CHAPTER 1 Basic Science of Pain

CASEY J. FISHER, MD • TONY L. YAKSH, PHD • KELLY BRUNO, MD • KELLY A. EDDINGER, BS, RVT

INTRODUCTION detect noxious stimuli, they are triggered to discharge Clinically, the most commonly referenced definition of when the range of temperature or pressure corresponds pain initially described by Harold Merskey in 1964, and to what would be considered painful. The distal termi- as adopted by the International Association in the Study nals of these small C fibers display large branching den- of Pain in 1979, defines pain as “an unpleasant sensory dritic trees and are characterized as being “free” nerve and emotional experience associated with actual or po- endings. These nerve endings can be activated further tential tissue damage, or described in terms of such by many specific agents in the periphery in response damage.”1 In this chapter, we will give an overview of to tissue injury, inflammation, or infection in a the pathways involved in pain processing as it occurs concentration-dependent fashion. Table 1.1 depicts in both the central and peripheral nervous systems as the source and nature of these agents as well as the is currently conceived. This overview will include: anat- eponymous receptor on C fibers that is activated with omy involved in the processing of nociceptive stimuli; each agent. The fact that there are multiple stimulus the fundamentals of systems underlying acute nocicep- modalities for these C fibers that can lead to a signal tion and persistent pain states; and the linkage to of pain is the reason they are known as C-polymodal chronic pain and how the immune system plays a role nociceptors. In fact, C fibers can be characterized further in this processing. by what provokes them to fire. There are some C fibers that do not respond to mechanical stimulation. These so-called silent nociceptors, or mechanically insensitive PERIPHERAL ANATOMY afferents (MIAs), only respond to very high levels of e Primary Afferents2 4 nonphysiologic mechanical stimulation and/or heat. The signal of acute nociceptive pain is propagated However, they can acquire sensitivity in the face of pa- along sensory neurons, which have cell bodies (somas) thology, such as inflammation, which leads to a sensi- that lie in the dorsal root ganglia (DRG) and send one tized state and activation by relatively low-intensity of their axon projections to the periphery and the mechanical/thermal stimuli. other to the dorsal horn of the spinal cord in the Like C fibers, A-d fibers are small and can be high central nervous system. The axons of peripheral affer- threshold. But, A-d fibers are myelinated, and there- ents can be classified by anatomical characteristics fore, faster, with conduction velocity between 10 and (Erlanger-Gasser), Conduction Velocity (Lloyd-Hunt), 40 m/s. As such, A-d fibers act as nociceptors and and by their respective thresholds for activation. Most mediate “first” or “fast” pain, whereas C fibers are commonly, they are known by anatomical classification responsible for “second” or “slow” pain. To put this into two types of A fibers (b and d) and C fibers. in context, consider what happens when you touch a C fibers are small, unmyelinated, and therefore, slow hot object. Your immediate reaction is to pull your conducting fibers (<2 m/s). These primary sensory hand away, which is mediated by noxious thermal afferent neurons represent the majority of afferent fibers sensation activating fast conducting A-v afferents. found in the periphery and are most commonly high Typically, there is also a slower sensation of pain trav- threshold fibers, meaning they are not activated unless eling over the slowly conducting C fibers, which relay the stimulus (thermal, mechanical, or chemical) is at tissue damage in the form of a burning sensation. an intensity sufficiently high enough to potentially Some populations of A-d fibers can also be lower cause tissue injury. As nociceptors, or receptors that threshold at times, meaning they begin to discharge

