonstrate that a better un-
derstanding of the mechanisms of pain generation will influence therapeutic
approaches and facilitate treatments.
Summary
This article has described the complexity of the pain phenomenon and ex-
plained mechanisms involved in the development and maintenance of pain
conditions. This knowledge is a strong foundation on which to develop
a therapeutic guide for the treatment of pain. Although there is commonal-
ity in the nociceptive pathways of our patients, each individual will respond
305THE PHYSIOLOGY OF PAIN MECHANISMS
differently to pain as a result of genetic and environmental background. This
variability in perception and response to pain may lead to physician bias or
even misinterpretation of an individual patient’s symptoms. Keeping in
mind the heterogeneity of the pain response and the unique characteristics
of an individual patient will lead to better patient care. Understanding the
neurophysiologic mechanisms underlying the development and maintenance
of pain will help focus treatments more efficiently toward the specific abnor-
mality causing pain. The knowledge of the science of pain has provided an
opportunity to address pain from a mechanistic approach, with the objective
of reinforcing inhibitory mechanisms or reducing the hyperactivity of the
nociceptive response.
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THE PHYSIOLOGY OF PAIN MECHANISMS
... Almost every human being may have experienced pain at some point in their life-time. There are various cellular, molecular, genetic and environmental factors which try to explain the mechanisms of pain [7]. ...
... Özgür elektronlar kristalin üzerinde hareket edebilirler. Buna fotovoltaik etki denir [7]. Çift yüzlü fotovoltaik panel hücreleri üzerine yapılan araştırmalar, fotovoltaik endüstrisinin başlangıcına kadar uzanmaktadır. ...
... The full understanding of the exact mechanism of attenuation for noise, vibration, and erosion due to air injection is important for the development of the aeration system. The main reason for this attenuation comes from the fact that the existence of non-condensable gas, air, inside the cavitation bubble reduces the rate at which the bubble will collapse [7]. A build-up of the non-condensable gas at the interface will create a barrier that the water vapour must diffuse through it to condense on the interface. ...
... 78,95 In response to a painful stimuli, sensitized nociceptors will generate an increased number of action potentials to be processed centrally and interpreted are more intense pain. 96,97 However, it is important to consider that the thermal stimulation was only conducted in a control area of the body. Sensitized nociceptors in the control area of the body may be associated with an overexpression of pronociceptive mediators, channels and receptors leading to pathological spontaneous discharges and lowered activation threshold for thermal and mechanical stimuli. ...
... 27 However, the dissociation of thermal and mechanical hyperalgesias may be explained by the differences in neural signaling of thermal and mechanical pain that starts with peripheral encoding in distinct subsets of nociceptors or central sensitization, which is more prominent for mechanical stimuli. 95,96 The increased excitability at the spinal level in response to stimuli may be associated with an increased in the receptive fields of the nociceptive spinal cord neurons. 96,97 The ongoing pain in this sensory profile may be due to spontaneous activity in the nociceptive system originating from the peripheral and/or central nervous system. ...
... 95,96 The increased excitability at the spinal level in response to stimuli may be associated with an increased in the receptive fields of the nociceptive spinal cord neurons. 96,97 The ongoing pain in this sensory profile may be due to spontaneous activity in the nociceptive system originating from the peripheral and/or central nervous system. Pain Modulation Phenotyping Improving the diagnostic process by identifying patients with chronic MSK pain based on the results of inhibitory and facilitatory pain modulation responses can provide additional standardized outcomes for clinical trials. ...
