Subtypes of nociceptive units in the rat temporomandibular joint

Response properties of nociceptors in the rat's temporomandibular joint (TMJ) were investigated using an in vitro TMJ-nerve preparation. Recordings were obtained from 33 nociceptive units that responded to mechanical, chemical, and/or thermal stimuli. According to both characteristics of nociceptors and afferent fibers, nociceptive units in the TMJ area were classified into the following four subtypes: Adelta-high-threshold mechanonociceptor (HTM) (12.1%), Adelta-polymodal nociceptor (POLY) (36.4%), C-HTM (12.1%), and C-POLY (39.4%). The mean mechanical threshold of the Adelta units was significantly lower than that of the C units. Bradykinin increased the discharge of Adelta- and C-POLY units. No significant differences of thermal thresholds between Adelta and C units were found. The percentage of Adelta units was 47.2% and of C units was 52.8%, respectively. In the TMJ area, POLY units were predominant (75.8%), suggesting that inflammatory reactions can easily evoke pain sensation.     

Leukotriene B4 induced decrease in mechanical and thermal thresholds of C-fiber mechanonociceptors in rat hairy skin

We have recently shown that leukotriene B4 (LTB4), a product of the 5-lipoxygenase pathway of arachidonic acid metabolism, sensitizes nociceptors to mechanical stimuli. The present study examined whether LTB4 also induces a thermal sensitization of cutaneous C-fiber high-threshold mechanonociceptors (C-HTMs). C-HTMs were characterized according to their responsiveness to noxious mechanical, thermal and chemical stimuli, including glacial acetic acid, bradykinin and capsaicin. C-HTMs were found to be either heat responsive (heat C-HTMs) or heat and chemically responsive (polymodal C-HTMs). Ninety-four percent of polymodal C-HTMs and 60% of C-HTMs were sensitized to thermal and mechanical stimuli by LTB4 (75 ng). All sensitized C-HTMs showed decreases in both thermal and mechanical thresholds. LTB4 lowered in both subclasses of C-HTMs average thermal threshold from 45 to 35 degrees C and produced an average decrease in the mechanical threshold of approximately 82-86%. For both heat and polymodal C-HTMs, the magnitude of LTB4-evoked decreases in thermal and mechanical thresholds was similar to that produced by 75 ng of PGE2. The possibility was discussed that LTB4 may contribute to the component of hyperalgesia that is resistant to non-steroidal anti-inflammatory agents.     

Evolution of nociception in vertebrates: comparative analysis of lower vertebrates

Nociception is an important sensory system of major fundamental and clinical relevance. The nociceptive system of higher vertebrates is well studied with a wealth of information about nociceptor properties, involvement of the central nervous system and the in vivo responses to a noxious experience are already characterised. However, relatively little is known about nociception in lower vertebrates and this review brings together a variety of studies to understand how this information can inform the evolution of nociception in vertebrates. It has been demonstrated that teleost fish possess nociceptors innervated by the trigeminal nerve and that these are physiologically similar to those found in higher vertebrates. Opioid receptors and endogenous opioids are found in the brain and spinal cord of the fishes and morphine blocks avoidance learning using electric shock as well as reducing nociceptive behavioural and physiological responses to noxious stimulation. Comparative analysis of the fishes and higher vertebrates show that fish possess less C fibres than higher vertebrates. The electrophysiological properties of fish nociceptors are almost identical to those found in higher vertebrates suggesting the evolution of these properties occurred before the emergence of the fish groups.     