TABLE 1.1 allodynia is low-intensity tactile or thermal stimuli causing a pain state. This can occur in scenarios where Summary of Agents, Tissue Source, and there is nerve damage (for example, carpal tunnel or Receptors Found at C Fibers12. sciatic nerve lesions). Agents Tissue Source Receptors All of the afferent nerve fibers share the following Amines Mast cells H1 important characteristics related to the pattern in which (histamine) 5HT3 they respond to a stimulus and the manner in which Platelets they fire: (serotonin) i) Afferent nerve fibers display little or no sponta- fi Bradykinin Clotting factors BK 1, BK2 neous ring. They do not spontaneously discharge (bradykinin) like other nerve cells of the brain or heart; Lipidic acids Prostanoids EP ii) Peripheral afferents typically display a monotonic (PGE2), increase in discharge frequency that covaries leukotrienes with stimulus intensity. This means that if the thermal or mechanical intensity increases, there Cytokines Macrophages IL-1, TNFR (interleukins, will be a monotonic increase in discharge fre- tumor necrosis quency because there will be a greater depolari- factor) zation of the terminal, which will increase frequency of axon discharge; and Primary afferent C fibers NK1, peptides [substance P (SP), CGRP iii) Afferents serve to encode modality by being able calcitonin gene- to transduce thermal, mechanical, and/or chemi- related peptide cal signals into a depolarization based on their (CGRP)] individual nerve ending transduction properties. b fi Proteinases Inflammatory cells PAR3, For the larger A- ber afferents, the nerve endings (thrombin, trypsin) PAR1 are highly specialized, e.g., Pacinian corpuscle, and fi þ only respond to speci c low threshold stimuli, Low pH or Tissue injury [(H ), ASIC3/ fi hyperkalemia (Kþ), adenosine] VR1, A2 whereas the free nerve endings of the small C bers respond to a more diverse array of signals at higher Lipopolysaccharide Bacteria (LPS, TLR4, threshold. (LPS), formyl formyl peptide) FPR1 peptide Somatic and Visceral Afferents4,5 The location of peripheral afferents is also important when it comes to the type of pain sensation. Peripheral when the range of temperature or pressure corresponds afferent axon projections to the periphery are found to what would be nontissue damaging. In the case of throughout the body. The axon projections to the thermal stimulus, it would be considered a mildly skin, joints, and muscles are involved in somatic pain. noxious warm/hot sensation. In the case of mechanical The axon projections to the organs are involved in stimulus, it would be considered touch or pressure that visceral pain. The main functional differences between is borderline painful. A-d fibers also differ from C fi- afferents in the viscera and somatic systems are that bers in that they express specialized nerve endings there is little distinction between nociceptive afferents that serve to define their response characteristics. This and nonnociceptive afferents, and there is significant relationship will be delineated further in the periph- prevalence of MIAs in the viscera. The visceral afferents eral physiology section. only exist as high threshold and low threshold afferents, In contrast, A-b fibers are large, myelinated fibers the latter of which responds to a range of stimulation with the fastest conduction velocity (>40 m/s) of pri- intensities. The main anatomical difference between mary afferent neurons. They are low threshold afferent afferents in the viscera and afferents in the somatic sys- fibers that fire in response to low threshold mechanical tem is that there are significantly fewer afferents in the stimulation, such as touch or pressure. Under normal viscera. Less than 10% of the total spinal cord afferent physiologic states, activation of these afferents does input comes from the visceral afferents. This often not generate a noxious sensation. However, there are means that visceral input travels to its more central certain conditions in which these afferents initiate a projections along with somatic input. The concept of pain sensation, or allodynia. The definition of referred pain, whereby organ pathology causes a CHAPTER 1 Basic Science of Pain 3