Article
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Purpose: A major limitation in treatment outcomes for chronic pain is the heterogeneity of the population. Therefore, a personalized approach to the assessment and treatment of children and adolescents with chronic pain conditions is needed. The objective of the study was to subgroup pediatric patients with chronic MSK pain that will be phenotypically different from each other based on their psychosocial profile, somatosensory function, and pain modulation. Patients and methods: This observational cohort study recruited 302 adolescents (10-18 years) with chronic musculoskeletal pain and 80 age-matched controls. After validated self-report questionnaires on psychosocial factors were completed, quantitative sensory tests (QST) and conditioned pain modulation (CPM) were performed. Results: Three psychosocial subgroups were identified: adaptive pain (n=125), high pain dysfunctional (n=115), high somatic symptoms (n=62). Based on QST, four somatosensory profiles were observed: normal QST (n=155), thermal hyperalgesia (n=98), mechanical hyperalgesia (n=34) and sensory loss (n=15). Based on CPM and temporal summation of pain (TSP), four distinct groups were formed, dysfunctional central processing group (n=27) had suboptimal CPM and present TSP, dysfunctional inhibition group (n=136) had suboptimal CPM and absent TSP, facilitation group (n=18) had optimal CPM and present TSP, and functional central processing (n=112) had optimal CPM and absent TSP. A significant association between the psychosocial and somatosensory profiles. However, no association was observed between the psychosocial or somatosensory profiles and pain modulatory profiles. Conclusion: Our results provide evidence that adolescents with chronic musculoskeletal pain are a heterogenous population comprising subgroups that may reflect distinct mechanisms and may benefit from different treatment approaches. The combination of screening self-reported questionnaires, QST, and CPM facilitate subgrouping of adolescents with chronic MSK pain in the clinical context and may ultimately contribute to personalized therapy.
... Nociceptive LBP stimulation of lumbar spine innervated structures (e.g the zygapophyseal joints, spinal ligaments or muscles, or the outer part of the lumbar disc) can induce the transduction of a noxious stimulus into an electric signal in the nervous system. This signal, often referred to as an warning signal, will be processed in the central nervous system with significant brain excitements [13] and can lead to pain [5,14,15]. In the case of nociceptive pain, the somatosensory nervous system functions normally [16]. ...
Article
Full-text available
Low back pain (LBP) that radiates to the leg is not always related to a lesion or a disease of the nervous system (neuropathic pain): it might be nociceptive (referred) pain. Unfortunately, patients with low-back related leg pain are often given a variety of diagnoses (e.g. 'sciatica'; 'radicular pain'; pseudoradicular pain"). This terminology causes confusion and challenges clinical reasoning. It is essential for clinicians to understand and recognize predominant pain mechanisms. This paper describes pain mechanisms related to low back-related leg pain and helps differentiate these mechanisms in practice using clinical based scenarios. We illustrate this by using two clinical scenarios including patients with the same symptoms in terms of pain localization (i.e. low-back related leg pain) but with different underlying pain mechanisms (i.e. nociceptive versus neuropathic pain).
... Higher organisms have developed the nociceptive pathway by which we can perceive pain attributes (e.g. location, type, intensity) which is part of the normal functioning of the body [1]. When the perception of pain persists over time and loses its capacity for sensory functional information is defined as pathological pain [2]. ...
Article
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Shikonin is an ointment produced from Lithospermun erythrorhizon which has been used in traditional medicine both in Europe and Asia for wound healing and is associated with anti-inflammatory properties. The goal of this work is to assess the analgesic properties of Shikonin in the CFA-induced inflammation model of pain. Rats were subjected to inflammation of the hind paw by CFA injection with a preventive injection of Shikonin and compared to either a control group or to a CFA-inflamed group with the vehicle drug solution. Inflammation of the hind paw by CFA was assessed by measurement of the dorsal to plantar diameter. Mechanical thresholds were established by means of the Von Frey filaments which are calibrated filaments that exert a defined force. Finally, the spinal cord of the studied animals was extracted to analyse the microglia population through immunohistochemistry using the specific marker Iba-1. Our results show that Shikonin reduces the paw oedema caused by CFA inflammation. Subsequently, there is a concomitant restoration of the mechanical thresholds reduced by CFA hind paw injection. Additionally, spinal microglia is activated after CFA-induced inflammation. Our results show that microglia is inhibited by Shikonin and has concomitant restoration of the mechanical thresholds. Our findings demonstrate for the first time that Shikonin inhibits microglia morphological changes and thereby ameliorates pain-like behaviour elicited by mechanical stimulation.
... The triggers are normally the same as those involved in pain of visceral origin, with direct trauma and chronic inflammatory processes being the most common. 20,21 Neuropathic pain results from demonstrable neurological damage "or a disease that satisfies established neurological diagnostic criteria." It may be central or peripheral depending on the site of the somatosensory nervous system affected. ...