The Classification and Properties of Nociceptive Afferent Units from the Skin of the Anaesthetized Pig

The afferent properties of nerve fibres innervating the hairy skin of the pig hind limb were investigated by recording from 142 single units from the saphenous nerve. Identified single units were isolated using maximal electrical stimulation of the nerve trunk. Afferent units were classified on the basis of their responses to a range of stimuli, both thermal (heating to 60°C and cooling to 10°C) and mechanical (air jet, von Frey type filaments with forces of 0.1–250 mN, and strong pressure with a blunt needle). A-fibre units (conduction velocity 6.3–64 m/s, n= 60) fell into categories that have been described in hairy skin in other mammalian species. Most were mechanoreceptors, although seven typical A-fibre mechanical nociceptors with large, multipoint fields were also isolated. No cutaneous receptive field could be found for 15% of A-fibre units. Out of 62 C-fibre units (conduction velocity 0.49–2 m/s) 40% had no cutaneous field for pressure, heat or cold. Of the C-fibre units with cutaneous fields, 42% were polymodal nociceptors, 38% were mechanoreceptors with a variety of properties, including some excited by noxious heat, and 19% were heat-only nociceptors. C-polymodal nociceptors had large receptive fields up to 12.5 mm across and did not sensitize following strong heating. Twenty units conducted at 2–6.3 m/s, between the main C- and A-fibre bands, and were varied in their responses. Some had properties identical to C-fibre mechanoreceptors whilst four were sensitive cold thermoreceptors and one was a polymodal nociceptor. Two units were mechanical nociceptors with small receptive fields. The innervation of pig skin thus has some features like that of primates, such as the presence of C-heat nociceptors and the large receptive fields of C-polymodal nociceptors. However, other features were like non-primates (e.g. the minimal heat-sensitivity of A-mechanical nociceptors) or were unique (the heat-sensitive mechanoreceptors and the lack of heat sensitization in C-polymodal nociceptors from hairy skin).     

Persistent nociceptor hyperactivity as a painful evolutionary adaptation

Chronic pain caused by injury or disease of the nervous system (neuropathic pain) has been linked to persistent electrical hyperactivity of the sensory neurons (nociceptors) specialized to detect damaging stimuli and/or inflammation. This pain and hyperactivity are considered maladaptive because both can persist long after injured tissues have healed and inflammation has resolved. While the assumption of maladaptiveness is appropriate in many diseases, accumulating evidence from diverse species, including humans, challenges the assumption that neuropathic pain and persistent nociceptor hyperactivity are always maladaptive. We review studies indicating that persistent nociceptor hyperactivity has undergone evolutionary selection in widespread, albeit selected, animal groups as a physiological response that can increase survival long after bodily injury, using both highly conserved and divergent underlying mechanisms.     

Caenorhabditis elegans nicotinic acetylcholine receptors are required for nociception

Polymodal nociceptors sense and integrate information on injurious mechanical, thermal, and chemical stimuli. Chemical signals either activate nociceptors or modulate their responses to other stimuli. One chemical known to activate or modulate responses of nociceptors is acetylcholine (ACh). Across evolution nociceptors express subunits of the nicotinic acetylcholine receptor (nAChR) family, a family of ACh-gated ion channels. The roles of ACh and nAChRs in nociceptor function are, however, poorly understood. Caenorhabditis elegans polymodal nociceptors, PVD, express nAChR subunits on their sensory arbor. Here we show that mutations reducing ACh synthesis and mutations in nAChR subunits lead to defects in PVD function and morphology. A likely cause for these defects is a reduction in cytosolic calcium measured in ACh and nAChR mutants. Indeed, overexpression of a calcium pump in PVD mimics defects in PVD function and morphology found in nAChR mutants. Our results demonstrate, for the first time, a central role for nAChRs and ACh in nociceptor function and suggest that calcium permeating via nAChRs facilitates activity of several signaling pathways within this neuron.     

Assessment of mechanical and thermal thresholds of human C nociceptors during increases in skin sympathetic nerve activity.