WDR

Visceral Afferent

Somatic Afferent

Coronary Left Arm Pain Ischemia

FIG. 1.1 Viscerosomatic convergence. (Credit: Kelly A. Eddinger.) concomitant dermatomal spread of pain, is thought to CENTRAL ANATOMY be caused by convergence of somatic and visceral affer- First-Order Neurons: Spinal Dorsal Horn ents onto the same wide dynamic range neurons Projections6,7 (WDRs) at the dorsal horn level (see Fig. 1.1). This leads If the signal generated by an acute nociceptive stimulus is to the message generated by a visceral afferent being followed anatomically, from distally to proximally, the fl con ated with the input generated by a particular so- peripheral afferents extend through the DRG, where the “ matic region, thereby accounting for the referred afferent cell body lies, to the axon terminals of the spinal ” fi pain pro le of a visceral stimulus. WDR neurons will cord. Then, they terminate at the dorsal horn, where the be discussed further in the central anatomy section. dorsal root entry zone (DREZ) is found. The smaller 4 Pain Care Essentials and Innovations afferents tend to enter the DREZ more laterally, and the a way to organize the types of second-order neurons larger afferents tend to enter the DREZ more medially. that give rise to the tracts of the spinal cord (See The small and large afferents collectively enter the dorsal Fig. 1.2). As discussed earlier, specific sensory afferents horn as the fascicles that make up the nerve root. Nerve project to specific locations within the dorsal horn. roots are divided into cervical, thoracic, lumbar, and The most superficial, dorsal level of the dorsal horn sacral segments in a rostrocaudal distribution and enter is known as the marginal zone or Lamina I, according to on the ipsilateral side of the dorsal horn. the Rexed Lamina(e) classification. The marginal zone High threshold, small, unmyelinated afferents (C fi- is where C fibers and A-d fibers terminate. As such, neu- bers) project to the more superficial regions of the dorsal rons in this layer respond selectively to high threshold horn: Lamina I (marginal layer), Lamina II (substantia stimulation and send projections into the spinal tracts gelatinosa), and the central canal (Lamina X). These located in the ventrolateral (or anterolateral) section segmental penetrating axons also send axons rostrally of the cord on the contralateral side of entry. These and caudally, traveling in the lateral tract of Lissauer, tracts will eventually project to the various nuclei in up to the level at which they enter the dorsal horn. the contralateral brainstem and thalamus, more This collateralization can be up to several segments formally known as the spinothalamic tracts. As dis- above or below the entry point into the dorsal horn. cussed before, some of the neurons in the marginal Low threshold, large, myelinated afferents (A-b fibers) zone project ipsilaterally for a few segments in the white project deeper into the dorsal horn (Laminae IV, V, and matter along the dorsolateral tract of Lissauer before VI), a region known as the nucleus proprius. Smaller entering the gray matter on the contralateral side and myelinated afferents (A-d fibers) project to the marginal joining the tracts in the ventrolateral quadrant of the layer and substantia gelatinosa layers (Laminae I and II) spinal cord. These projections are otherwise known as as well as the deeper layer of the nucleus proprius part of the intersegmental system. (Lamina V). Larger A type fibers also collateralize up to Lamina II, also known as the substantia gelatinosa, is 1 to 2 segments rostral or caudal to their entry point, where C fibers and A-d fibers terminate. However, it is but they collateralize in the dorsal columns to travel up also where many local interneurons are located. Inter- to the dorsal column nuclei (see Fig. 1.2). neurons are second-order neurons that project from the more superficial dorsal horn layers to the deeper Second-Order Neurons: Dorsal Horn layers of the Nucleus Proprius (Laminae III, IV, V, and 8,9 Anatomy and Functional Organization VI). These interneurons are classified as excitatory or The dorsal horn is made up principally of second-order inhibitory and serve to locally regulate the excitability neurons that will send projections into the ascending of dorsal horn projection neurons. The complexity of spinal tracts to transmit sensory information to the this layer is due in part to the fact that second-order in- brainstem and cortex. These second-order neurons can terneurons are in direct communication with second- be classified according to their functional characteristics order neurons, which make up the ascending spinal as well as their anatomic location within the dorsal tracts that are all in communication, either directly or horn itself. Cross-sectional anatomy of the gray matter indirectly, with the original primary afferents, including of the dorsal horn has been well characterized and is C fibers and A-d fibers.

Substantia Marginal Zone Gelatinosa

Nucleus Proprius

Motor Horn

Central Canal FIG. 1.2 Dorsal horn cross section. See text for further discussion. (Credit: Kelly A. Eddinger.) CHAPTER 1 Basic Science of Pain 5

The nucleus proprius (Laminae III, IV, V, and VI), squeeze. The location of these neurons is principally where large, myelinated, primary afferents (A-b fibers) in the nucleus proprius (Laminae III, IV, V, and VI). terminate, has large-soma neurons, which send their WDR neurons are also known as modality conver- dendrites up into the upper lamina to make contact gent neurons because they do not necessarily discrimi- with small, high threshold afferent input. Large, low nate between the source of the primary afferent from threshold afferents make synaptic contact on the cell which they receive input. As mentioned previously, bodies and the ascending dendrites. Thus, they receive the somatic and visceral afferents can cause a referred convergent low and high threshold input. This enables pain due to viscerosomatic convergence. This explains them to respond to low threshold and show increasing why pain caused by coronary artery occlusion during discharge rates with increasing stimulus intensities as a myocardial infarction can cause a concomitant derma- small afferents are engaged. Accordingly, these cells tomal pattern of pain. This convergence can also occur are referred to as WDR neurons. between deeper and more superficial somatic structures, The central canal (Lamina X) is the deepest layer of which explains why deep muscle or bone pain can also the dorsal horn. This location is where a large amount show a pattern of dermatomal spread of pain similar to of visceral afferent input occurs and where smaller so- the referred pain of coronary artery ischemia. matic afferents (C fibers and A-d fibers) also terminate. Another characteristic of WDR neurons is that they Signals received here are high threshold temperature can be prompted to exist in a state of ongoing discharge and noxious mechanical stimulation. The tracts located by low frequency, repetitive stimulation of C fiber here are the second-order neurons that are crossing primary afferents. This continuous discharge has been midline and ascending to form the anterolateral termed “wind-up” by Mendell and Wall in 1965. portion of the spinothalamic tracts. Wind up, also known now as central sensitization, Organization of the dorsal horn is not strictly based will be discussed in detail later in the physiology section on anatomic location of the second-order neurons. of this chapter. Second-order neurons can also be organized by how they respond to stimuli and, therefore, by their func- Third-Order Neurons: Ascending and tion. There are high threshold neurons in Lamina I Descending Spinal Tracts and Supraspinal 9e11 and II that are nociceptive specific and respond only Projections to highly intense stimulation. The other type of In order to understand where the third-order neurons second-order neuron, described above, which is based project, it is first necessary to understand the different on how it responds is the WDR neuron (see Table 1.2). individual ascending projections that make up the As the name suggests, these neurons respond to a wide ascending sensory spinal tracts. The four major projec- range of stimulus intensities and they are able to tions identified in the ventrolateral quadrant of the spi- respond with increased frequency based on how intense nal cord are the spinothalamic, the spinoparabrachial, the stimulation is that they are receiving. This concept the spinoreticulothalamic, and the spinomesencephalic of a graded response to stimulation is how innocuous projections. A fifth ascending projection that transmits light touch is differentiated from a noxious pinch or sensory information, but not necessarily pain unless

