Electrophysiologic studies of penile mechanoreceptors in the rat

The neural activity of afferent fibers in the dorsal nerve of the penis was studied in 16 barbiturate-anesthetized rats. Sixty-one single units responsive to mechanical stimulation were isolated. Each receptor was tested for critical displacement, force, and velocity thresholds as well as its sinusoidal stimulation frequency-following characteristics and fiber conduction velocity. Based on adaptation responses to sustained mechanical stimuli, 36 of the 61 fibers were classified as rapidly adapting (RA) units and 25 as slowly adapting (SA) units, whereas previous workers did not find any slowly adapting receptors. We believe our ability to find SA units is due to mechanical and thermal stabilization of the penis and to our use of a wellcontrolled mechanostimulator.     

Responses of single units in the inferior olive nucleus to stimulation of the splanchnic afferents in the cat

The activity of olivary neurons in the caudal parts of the medial accessory olive (MAO) and dorsal accessory olive (DAO) was recorded with extracellular microelectrodes in anesthetized cats. The responses were obtained by electrical stimulation of the contralateral splanchnic nerve and mechanical stimulations of gastro-intestinal and peritoneal receptors. The stimulated mechanoreceptors were slowly adapting muscular receptors of the stomach (connected with C fibers) and slowly adapting peritoneal movement receptors (connected with A gamma delta or B fibers). Splanchno-visceral, splanchno-somatic and splanchno-cortical convergences were observed, as in the cerebellum (lobule V and VI). So, the inferior olive must be considered as an integrating structure not only for central and somatic messages, but also for visceral ones.     

Suppression of C-fibre discharges upon repeated heat stimulation may explain characteristics of concomitant pain sensations

Nociceptors with unmyelinated axons were recorded from the superficial radial nerves of 7 volunteers. A sequence of uniform radiant heat stimuli of 18 s duration, starting from an individually adjusted adapting temperature were used to raise the skin surface temperature by 6°C to a painful level (41–43 °C). These stimuli followed each other at 3 different interstimulus intervals of 35 s, 70 s and 105 s, occurring in a random order. The subjects were asked to track the time course of the stimulus-evoked sensation by manipulating the length of a light bar. Adaptation and stimulus temperatures were chosen to induce sensations of heat and/or pain. All nociceptors studied responded to these stimuli with a phasic response of 3–5 s duration, often followed by a low frequency tonic discharge, lasting as long as the stimulus. No discharges were seen in interstimulus periods. Discharge rates during the phasic responses were linearly related to interval duration, whereas tonic discharges were not influenced by the preceding interval. In parallel readings of pain responses were lower up to the 10th second of the stimulus after short rather than after long intervals. These results indicate that the suppression of C-fibre nociceptor discharges during repetitive stimulation may explain concomitant reductions in the magnitude of human pain sensations.     

Properties of functionally identified nociceptive and nonnociceptive facial primary afferents and presynaptic excitability changes induced in their brain stem endings by raphe and orofacial stimuli in cats

The activity of 221 single primary afferent units was recorded extracellularly in the trigeminal (V) ganglion of chloralose-anaesthetized cats to examine the receptive field properties of nonnociceptive and nociceptive cutaneous afferents and the effect of conditioning stimulation of the raphe system and orofacial afferents on the antidromic excitability of their brain stem endings in V subnucleus caudalis. In addition to slowly adapting and rapidly adapting low-threshold mechanosensitive afferents, we functionally identified three classes of cutaneous nociceptive afferents: these included A-delta high-threshold mechanoreceptive afferents (A-delta HTMs), C-fiber high-threshold mechanoreceptive afferents (C-HTMs), and C-polymodal nociceptive afferents (CPNs). Most of the CPNs could be activated by light tactile stimuli as well as by heavy pressure and pinch and noxious radiant heat applied to their mechanoreceptive field which usually involved a localized spot (approximately 1 mm in diameter) of skin. In contrast, the C-HTMs and A-delta HTMs could not be activated by radiant heat stimuli although some did show sensitization which was also a feature of the CPNs; they did respond to noxious mechanical stimulation of a localized area of skin. We noted that orofacial conditioning stimulation could produce an increase in antidromic excitability which was considered a reflection of primary afferent depolarization (PAD) in both nociceptive and nonnociceptive afferents innervating the cat's facial skin; nonnoxious mechanical stimuli and electrical stimuli were particularly effective in the low-threshold mechanosensitive afferents and noxious mechanical and high-intensity electrical stimuli were especially effective in the cutaneous nociceptive afferents. Raphe conditioning stimulation also was very effective in inducing PAD in these nociceptive afferents; however, the raphe conditioning effects were not limited to these nociceptive afferents since PAD was also frequently demonstrated in the low-threshold mechanosensitive afferents.     