OBJECTIVES: The objective of the present study is to investigate the relationship between C-fiber nociceptor sensitivity and skin sympathetic nerve activity during mental arithmetic. METHODS: Single afferent C-fibers were identified simultaneously with spontaneous postganglionic sympathetic discharges and recorded from the peroneal nerve using microneurography in 23 normal subjects. Mechanical and heat thresholds were measured by 'marking' the nociceptor with suprathreshold stimuli, causing increased latency after a subsequent threshold stimulus at rest and during mental arithmetic. Skin sympathetic nerve activity was estimated by counting the number of bursts per minute. RESULTS: Thirty-two single C-fibers were identified. Eleven had polymodal receptors (mechanical and heat sensitive), eight were only sensitive to mechanical stimuli, two were only sensitive to heat stimuli, and 11 were insensitive to both. C-fibers were selected when the ratio of skin sympathetic nerve activity during mental arithmetic to that at rest was over 1.00. In 19 selected mechanical sensitive units, average mechanical threshold was 4.86 at rest and 4.84 during mental arithmetic. In 6 selected heat sensitive units, average heat threshold was 45.0 degrees C at rest and 43.4 degrees C during mental arithmetic. However, differences were not statistically significant. CONCLUSIONS: Physiological sympathetic stimulation did not affect afferent C-fiber nociceptor sensitivity to mechanical and heat stimuli in healthy subjects.     

Hyperexcitable polymodal and insensitive nociceptors in painful human neuropathy

Six patients with chronic pain, mechanical and thermal hyperalgesia/allodynia, and cutaneous vasodilatation starting distally in their extremities, were evaluated using clinical and neurophysiological methods and microneurography. Evidence of small-fiber polyneuropathy was documented in all, but the etiology remained cryptogenic in several. Different forms of hyperexcitability were detected by microneurography in both common polymodal and mechanically insensitive C nociceptors, which explain all the somatosensory abnormalities. Signs of hyperexcitability included reduced receptor threshold (accounting for mechanical and heat allodynias), spontaneous C nociceptor discharge (explaining spontaneous "burning" pain and antidromic vasodilatation), and multiplied nociceptor responses to stimulation (accounting for hyperalgesia). The clinical and electrophysiological profiles of these patients resemble the experimental syndrome evoked by application of capsaicin to the skin. This similarity, and the striking heat dependence of the spontaneous pain, suggest that a common feature may be altered expression or modulation of vanilloid 1 receptor, provoking abnormal nociceptor discharges.     

Nerve growth factor‐evoked nociceptor sensitization in pig skin in vivo

Peripheral sensitization of skin nociceptors by nerve growth factor (NGF) was explored in pig skin in vivo. As an objective output measure, the area of axon‐reflex‐mediated erythema was assessed upon mechanical, thermal, chemical, and electrical stimuli delivered at 1, 3, and 7 days after i.d. injection of 1 μg NGF into the pig's back skin (n = 8). Pretreatment with NGF provoked a sensitization to mechanical (600 mN), thermal (10 sec 49°C) and chemical (15 μl, pH 3) stimuli that lasted for 7 days. No sensitization, however, was found in response to weak mechanical (100 mN), weak thermal (10 sec 45°C), or electrical stimuli. Irrespective of the skin pretreatment (NGF or PBS vehicle control), the area of electrically induced erythema decreased upon repetition (days 1–7) by 70% (P < 0.05). Sensitization of sensory endings by NGF upon mechanical, heat, and chemical stimuli suggests recruitment of sensory transducer molecules [e.g., TRPV1, acid‐sensing ion channels (ASICs)]. In contrast, the gradual decrease in electrically induced erythema over 7 days might be attributable to axonal desensitization and possibly activity‐dependent down‐regulation of sodium channels. Thus, long‐lasting sensitization processes of nociceptor endings or axonal sodium channel desensitization mechanisms can be explored in the pig as a translational experimental animal model. © 2010 Wiley‐Liss, Inc.     

Spinal termination of functionally identified primary afferent neurons with slowly conducting myelinated fibers.