TABLE 1.2 Organization of Primary, Secondary, and Tertiary Neurons. Primary Afferents Secondary Projections Tertiary Projections Results C fibers Dorsal horndLaminae I and Spinothalamic tracts to Anterior cingulate II mediodorsalis nucleus of cortexdemotional pain response thalamus A-d fibers Dorsal horndLaminae II and Spinothalamic tracts to Somatosensory V ventrobasal thalamus cortexdprecision mapping of pain response A-b fibers Dorsal columns Dorsal columns to medial Somatosensory cortexdtactile collateralization lemniscus sensation and proprioception 6 Pain Care Essentials and Innovations the system is in a state of evoked hyperpathia, is the dor- ventrobasal nucleus project to the somatosensory sal column medial lemniscal pathway. These projec- cortex and input in these areas follows a strict pattern tions terminate in three main brainstem regions: the that is consistent with the sensory homunculus in the diencephalon, the mesencephalon, and the medulla. cortex. In other words, input in this area can be inter- The third-order neurons in these regions then project preted over a range of intensities with precision as to further to the cortex or within the diencephalon or what part of the body is affected somatotopically and mesencephalon. what is the modality of the injury. The spinothalamic projections consist of both WDR The spinoparabrachial projections start as ascending neurons coming from the Lamina V portion of the dor- tracts in the contralateral ventrolateral section of the sal horn and high threshold, pain-specificLaminaI dorsal horn and they terminate in the parabrachial nu- neurons. The pain-specific, high threshold neurons cleus of the pons (see Fig. 1.4). Third-order neurons in make up the ascending tracts that will project into this area then project to the amygdala and the VMpo. As the posterior ventral medial nucleus (VMpo) and the noted earlier, the VMpo also projects to the insula, medial dorsal nucleus of the thalamus. The third- meaning that these projections appear to have a role order neurons in the medial dorsal nucleus then proj- in the “affective” aspect of pain. Previous animal and ect to the anterior cingulate cortex. The third-order human studies of lesions in the areas of the temporal neurons in the VMpo then project to the insula. These lobe and amygdala show a dysfunction in the associa- pathways are both known to be less precise about the tion of the stimulus intensity of pain with its affective actual intensity and localization of the pain and more component (suffering). involved in the emotional and affectual aspects of The spinoreticulothalamic projections start as pain. In contrast, the third-order neurons in the ven- ascending tracts in the ipsilateral dorsal horn just like trobasal nucleus of the thalamus, where the WDR the WDR neurons of the spinothalamic tract, but these neurons from Lamina V terminate, are more precise tracts ascend to the reticular formation of the medulla. about intensity and localization of the pain (see Here, the third-order neurons project to the intralami- Fig. 1.3 and 1.5). Third-order neurons in the nar nucleus of the thalamus that further relays

Somatosensory Cortex

Anterior Cingulate Cortex

Inferior Insula

Diencephalon Mediodorsal thalamic projection VMpo thalamic projection

Ventrobasal Thalamus

Spiniothalamic Projections FIG. 1.3 Spinothalamic projections: see text for further discussion. (Credit: Nancy Dinh.) CHAPTER 1 Basic Science of Pain 7

Cortex As mentioned previously, the dorsal column projections are made up, in large part, due to the collateralization of the large, low threshold, primary afferents, A-b ﬁbers (see Table 1.2). These ascending tracts stay Diencephalon VMpo ipsilateral and ascend along the dorsal columns to the nuclei in the medulla, where they then project to second-order neurons, which will cross over and continue to ascend as a part of the medial lemniscus Amygdala Pons to the thalamus. The third-order neurons there then project to the cortex. This system is not responsible ﬁ Parabrachial speci cally for pain, but it is responsible for tactile Nucleus sensation and limb proprioception which can be painful in certain pathologic states that will be considered in the next sections (see Fig. 1.6).