Structure and function relationship in the abdominal stretch receptor organs of the crayfish

The structure/function relationship in the rapidly and slowly adapting stretch receptor organs of the crayfish (Astacus leptodactylus) was investigated using confocal microscopy and neuronal modeling methods. Both receptor muscles were single muscle fibers with structural properties closely related to the function of the receptors. Dendrites of the rapidly adapting neuron terminated in a common pile of nerve endings going in all directions. Dendrites of the slowly adapting neuron terminated in a characteristic T shape in multiple regions of the receptor muscle. The slowly adapting main dendrite, which was on average 2.1 times longer and 21% thinner than the rapidly adapting main dendrite, induced larger voltage attenuation. The somal surface area of the slowly adapting neuron was on average 51% larger than that of the rapidly adapting neuron. Variation in the neuronal geometry was greatest among the slowly adapting neurons. A computational model of a neuron pair demonstrated that the rapidly and the slowly adapting neurons attenuated the dendritic receptor potential like low-pass filters with cut-off frequencies at 100 and 20 Hz, respectively. Recurrent dendrites were observed mostly in the slowly adapting neurons. Voltage signals were calculated to be propagated 23% faster in the rapidly adapting axon, which is 51% thicker than the slowly adapting axon. The present findings support the idea that the morphology of the rapidly and the slowly adapting neurons evolved to optimally sense the dynamic and the static features of the mechanical stimulus, respectively.     

Functional properties of mechanoreceptors in glabrous skin of the raccoon's forepaw

Utilizing a mechanical stimulator which permits precise control over the velocity and extent of skin displacement, response properties of 76 single myelinated median nerve fibers innervating mechanoreceptors in raccoon glabrous forepaw skin were examined. All units fell into one of two categories, rapidly adapting (34 units), responding only during skin movement, and slowly adapting (42 units), responding during both moving and static phases of skin displacement. Both types had single receptive fields, 85% of which were less than 1 mm² in diameter, and low thresholds to mechanical displacement. Frequency of discharge during the moving phase of stimulation was a power function of velocity for both types, but discharge patterns varied. Power function slope constants were greater for the rapidly adapting units, but the slowly adapting units exhibited a higher maximum discharge rate. Results are discussed with particular reference to the possible roles of these receptors in tactile exploratory behavior.     

Frequency discrimination in the sense of flutter: psychophysical measurements correlated with postcentral events in behaving monkeys

The capacities of humans and monkeys to discriminate between the frequencies of mechanical sinusoids delivered to the glabrous skin of the hand have been measured in psychophysical experiments. The 2 primates have similar capacities; they make discriminations with Weber fractions that change little over the frequency range from 20 to 200 Hz. The discriminatory capacities are similar whether stimuli are received passively or acquired actively. Combined experiments have been made in monkeys in which the electrical signs of the activity of quickly adapting (QA) and slowly adapting (SA) neurons of postcentral areas 3b and 1 were recorded, both in the working state as the animal made discriminations and in the irrelevant state in which the stimuli did not guide behavior. The neuronal responses were analyzed in terms of discharge rates, periodicities in the neuronal discharges, and harmonic contents. It was shown that discriminatory capacity depends upon the period lengths in the sets of periodically entrained activity evoked by stimuli readily discriminated, and not upon the small differences in rates of discharge evoked by those stimuli. The periodicities were shown by harmonic analysis to be sharply limited to stimulus frequencies. Low-frequency stimuli evoke periodicities at the second and third harmonics in some neurons, in addition to strongly periodic signals at the fundamental frequency of the stimuli. Their presence does not appear to interfere with frequency discrimination. Neuronal responses recorded in the stimulus-irrelevant state were not distinguishable from those recorded as monkeys made discriminations. The responses of SA neurons, recorded under similar conditions, resembled those of QA neurons in almost every feature, but reasons are given for concluding that the SA system plays no role in frequency discrimination in the sense of flutter.     