Single primary afferent myelinated fibers from cutaneous receptors of cat and monkey were functionally identified by recording from the spinal cord with micropipettes filled with horseradish peroxidase (HRP). Relatively slowly conducting fibers (less than 40 m/sec) from high threshold mechanoreceptors (mechanical nociceptors) and two types of low threshold mechanoreceptor (D-hair and field) were selected for staining. Iontophoresis of the HRP and subsequent histochemical reaction stained the axons recorded from and their collaterals, including terminations, for several millimeters. The termination patterns in the two species proved essentially identical. Ipsilaterally, the mechanical nociceptor fibers terminated principally in the dorsal horn's marginal zone and in the ventral parts of the nucleus proprius (lamina V in the cat). Some of these nociceptors also had terminals in the midline just dorsal to the central canal, contralaterally in the marginal zone, and at the base of the opposite nucleus proprius. In contrast, the D-hair primary afferent axons terminated in the dorsal part of the nucleus proprius overlapping into the innermost portion of the substantia gelatinosa. The field receptor fibers terminated predominantly in the middle part of the nucleus proprius. These results suggest that there is a highly specialized central projection of primary afferent endings which is related to sensory function and not to fiber diameter. The marginal zone and most dorsal parts of the substantia gelatinosa receive direct projections from cutaneous nociceptors but do not have direct input from cutaneous receptors transmitting activity initiated by innocuous stimulation.     

Postherpetic neuralgia: Topical lidocaine is effective in nociceptor–deprived skin

Objectives Topical lidocaine is effective in postherpetic neuralgia (PHN). The aim of the present investigation was to classify patients according to their predominant peripheral nociceptor function and to compare these data with the results of a controlled study using dermal lidocaine patch.Methods Within the skin area of maximal pain QST (thermotest) and QCART (histamine iontophoresis and laser Doppler flowmetry) were performed prospectively in 18 PHN patients. A controlled study using cutaneous lidocaine (lidocaine 5% patch, IBSA) followed.Results Six patients (group I, sensitised nociceptors) had no sensory loss. Heat pain thresholds were equal or lower than on the contralateral side. Histamine–induced flare and axon reflex vasodilatation were not different on both sides. Histamine evoked pain increased. In 12 patients (group II, nociceptor impairment) heat pain thresholds were higher than contralateral. Histamine–induced flare was impaired or abolished. Histamine did not induce any sensation. Lidocaine was efficacious in the entire group of patients. Subgroup analysis revealed that patients with impairment of nociceptor function had significantly greater pain reduction under lidocaine vs placebo. Patients with preserved and sensitised nociceptors demonstrated no significant pain relief.Conclusions PHN patients differ concerning their cutaneous nociceptor function: In the group I pain is caused by pathologically sensitised nociceptors. In subset II there is a loss of function of cutaneous C–nociceptors within the allodynic skin. Patients responded well to topical lidocaine even if the skin was completely deprived of nociceptors. Different underlying mechanims of lidocaine action in nociceptor–deprived skin are discussed.     

Response properties of hypogastric afferent fibers supplying the uterus in the cat

The present study was undertaken to examine the sensory function of uterine afferent fibers in cats at unknown stages of the estrous cycle. Single unit activity was recorded from strands of the hypogastric nerve of the anesthetized cat. Once a unit was found, the conduction velocity was determined and the mechanical receptive field localized on the uterus. The response properties of the unit to mechanical stimuli applied to the receptive field and to chemical stimuli applied by intra-arterial injection of algesic chemicals (bradykinin, KCl and capsaicin) into the uterine artery were studied. Single unit activity from a total of 52 units was examined in this study. Based on the conduction velocities, about2/3 of these fibers were found to be unmyelinated C fibers and the remaining1/3 were thinly myelinated Aδ fibers. The receptive fields of most of these fibers were located at different parts of the uterine horn and body while a few were at the uterine cervix. Mechanical thresholds, as determined by von Frey filaments applied to the external surface of the uterus, varied more than 150-fold among mechanically sensitive units, ranging from extremely low to high thresholds. In addition, most of these afferents were activated by intra-arterially injected algesic chemicals, often by more than one chemical. The data in the present study suggest that a large portion of the cat uterus is innervated by the hypogastric nerve and that these afferents originate from sensory receptors that have potentially a wide range of functions. Their potential functions as low threshold mechanoreceptors and nociceptors are discussed.     