Article
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Chronic pelvic pain (CPP) affects a significant proportion of women worldwide And has a negative impact on several aspects of these women's lives including mental health, work, relationships and sexual function, among others. This set of factors ultimately reflects negatively on quality Of life. The physiopathology of CPP is complex and remains to be fully clarified; however, recent advances have increased understanding of the mechanisms involved in chronic pain in general, and more specifically, CPP. Nonetheless, even when a detailed clinical history is obtained, meticulous physical examination is performed and imaging resources are appropriately used, the organic cause of the pain may still fail to be identified in a substantial number of women with CPP. Management of CPP may therefore be challenging. This narrative review was aimed at adding to the available literature on the subject, presenting and discussing the principal characteristics of CPP in women. The paper highlights gaps in the literature while providing the most up-to-date evidence associated with the physiopathology and classification of pain, its diagnosis and treatment. In addition, current challenges in the management of women with CPP are discussed.
... In fibromyalgia, myofascial pain syndrome and the TM (which occurs as the component of both) patients detect some stimulators as a pain, although these are just stimulators caused by the change in the processing the pain and senses. This situation may cause an increase in the using analgesic caused by a decreased sensitivity to peripheral and central sensitization (10)(11)(12)(13). In our study, we considered this situation and planned to work on the effects of the existence of a chronic TM pain on a postoperative pain in laparoscopic cholecystectomy patients. ...
Article
Introduction: Trapezius myalgia (TM) is the chronic pain of trapezius muscle, one of the most common reasons for shoulder pain seen in a common population. The aim of our study is to determine the effects of an existence of preoperative TM to the postoperative pain in the patients, who underwent a laparoscopic cholecystectomy. Methods: After receiving an ethical committee approval, we have included 60 ASA 1-2 patients who underwent a laparoscopic cholecystectomy surgery in the general surgery operating room in University of Health Sciences Turkey, İstanbul Training and Research Hospital between January and June 2015. The patients who were classified as ASA 3-4, had head and neck surgery or trauma, operations which had started as the laparoscopic procedures but then had to be performed as an open surgery, patients who had rheumatological and neurological diseases, tendinitis that affects the shoulder joints, and patients who had to have the emergency surgeries were excluded in this study. Patients were divided into two groups, one with (TM+) and who were not having (TM), when evaluated preoperatively. Each group had analgesia by a patient-controlled device. Using Numeral Rating scale demographical data, shoulder and umbilical pain levels were evaluated, and postoperative analgesic needs and doses were recorded in the case files. Results: Five patients who were excluded; n=26 in TM+ group and n=29 in TM group were taken. There was no significant demographic difference found between two groups. No statistically significant difference was found between TM+ and TM- groups (p>0.05). While no statistically significant difference was established in the tramadol doses between two groups, a total of ten patients out of 26 TM+ had shoulder pain, and additional analgesic need was found statistically increased in TM+ group (p
... From the clinical efficacy standpoint these results are critical becausehigher redox potential are typically correlated with immediate esthetic outcomes that are more intense and durable, as observed in HP35. Despite these results, it is anticipated that patients treated with highly concentrated bleaching gels will experience BS (either trans-or post-operatory) that is more intense and durable due to this material's ability to generate a great ionic displacement flows that are capable of promoting strong depolarization of pulpal tissues[32].The present study represents an effort to provide relevant information regarding how variations in pH and EP may influence the attainment of immediate esthetic results and the occurrence of BS. It is anticipated that the result of the present manuscript will positively impact subsequent research (either in vitro, in situ or clinical trials) and the development of novel dental bleaching materials. ...