Postnatal loss of Merkel cells, but not of slowly adapting mechanoreceptors in mice lacking the neurotrophin receptor p75

Merkel cells are specialized epidermal cells which are abundantly found in touch-sensitive areas and which are innervated by slowly adapting mechanosensitive afferent fibres with large myelinated (Abeta) axons. The role of Merkel cells in mechanosensation, their developmental regulation and their influence on sensory neuron function are, however, incompletely understood. Here, we used mice lacking the neurotrophin receptor p75 which is expressed on Merkel cells to investigate their postnatal development and that of their innervating sensory neurons. Using morphological studies we now show that Merkel cells develop normally in both hairy and glabrous skin in these animals until 2 weeks old, but are progressively lost thereafter and have almost completely disappeared 2 months after birth. Using standard extracellular electrophysiological recording techniques we find that despite the profound loss of Merkel cells there is no corresponding reduction in the number of myelinated slowly adapting afferent fibres. Moreover, the mean mechanical threshold of these neurons and their average stimulus response function to suprathreshold mechanical stimuli does not change during the time period when more than 99% of Merkel cells are lost. We conclude that Merkel cells require p75 during the late postnatal development. However, neither the survival nor the mechanical sensitivity of slowly adapting mechanoreceptive Abeta-fibres depends on the presence of Merkel cells.     

Identification of afferent C units in intact human skin nerves

Single unit potentials were recorded with microelectrodes from intact human skin nerves, and the unitary responses to electrical and natural skin stimuli were studied. The unitary discharges were derived from afferent C fibres since the impulses were conducted at C velocity and persisted after preferential blocking of activity in myelinated fibres by nerve compression, whereas they were abolished before the A fibre discharges by lidocaine.The afferent C units adapted slowly to mechanical stimuli and some of them exhibited afterdischarges following withdrawal of the stimuli. The excitability decreased as a consequence of repeated mechanical stimulation. The most vigorous responses were elicited by various intense stimuli such as pinpricks or heat stimulation. Although no definite classification was made, the responses of the human C units to different stimuli reminded of responses in ‘polymodal’ C receptors identified in the cat and the monkey.Painful stimuli elicited the most intense discharges in the units, but considerable activity was also elicited by non-painful stimuli. Some evidence suggested that single or a few repeated impulses in an afferent C unit need not reach consciousness.     

Effects of gadolinium and tetrodotoxin on the response of slowly adapting type I mechanoreceptors to mechanical stimulation in frog dorsal skin

To elucidate the excitatory mechanism of mechanoreceptors innervating the frog skin, we examined the effects of gadolinium (Gd3+) and tetrodotoxin (TTX) on the response of single-unit activity of slowly adapting type I mechanoreceptors to mechanical stimulation topically applied to the receptive field (RF). Recordings were made from 46 fibers responding to mechanical stimulation with von Frey hairs, which caused an irregular firing pattern with slow adaptation. Application of a mechanically gated channel blocker, Gd3+ (30 microM), and a Na+ channel blocker, TTX (3 microM), caused the suppression of discharge rates, which was characterized by the conversion of a slowly adapting to a rapidly adapting discharge pattern. The administration of a high-voltage-activated (HVA) Ca2+ channel blocker, Cd2+ (100 microm), inhibited the unit discharge and caused the conversion of a slowly adapting to a rapidly adapting discharge pattern. Tonic discharges evoked by anodal electrical stimulation were inhibited by the application of Gd3+ or TTX. Electron microscopic examination showed that the cytoplasm of Merkel cells seen in the RF contained numerous Merkel granules. These results suggest that the excitatory mechanism of frog cutaneous mechanoreceptors may be mediated by the activation of Gd(3+)-sensitive stretch-activated channels in the Merkel cell-neurite complex, which are related to the Na+ influx via voltage-gated Na+ channels and/or the Ca2+ influx through HVA Ca2+ channels.     