Tweety-Homolog 1 Facilitates Pain via Enhancement of Nociceptor Excitability and Spinal Synaptic Transmission

Tweety-homolog 1 (Ttyh1) is expressed in neural tissue and has been implicated in the generation of several brain diseases. However, its functional significance in pain processing is not understood. By disrupting the gene encoding Ttyh1, we found a loss of Ttyh1 in nociceptors and their central terminals in Ttyh1-deficient mice, along with a reduction in nociceptor excitability and synaptic transmission at identified synapses between nociceptors and spinal neurons projecting to the periaqueductal grey (PAG) in the basal state. More importantly, the peripheral inflammation-evoked nociceptor hyperexcitability and spinal synaptic potentiation recorded in spinal-PAG projection neurons were compromised in Ttyh1-deficient mice. Analysis of the paired-pulse ratio and miniature excitatory postsynaptic currents indicated a role of presynaptic Ttyh1 from spinal nociceptor terminals in the regulation of neurotransmitter release. Interfering with Ttyh1 specifically in nociceptors produces a comparable pain relief. Thus, in this study we demonstrated that Ttyh1 is a critical determinant of acute nociception and pain sensitization caused by peripheral inflammation.     

A new tr(i)p to sense pain: TRPA1 channel as a target for novel analgesics

Pain sensation occurs at multiple levels of the nervous system, involving a myriad of molecules and signaling pathways that contribute to the detection of noxious stimuli, and the modulation, initiation and propagation of electrical activity in nociceptors, a subset of primary sensory neurons. The fact that several types of ion channels or their splice variants are highly expressed in nociceptors and are critical for pain transduction has prompted scientists to examine their physiological roles in order to better understand the neuromolecular mechanisms of the pain pathway. Recent reports have demonstrated that TRPA1, a member of the mammalian transient receptor potential cation channel family, acts as a key chemical nocisensor in response to diverse chemical stimuli. The TRPA1 channel represents a new target for novel analgesics to specifically eliminate pain sensation of various stimuli.     

Endogenous nerve growth factor regulates the sensitivity of nociceptors in the adult rat

Nerve growth factor (NGF) has a well characterized role in the development of the nervous system and there is evidence that it interacts with nociceptive primary afferent fibres. Here we applied a synthetic tyrosine kinase A IgG (trkA-IgG) fusion molecule for 10–12 days to the innervation territory of the purely cutaneous saphenous nerve in order to bind, and thereby neutralize endogenous NGF in adult rats. Using neurophysiological analysis of 152 nociceptors we now show that sequestration of NGF results in specific changes of their receptive field properties. The percentage of nociceptors responding to heat dropped significantly from a normal 57% to 32%. This was accompanied by a rightward shift and a reduced slope of the stimulus response function relating the intracutaneous temperature to the neural response. The number of nociceptors responding to application of bradykinin was also significantly reduced from a normal of 28% to 8%. In contrast, the threshold for mechanical stimuli and the response to suprathreshold stimuli remained unaltered, as did the percentage of nociceptors responding to noxious cold. The reduced sensitivity of primary afferent nociceptors was accompanied by a reduction in the innervation density of the epidermis by 44% as assessed with quantitative immunocytochemical analysis of the panaxonal marker PGP 9.5. This demonstrates that endogenous NGF in the adult specifically modulates the terminal arborization of unmyelinated fibres and the sensitivity of primary afferent nociceptors to thermal and chemical stimuli in vivo.     

A novel mechanosensitive channel identified in sensory neurons

Mechanosensitive (MS) channels are ion channels gated by different types of mechanical stimuli. MS channels in sensory neurons are thought to be molecular transducers for somatic sensations such as touch, pressure, proprioception and pain. Previously, we reported that two types of MS channels are present in sensory neurons. These channels are termed low threshold (LT) and high threshold (HT) MS channels based on their pressure threshold for activation. Here, we report another type of MS channel present in sensory neurons. The channel is activated by low pressure applied to a patch (threshold approximately 20 mmHg, similar to that in the LT channel). However, because this channel has a smaller single-channel conductance than that of the LT channel, the newly classified MS channel is now called a low threshold small conductance (LTSC) channel. Unlike the LT channel, which has outwardly rectifying currents, the current-voltage relationship of the LTSC is linear. The LTSC was permeable to monovalent cations and Ca2+, and reversibly blocked by gadolinium, a blocker of MS channels. Unlike the LT channel, the LTSC was sensitized by prostaglandin E2, an inflammatory mediator that is known to sensitize nociceptors to mechanical stimuli. LTSC channels were found mostly in small cultured sensory neurons. Thus, these results suggest that the LTSC is a distinct type of MS channel that is different from the LT and HT channels in sensory neurons, and that LTSCs might play a role in mediating somatosensations, including pain.     