Medulla PHYSIOLOGY OF NOCICEPTION Acute Pain12,13 The initial report of pain and the physical movement to avoid a certain stimulus are responses that stem from Spinal Cord activity that is initiated at the peripheral sensory termi- Dorsal Horn nal. This activity, in the form of a signal, sent via terminal depolarization of small unmyelinated (C fibers) and myelinated (A-d fibers) primary afferent sensory neurons is due to the presence of potential damaging FIG. 1.4 Spinoparabrachial projections: see text for further stimuli. This noxious stimulus, which can be thermal, discussion. (Credit: Calvin Nguyen.) chemical, or mechanical, creates a signal that the body identifies as pain. information to many areas of the cortex. The reticular In most cases, acute pain is considered to be protec- formation and its projections to the cortex are a part tive, with the intention to remove the body from of the reticular activating system, which is in part further harm. The initial noxious stimulus is sensed responsible for regulating wakefulness and sleep-wake by peripheral nociceptors that communicate with the transition. This serves as a plausible pathway for pain spinal cord and ultimately send the signal to cortical interpretation to effect sleep and the sleep-wake pattern regions of the brain. The most immediate and direct (see Fig. 1.5). response to a noxious stimulus is the flexor withdrawal The spinomesencephalic projections start as reflex. When a nociceptor is activated by a noxious ascending tracts as a part of the anterolateral system stimulus, the signal travels through the primary sen- but terminate at the periaqueductal gray (PAG) matter sory neuron to the dorsal horn where it synapses and the reticular formation of the mesencephalon. with an interneuron. The interneuron synapses with From there, third-order neurons project to the lateral an alpha motor neuron. Once the signal passes to thalamus. The PAG is not well understood, but it does the motor neuron, a motor command is sent from play a role in descending inhibition of pain through the ventral horn of the spinal cord to the flexor and the bulbospinal pathway and it also causes autonomic extensor muscles resulting in removal of the body responses in humans when stimulated (see Fig. 1.5). part from the noxious stimulus. This reflex does not While the transmission of the pain signal is known require processing at the cortical level, allowing for a to be produced by the ascending tracts, there is some rapid response to avoid further injury. evidence that there are descending tracts that can The signal of nociception in this case is activation facilitate the signal. The bulbospinal projections are of C fibers. Activation of C fibers occurs with tissue descending tracts that transmit a signal in from the nu- pathology in the form of injury, inflammation, or cleus raphe magnus of the medulla to the dorsal horn infection. These disease states cause increase in that is excitatory in nature, but it serves to modulate inflammatory mediators including protons, prosta- the central sensitization at the WDR neuron level using glandins (PGE2), thromboxanes, leukotrienes, growth serotonergic receptors. factors, cytokines, chemokines, and neuropeptides 8 Pain Care Essentials and Innovations

Cortex Reticular Activating System

Thalamus

Midbrain Bulbospinal Tract Spinothalamic Tract Spinomesencephalic Tract Periaqueductal gray matter Pons

Reticular Upper Formation Medula Spinoreticular Tract

Lower Medulla

Spinal Cord

FIG. 1.5 Spinothalamic, Spinoreticular, Spinomesencephalic, and Bulbospinal projections. See text for further discussion. (Credit: Nancy Dinh.)

Dorsal Column- Medial Spinothalamic tract Lemiscus pathway

3rd order 3rd order neuron neuron

Thalamus 2nd order Dorsal column neuron nuclei

Decussation of Medulla medial lemiscus

2nd order 1st order neuron neuron Dorsal columns

Spinal Cord

Anteriorlateral quadrant 1st order neuron

FIG. 1.6 Comparison between dorsal column and spinothalamic tracts. (Credit: Vanessa Tran.) CHAPTER 1 Basic Science of Pain 9