Thermosensitivity and its possible fine-structural basis in mechanoreceptors in the beak skin of geese

An electrophysiological study on single afferent nerve fibers in the ophthalmic nerve of the goose revealed two kinds of temperature-sensitive mechanoreceptors in the beak, which are innervated by large-diameter myelinated axons. Some Herbst corpuscles, which display rapidly adapting responses to mechanical stimulation, discharge tonically to cooling the receptive field. A static response maximum occurs at temperatures between 15 and 25°C. The vibration sensitivity of Herbst corpuscles decreases with temperature. Many slowly adapting Ruffini endings also respond with sustained discharges to cooling but behave like rapidly adapting mechanoreceptors on warming the receptive field. The temperature at which the maximal static response occurs in different slowly adapting units varies between 10 and 30°C. The fine structure of Herbst corpuscles and Ruffini endings was investigated electron microscopically in order to elucidate possible morphological substrates for the specific functional properties of the two receptor types. Both Herbst corpuscles and Ruffini endings are characterized by a distinct but different combination of nervous and nonnervous (auxiliary) tissue elements and by the occurrence of special structural units at the sensory nerve endings. Such “transducer sites” presumably correspond to those areas of the receptor membrane where the mechanoelectric transduction process takes place. The most prominent constituent of a transducer site is a spurlike axon process, which in the Herbst corpuscle projects between the lamellae of the inner core, and in the Ruffini endings comes into contact with collagenous microfibrils, either directly or indirectly through the mediation of Schwann cell processes. Two morphological variants of Ruffini endings were recognized and were found to occur at different locations in the dermis underlying the horny covering of the bill tip. One type is distinguished by the presence of a specialized terminal cell making contact with the initial branches of the arborizing receptor axon. The other type of Ruffini ending lacks the terminal cell. The structure-function relations in Herbst corpuscles and Ruffini endings are discussed on the basis of the electrophysiological and ultrastructural observations. It is argued that the different response characteristics of both receptor types to mechanical stimuli result from the specific kind and arrangements of the auxiliary receptor structures and from the mechanical properties of the latter. On this basis the thermosensitivity of Herbst corpuscles and Ruffini endings can be explained as a consequence of the differential effects which temperature changes have on one hand on the auxillary receptor structures and on the other hand on the mechanically elicited receptor potential. It is concluded that the temperature sensitivity of some types of mechanoreceptors may be a secondary result of that differentiation of the auxiliary receptor structures which makes the discriminative function of the different types of mechanoreceptors possible and, therefore, does not necessarily conflict with the principle of modality specificity in sensory receptors.     

Afferent inhibition on various types of cat's cuneate neurons induced by dynamic and steady tactile stimuli