Opioid-Induced Hyperalgesic Priming in Single Nociceptors

Clinical µ-opioid receptor (MOR) agonists produce hyperalgesic priming, a form of maladaptive nociceptor neuroplasticity, resulting in pain chronification. We have established an in vitro model of opioid-induced hyperalgesic priming (OIHP), in male rats, to identify nociceptor populations involved and its maintenance mechanisms. OIHP was induced in vivo by systemic administration of fentanyl and confirmed by prolongation of prostaglandin E₂ (PGE₂) hyperalgesia. Intrathecal cordycepin, which reverses Type I priming, or the combination of Src and mitogen-activated protein kinase (MAPK) inhibitors, which reverses Type II priming, both partially attenuated OIHP. Parallel in vitro experiments were performed on small-diameter (<30 µm) dorsal root ganglion (DRG) neurons, cultured from fentanyl-primed rats, and rats with OIHP treated with agents that reverse Type I or Type II priming. Enhancement of the sensitizing effect of a low concentration of PGE₂ (10 nm), another characteristic feature of priming, measured as reduction in action potential (AP) rheobase, was found in weakly isolectin B4 (IB4)-positive and IB4-negative (IB4–) neurons. In strongly IB4-positive (IB4+) neurons, only the response to a higher concentration of PGE₂ (100 nm) was enhanced. The sensitizing effect of 10 nm PGE₂ was attenuated in weakly IB4+ and IB4– neurons cultured from rats whose OIHP was reversed in vivo. Thus, in vivo administration of fentanyl induces neuroplasticity in weakly IB4+ and IB4– nociceptors that persists in vitro and has properties of Type I and Type II priming. The mechanism underlying the enhanced sensitizing effect of 100 nm PGE₂ in strongly IB4+ nociceptors, not attenuated by inhibitors of Type I and Type II priming, remains to be elucidated.SIGNIFICANCE STATEMENT Commonly used clinical opioid analgesics, such as fentanyl and morphine, can produce hyperalgesia and chronification of pain. To uncover the nociceptor population mediating opioid-induced hyperalgesic priming (OIHP), a model of pain chronification, and elucidate its underlying mechanism, at the cellular level, we established an in vitro model of OIHP. In dorsal root ganglion (DRG) neurons cultured from rats primed with fentanyl, robust nociceptor population-specific changes in sensitization by prostaglandin E₂ (PGE₂) were observed, when compared with nociceptors from opioid naive rats. In DRG neurons cultured from rats with OIHP, enhanced PGE₂-induced sensitization was observed in vitro, with differences identified in non-peptidergic [strongly isolectin B4 (IB4)-positive] and peptidergic [weakly IB4-positive (IB4+) and IB4-negative (IB4–)] nociceptors.     

Configuration-sensitive visual responses in the superior colliculus of the house mouse (Mus musculus domesticus)

In order to compare visual pattern discrimination by tectal neurons in distantly related vertebrate groups, collicular cells of mice were examined for their responses to each of three simple configurational stimuli commonly used in studies of amphibians. The stimuli consisted of a large square, a horizontal bar and a vertical bar moved at various velocities. Of the recorded units (n = 51), 30-50% significantly preferred the square to the other stimuli at medium (10 degrees/s) and high (67 degrees/s) velocities. Approximately 10% preferred the horizontal bar at these velocities. A significant discrimination between the horizontal and the vertical bar was found in 39% of the units at a velocity of 10 degrees/s, and in 61% at a velocity of 67 degrees/s. These response types are very similar to those found in amphibians; therefore, it is concluded that tectal configurational sensitivity may be a plesiomorphic tetrapod character resulting from basic properties of tectal neuronal circuitry.     