(see Table 1.1). The presence of inflammatory Binding of these mediators to the respective receptor mediators in the periphery causes sensitization of the on the sensory afferent terminal leads to cellular depolari- primary afferents and leads to hyperexcitable states. zation, increased intracellular calcium, and activation of This hyperexcitable state can occur in all innervated protein kinases (e.g., protein kinase C and mitogen- tissues. activated protein kinases). These kinases serve to alter transducer molecules (e.g., TRPV1) and ion channels 12 Persistent Pain (e.g., voltage-sensitive sodium channels)causing increased For all intents and purposes, the hyperexcitable state in excitability of the nociceptor. In some cases, the nocicep- the periphery resolves when the inflammatory process tors are sensitized in such a way to become spontaneously resolves, except in certain cases where it does not active (as initiated by the local chemical milieu). When this resolve. Persistent pain states do occur due to sensitiza- occurs, the nociceptor may be activated by a less intense tion in the periphery and at the dorsal horn, which is physical stimulus resulting in an allodynic state. Peripheral known as peripheral sensitization. When the sensitiza- sensitization leads to ongoing pain and primary tion occurs at the dorsal horn level or above, it is hyperalgesia. referred to as central sensitization. Dorsal Root Ganglia15 12,14 Peripheral Sensitization It has become increasingly apparent that the DRG is a In physiological states, the activation and firing of complex local neural network. Aside from the presence sensory nerve endings in response to nonnoxious of neuronal cell body of the afferent, the DRG is stimuli occurs at a defined range. In states of chronic composed of astrocyte-like satellite cells that invest pain, the sensory nerve endings become sensitized each cell body. The innervation provided by postgan- such that they exhibit a reduced firing threshold and/ glionic sympathetic axons reflects the fact that the or an increased response to activation. This hyperexcit- DRG lies outside the conventional blood-brain barrier able response to noxious and nonnoxious stimuli is and has a sympathetically innervated vasculature. They termed peripheral sensitization. Following tissue also have a large population of macrophages that are injury, cell damage occurs with activation of afferent activated by neuroinflammatory stimuli, blood ves- nociceptive neurons and the aggregation of inflamma- sels, and of note, significant axon collaterals which tory cells at the site of injury. Activated nociceptors arise from the DRG cell body to release neurotransmit- and nonneuronal cells release chemical mediators ters in the DRG (see Fig. 1.7). The importance of this including protons, prostaglandins (PGE2), thrombox- complexity is that the DRG neuron can be the initiator anes, leukotrienes, growth factors, cytokines, chemo- of ectopic activity which drives action potentials down kines, and neuropeptides that mediate peripheral the glomerulus (connecting the cell body to its axon) sensitization. Each of these extracellular products in- and generate an action potential traveling both teracts with specific receptors found on the small orthodromically as well as antidromically. The DRG afferent nerve ending (see Table 1.1). also express the same receptors and channels that are