Article
Full-text available
Introduction Tooth whitening procedures are under continuous investigation to improve esthetic outcomes and reduce bleaching sensitivity (BS) precipitating from treatments. During the dental bleaching process it is known that the release of free radicals degrades the organic pigment molecules of the tooth and with this an amount of energy is released. Nonetheless, previous studies have never investigated the temporal correlation between of pH and electric potential (EP) generated in this treatment. Objectives Therefore, the objective of the present study was to investigate temporal variations of pH and EP associated with three different commercially available bleaching gels and the correlation levels between parameters of interest to provide relevant information regarding the kinetics of oxidation reactions in dental bleaching procedures. Methods The study was divided into 3 groups (n = 9) in function of hydrogen peroxide concentration (either 6%, 15% and 35%). The temporal evolution of pH and EP values were determined using a highly-accurate and previously calibrated pH meter at specific time-points (5, 10, 15 and 30 min). Results Data obtained were submitted to one-way ANOVA of repeated measures with Bonferroni post-test (α = 0.05). The results of the study showed difference in the factor gel concentration (p <0,0001), time (p <0,0001) and interaction (gel/time) (p = 0.002) while throughout the intervals evaluated the groups remained relatively stable and without significant difference in the intragroup variation of pH (p < 0.05) and in EDP only with significant difference in the 5 min interval of the 35% concentration. A 2nd order polynomial relationship test showed high correlation levels. Conclusion It can thus be concluded that there is a negative relationship between EP and pH variation in the different gel concentrations. Clinical Significance The findings of the present study suggest that bleaching gels of higher concentration may provoke BS that are more intense and durable due to significant electric depolarization of neuronal extensions of pulpal tissues.
Article
Several reports indicate either increased or decreased pain sensitivity associated with psychiatric disorders. Chronic pain is highly prevalent in many of these conditions. We reviewed the literature regarding experimental pain sensitivity in patients with major depression, bipolar disorder, posttraumatic stress disorder (PTSD), generalized anxiety disorder, panic disorder, obsessive-compulsive disorder and schizophrenia. Electronic searches were performed to identify studies comparing experimental pain in patients with these conditions and controls. Across 31 depression studies, reduced pain threshold was noted except for ischemic stimuli, where increased pain tolerance and elevated sensitivity to ischemic pain was observed. A more pervasive pattern of low pain sensitivity was found across 20 schizophrenia studies. The majority of PTSD studies (n = 20) showed no significant differences compared with controls. The limited number of bipolar disorder (n = 4) and anxiety (n = 9) studies precluded identification of clear trends. Wide data variability was observed. Awareness of psychiatric patients’ pain perception abnormalities is needed for active screening and addressing physical comorbidities, in order to enhance quality of life, life expectancy and mental health.
Chapter
Neuromodulatory treatments such as motor cortex stimulation, spinal cord stimulation, or transcranial magnetic stimulation are promising options for chronic pain refractory to conventional therapy. Deep brain stimulation (DBS) is still deemed investigational for chronic pain therapy due to inconsistent pain reduction reported in the literature. The targeting of specific areas of the pain matrix for DBS is usually performed according to standard coordinates using stereotactic atlases. Functional magnetic resonance imaging (fMRI) and diffusion tensor imaging (DTI), however, allow for direct visualization of DBS targets. Combining both methods provides the opportunity of examining the relationship between different structures and their functions. This way, it is possible to visualize patient-individual networks that may be pathologically altered. Future connectivity studies based on DTI and fMRI data may result in an advanced understanding of functional and dysfunctional networks within the brain and lead to more efficient targeting.
Conference Paper
Background. The authors compiled information about recent advances in screening for psychosocial risk factors considered to be yellow flags for potentially poor outcomes among patients with chronic orofacial pain (most commonly temporomandibular disorders). Types of Studies Reviewed. The authors conducted MEDLINE searches for the period 1995 through 2002 using the terms "temporomandibular disorders", "assessment" and "psychological", as well as "primary care," "screening" and "psychological disorders. They also searched personal files for relevant articles. Results. Psychosocial dysfunction is prevalent among patients with chronic orofacial pain. Yellow flags include high levels of disability; psychological disorders; and prolonged or excessive use of opiates, benzodiazepines, alcohol or other drugs. The authors identified several reliable, valid and brief patient self-administered questionnaires that can be use to screen for these yellow flags. Some of these are the Research Diagnostic Criteria/Temporomandibular Disorders Axis II, Alcohol Use Disorders Identification Test and Patient Health Questionnaire. Clinical Implications. Dentists can improve the quality of care for patients with chronic orofacial pain by screening for psychosocial risk factors and by referring patients with risk factors for psychological or psychiatric assessment and treatment.