Synaptic complexes formed by functionally defined primary afferent units with fine myelinated fibers

The individual fine myelinated fibers of cutaneous mechanical nociceptors and "D-hair" receptors were identified by electrophysiological recording with micropipette electrodes in cats and monkeys. Their intraspinal terminations were labeled by iontophoresing horseradish peroxidase intracellularly and subsequent diaminobenzidine histochemistry. These terminations were examined with light and electron microscopy to determine the nature and organization of their synaptic contacts. Myelinated fibers of the mechanical nociceptors became unmyelinated before exhibiting many enlargements that made multiple synaptic contacts in the marginal zone (lamina I) and lamina V. Pre- or postsynaptic contacts were found only on enlargements. In the marginal zone of the cat, enlargements made simple axodendritic contacts or were scalloped, central terminals in glomeruli. In glomeruli, myelinated mechanical nociceptor enlargements were presynaptic to several dendritic appendages and postsynaptic to two different types of profiles. One type was interpreted as a presynaptic axon terminal, the other as a presynaptic, vesicle-containing, dendritic appendage. In lamina V of the cat the nociceptor synaptic complexes were similar, but simpler, and only axonal profiles were found to be presynaptic to them. In the monkey marginal zone and deep nucleus proprius, myelinated nociceptor terminations formed the central element of glomeruli, which consisted of postsynaptic dendritic appendages and presynaptic axon terminals. D-hair axons terminated in large numbers of enlargements in the nucleus proprius (laminae III and IV) and inner substantia gelatinosa (lamina IIi). Their large rounded enlargements formed the central terminals in glomeruli and were presynaptic to both ordinary and vesicle-containing dendritic appendages; the presynaptic dendritic profiles also often contacted each other. Profiles interpreted as axonal in origin were the only terminals presynaptic to the primary ending within the D-hair glomeruli. The results suggest that transfer of primary afferent information occurs only at enlargements of the primary fiber and that each primary fiber enters into more than one kind of synaptic arrangement. They also point out that synaptic glomeruli are common to functionally different types of primary afferent fibers and that the internal organization of glomeruli varies with the kind of primary fiber and the locus of the complex.     

Ultrastructural features of functionally identified primary afferent neurons with C (unmyelinated) fibers of the guinea pig: classification of dorsal root ganglion cell type with reference to sensory modality

Intracellular labeling of neurons permitted a direct correlation of neuronal profiles with sensory modality of cutaneous receptors. In the guinea pig, 47 neurons in the dorsal root ganglion (displaying C-fiber conduction velocities) were labeled with horseradish peroxidase (HRP) by iontophoresis after determining the sensory modality. Receptive field were explored with systematic "natural" stimuli. Cell areas of all C-fiber units were measured by tracing the cellular contour in light microscopy. The mean cellular diameter calculated from cell areas was 21.8 microns in the second cervical ganglion of the guinea pig. Mean cell diameter for high-threshold mechanoreceptors was 20.9 microns, 24.6 microns for polymodal nociceptors, and 25.7 microns for mechanical-cold nociceptors. Electron microscopic observations showed that all labeled neurons of C-fiber units had profiles of small, dark type-B neurons. Neurons representative of each sensory modality exhibited different cell features, each belonging to a distinct subtype of small B neurons. High-threshold mechanoreceptor and mechanical-cold nociceptor displayed a peripheral lamellar arrangement of cisternae of endoplasmic reticulum (ER), corresponding to the B1 subtype. Polymodal nociceptor units were characteristically of the B2 subtype, in which stacks of long and short cisternae of ER were distributed randomly throughout the cytoplasm, and the arrangement of Golgi bodies varied among these cells. Cooling receptors displayed poorly developed, flattened cisternae of ER and numerous vesicles, typical of the B3 subtype. These results imply that all C-fiber cells belong to the small B-type cell category and that the ultrastructural features of the neuron in the dorsal root ganglion may reflect the sensory modality of the receptive field.