FIG. 1.7 Dorsal root ganglion structure. See text for further detail. (Credit: Kelly A. Eddinger.) 10 Pain Care Essentials and Innovations expressed on the afferent neuron terminals and corre- undergoes progressive depolarization as produced spondingly respond to the same mediators that act in response to repetitive stimulation via activation on these terminals. Following peripheral tissue injury, of AMPA and neurokinin 1 (NK1) receptors by the DRG may initiate upregulation of voltage-gated Na glutamate and substance P, respectively, the Mg and Ca channels as well as an increase in macrophages blockade is removed, and glutamate is able to acti- that release proinflammatory mediators that drive vate the receptor. In response, there is an influx of Ca ectopic DRG activity. that activates voltage-gated Ca channels and phos- phorylating enzymes including protein kinases A 12,16,17 Central Sensitization and C (PKA, PKC) and mitogen-activated protein Central sensitization is the amplification and/or main- kinases (MAPK). Thus, excitation of afferent input tenance of peripheral nociceptive input at the spinal along the collaterals of distal segments is insufficient and supraspinal levels. Sensitization occurs due to to cause the neuron to fire, but once neurons in that increased excitation or reduced inhibition of excitatory distal segment become sensitized (as with “wind- primary afferent neurons. This occurs in response to up”) after injury, the input is sufficient to allow persistent nociceptive input resulting in increased that distant segmental neuron to fire. release of glutamate, calcitonin gene-related peptide Specifically, PKC activates the NMDA receptor and (CGRP), brain-derived growth factor (BDGF), and sub- Na channels, causing further depolarization contrib- stance P from primary afferent terminals in the spinal uting to “wind up.” P38 MAPK phosphorylates en- cord and trigeminal nucleus that results in activity- zymes including phospholipase A2, which initiates dependent changes in dorsal horn spinal function. release of arachidonic acid and provides the substrate Second-order neurons receive input from both low for cyclooxygenase (COX) to synthesize prostaglandins and high threshold primary afferents. The input is (PGE). PGE acts presynaptically to enhance opening of received in a stimulus intensity-dependent manner so voltage-gated Ca channels and postsynaptically to block that an increase in discharge of WDR neurons results glycinergic inhibition at the interneuron level. The acti- in an increase in output frequency. Repetitive stimula- vation of afferent input and second-order neurons is tion of C fibers causes progressive and sustained partial regulated by local inhibitory interneurons containing depolarization of the cell making it more susceptible to inhibitory amino acids including GABA and glycine. future afferent input. Under normal conditions, the When high frequency afferent input occurs, inhibition WDR neuron may be activated by a natural stimulus is reduced leading to increased response of WDR neu- at a discrete location. After C fiber conditioning, a rons. The loss of inhibition via GABA or glycine input natural stimulus applied over a larger area now augments the response of WDR neurons leading to displays the ability to activate the same WDR neuron. facilitation of dorsal horn excitability (see Fig. 1.8). As previously mentioned, this exaggerated discharge 18e21 of WDR neurons evoked by repetitive stimulation of Neuropathic Pain primary afferent neurons was termed “wind-up” by Neuropathic pain, defined as damage to the peripheral Mendell and Wall. afferent nerve axon itself as opposed to other structures The result of central sensitization is an increased in the periphery around the primary afferent, causes receptive field of the second-order neuron known as an initial retrograde degeneration of the nerve axon secondary hyperpathia by which pain can be evoked in a process called Wallerian degeneration. This degen- by a nonnoxious stimulus in adjacent noninjured tis- eration leads to axon sprouting, whereby the axons will sue. Two mechanisms contribute to the increase in attempt to reconnect causing a neuroma formation. receptive field: This injury can cause peripheral and central sensitiza- i) As reviewed above, primary sensory afferent neu- tion just like injury in other areas, but it also causes a rons collateralize upon entering the spinal cord, reorganization in the central processing of pain which sending segments rostrally and caudally up to should be differentiated from central or peripheral several segments; and sensitization because it is specific to nerve cell damage. ii) The facilitation of central sensitization is greatly The reorganization of central processing that dependent on phosphorylation of the glutamate- occurs with neuropathic injury and pain is seen clini- activated n-methyl-D-aspartate (NMDA) receptor. cally with the phenomenon of tactile allodynia. This In physiological conditions, the NMDA channel re- anomaly whereby large, low threshold, sensory primary mains quiescent due to continuous blockage by afferents (A-b fibers) that are normally only activated magnesium (Mg). However, when the membrane causing a sensation of light touch can produce a CHAPTER 1 Basic Science of Pain 11 nociceptive signal giving the sensation of pain reflects input. As discussed earlier, glutamate is responsible several events in two main areas of pain transmission: for increased NMDA activation causing neuronal i) Injured afferent axons will develop spontaneous excitability. Astrocyte and microglia activation lead activity. This ectopic activity arises both at the site of to increased COX, NOS, and glutamate, which injury and from the DRG of the injured afferent contribute to a hyperexcitable state. neuron. Normally, glycine agonism and GABA antagonism ii) The dorsal horn, following injury, will also show serve to regulate the excitatory potential of A-b fibers reorganization through multiple mechanisms in the dorsal horn. Loss of that regulatory inhibition including spinal glutamate release, microglia and causes A-b fibers to produce an aggressive depolariza- astrocyte activity increase, loss of GABAergic and tion of the WDR neurons. Of note, after nerve injury, glycinergic control, and increased sympathetic there does not appear to be a loss of GABA/glycine

FIG. 1.8 Central Sensitization: Excitation of primary afferents produces glutamate and substance P release, which acts post synaptically on second-order neurons and excitation increases intracellular Calcium and activates a myriad of protein kinases. See text for further discussion. Sp, substance P; MAPK, mitogen- activated protein kinase; PLA2, phospholipase A2; PGE2, prostaglandin E2; Gs, stimulatory signaling protein; NA, noradrenalin, Caþþ, calcium; 5-HT, serotonin; Naþ, sodium; COX, cyclooxygenase; PKA, protein kinase A; PKC, protein kinase C; EP, prostaglandin receptor; CaV, voltage-gated calcium channel; NaV, voltage-gated sodium channel; NK-1, Neurokinin 1 receptor; AMPA receptor, a-amino-3-hydroxy-5methyl-4- isoxazolepropionic acid receptor; CP-AMPA, calcium permeable AMPA receptor; NMDA, N-methyl-D- aspartate receptor; WDR, wide dynamic range neuron. (Credit: Vanessa Tran.) 12 Pain Care Essentials and Innovations content release of receptors. However, it is now appreci- inflammatory cascade. Neutrophils are the predomi- ated that after nerve injury, the chloride (Cl) gradient is nant recruited cell type in an acute and early inflamma- altered due to the loss of cellular chloride transporters. tory response causing proinflammatory effects through As a result, at this point, GABA or glycine receptor the release of lipoxygenase products, prostaglandins, activation results in an increased permeability to Cl, NO, cytokines, and chemokines as well as possible anti- and in contrast to the normal state, Cl in the nerve nociceptive effects through the expression of opioids. injury animal will flow out carrying negative charge The inflammatory mediators released by innate im- and resulting in a paradoxical depolarization. Hence, mune cells modulate peripheral and central sensitiza- GABA and glycine release become an excitatory linkage tion that contributes to pain hypersensitivity. rather that an inhibitory linkage. Recent evidence suggests that adaptive immunity Additionally, the presence of a neuroma will cause may also play a role in chronic pain; however, this sympathetic postganglionic afferents to sprout and be role is less clear. The adaptive immune system is present at the neuroma site and in the DRG. Stimula- comprised of B and T cells. Although some recent tion of these sympathetics will drive ectopic activity at data suggest that B cells may play a role through anti- both the neuroma and at the DRG at that level. body production, the majority of recent literature has focused on the involvement of T cells in the produc- Immune and Inflammatory tion and resolution of chronic pain. Studies have 12,22e24 Mechanisms found that infiltration of T cells occurs days to weeks It has been well established that the process of neuro- postinjury, firstatthesiteofinjuryanddistalendof pathic and inflammatory pain not only involves the nerve, then within the DRG, and last within the neuronal pathways that transmit signals from peripheral dorsal horn of the spinal cord. T cells have been found tissue via the spinal cord to the brain but also immune to both suppress and promote pain via multiple cells that release and modulate a range of inflammatory mechanisms and variations in expression. T cells mediators. Interestingly, evidence suggests that proin- may indirectly modulate neuroinflammation via the flammatory cytokines are able to act directly on nocicep- antiinflammatory reflex. In response to norepineph- tors in the periphery as well as the dorsal horn of the rine, b2-adrenergic receptor-expressing T cells release spinal cord resulting in increased afferent input and sub- acetylcholine, which signals macrophages to switch sequent peripheral and central sensitization, respectively. from producing proinflammatory to antiinflammatory The immune system is comprised of two independent, products causing dampening of the immune system. but intricately connected systemsdthe innate immune T cells present in the periphery also express receptors system and the adaptive immune system. The innate im- for glutamate, substance P, and CGRP, which regulate mune system is continuously active and monitoring for T-cell adhesion, migration, and immunological foreign pathogens or injury to which it mounts a gener- phenotype that drives neuroinflammation. alized response. Alternatively, the adaptive immune sys- 12,25e28 tem is an acquired and specific immunity that retains Transition of Acute to Chronic Pain memory from prior exposures. Emerging evidence has Normally, pain resolves with termination of the acute implicated both of these systems in the development noxious stimulus and resolution of tissue injury. In of and maintenance of chronic pain. some cases, pain persists after the resolution of the acute Injury and inflammation give rise to the release of a inflammatory response. There is increased appreciation variety of products that can activate sensory compo- that development of persistent pain after an acute injury nents of the innate immune system, through recogni- or inflammation may reflect a mechanistic transition tion sites such as the Toll-like receptors (TLRs). While from an acute to a chronic pain state reflecting long- classically expressed on inflammatory cells (macro- term changes that sustain and amplify pain signaling. phages), it became appreciated that they were also pre- The mechanisms underlying this transition are sent on microglia and astrocytes in the neuraxis. complex and at present poorly understood. However, Further, these signaling receptors are present on DRG this process has been demonstrated in a variety of neurons and of course on macrophages that are widely clinical pain states including after tissue injury, nerve expressed in the DRG. Activated macrophages release damage, and inflammatory pain. Studies performed inflammatory mediators, most notably tumor necrosis using experimental pain models such as K/BxN factor-a (TNF-a), interleukin-1b (IL-1b), nerve growth serum-transfer arthritis and collagen antibody-induced factor (NGF), nitric oxide (NO), and prostanoids as arthritis (CAIA) have found that the animals experience well as complement proteins that initiate the innate tactile allodynia that persists long after the localized CHAPTER 1 Basic Science of Pain 13 swelling and inflammation has resolved. This process Recent studies have also identified Toll-like receptor appears to involve discrete pathophysiological changes 4 (TLR4) as a potential driver of the transition from that are mediated by a combination of localized inflam- acute to chronic pain. Using the K/BxN model of matory mediators that drive peripheral and central arthritis, researchers found that TLR4 knockout mice sensitization and neuroimmune mechanisms. showed a resolution in pain that corresponded with As mentioned earlier, following tissue injury, the resolution of inflammation. Further work has damaged cells release factors that attract mast cells, mac- demonstrated that administration of a TLR4 antagonist rophages, and neutrophils that release proinflammatory can prevent the development of persistent pain state in mediators and NGF. Proinflammatory molecules acti- wild-type mice suggesting that spinal TLR4 signaling vate primary afferent neurons, including A-b fibers plays a significant role in mediating the transition from and C fibers, that initiate a process resulting in acute to chronic pain. 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