Afferent inhibition in several distinct types of cuneate neurons was studied using controlled natural stimuli in 35 lightly anesthetized cats. Mechanoreceptive cuneate neurons were recorded extracellularly with microelectrodes from the middle and caudal divisions of the main nucleus. They were classified into several modality subtypes based on their response to adequate mechanical stimuli. Emphasis was laid on the neurons which had their receptive fields (RFs) in the forepaw. Afferent inhibition was induced by conditioning tactile stimuli in 31 out of 168 neurons (18%) tested. There were particular combinations between the neuron types inhibited and conditioning stimulus modalities. Dynamic stimuli such as high frequency vibration and hair movement by air-jet stimuli applied to areas beyond the excitatory RFs induced inhibition on touch (T), hair (H) and slowly adapting pad (SA) units predominantly in the paw region. In contrast, steady pressure stimulation on the skin adjacent to the excitatory RFs induced inhibition in exclusively slowly adapting neurons receiving afferent inputs from hairy skin such as touch (T), joint (J) and subcutaneous (Deep) units in the paw, elbow and shoulder regions. Most of the inhibitory RFs were organized laterally or eccentrically rather than concentrically around the excitatory RF. Two J units were found to be inhibited by steady pressure applied to the shoulder region of the contralateral forelimb. Functional significance of the intramodality and cross modality inhibition of cuneate neurons is discussed.     

Hyper-excitability in low threshold mechanical A fibers is potentially involved in scorpion BmK sting pain

In the present study, using the single fiber recording technique, we found that BmK I, the main toxic component in scorpion Buthus martensi Karsch (BmK) venom, induced dramatic increase in excitability of rapidly adapting (RA) and type I slowly adapting (SAI) low threshold mechanical A fibers of rat. Five micrograms BmK I (691 nmol, in 10 μl saline) administrated to the receptive fields induced spontaneous activity in 80% of RA and SAI fibers, increased the response to 10 g–10 s stimulation at about 20 times and altered the firing pattern to burst mode with maximal NS (number of spikes in burst) averaging from all fibers studied as many as 59. The increase in the excitability of RA and SAI fibers did not recover completely in 2 h.Our finding suggests that the gigantic abnormal activity in low threshold mechanical A fibers is involved in BmK scorpion sting pain, and the experimental model of BmK scorpion sting pain can be used to study A-fiber related central pathway which is important for relief of refractory neuropathic pain likewise.     

Reduced responsiveness of touch (SA I) receptors in the cat following close arterial infusion of neomycin

Electrophysiological recordings were made from afferent nerve fibres supplying slowly adapting type I (SA I) cutaneous mechano-receptors in the cat during application of mechanical stimuli (20 mN constant force, 0.2 s rise time and 2 s plateau phase, every 30 s). Close arterial infusion of neomycin (2.5 mg/min for 5 min) strongly depressed the responsiveness of SA I receptors. A direct effect on the receptor is thought to be the mode of action.     

Response to an itch-producing substance in cat. II. Cutaneous receptor populations with unmyelinated axons

Within the sampled population of cutaneous unmyelinated afferent neurons (n = 94), only the C-polymodal nociceptor population was reactive to the pruritogen cowhage. Of 62 C-polymodal neurons tested, 11 were unresponsive to cowhage. No C-polymodal neurons were more responsive to inactive, than to active, cowhage (n = 17) and all were responsive to mechanical (n = 62) stimuli and noxious heat (n = 24). The range of conduction velocities obtained by single-unit recording techniques was similar to that found by signal averaging the activity from larger strands of nerve. Hence, it is concluded that our recording technique was capable of recording from the smallest afferent fibers in a cutaneous nerve and it was unlikely that we would have missed finding a slowly conducting, pruritus-signaling neuron due to sampling bias. A search of slowly conducting afferents (n = 314) using electrocutaneous stimulation gave no evidence to suggest the existence of an unknown population of unmyelinated fibers that might signal pruritus. A number of alternative mechanisms by which the sensation of itch might be encoded were discussed, the most favored being the activation of a subset of the C-polymodal nociceptive population.     

Botulinum toxin‐a treatment reduces human mechanical pain sensitivity and mechanotransduction

The mechanisms underlying the analgesic effects of botulinum toxin serotype A (BoNT‐A) are not well understood. We have tested the hypothesis that BoNT‐A can block nociceptor transduction. Intradermal administration of BoNT‐A to healthy volunteers produced a marked and specific decrease in noxious mechanical pain sensitivity, whereas sensitivity to low‐threshold mechanical and thermal stimuli was unchanged. BoNT‐A did not affect cutaneous innervation. In cultured rodent primary sensory neurons, BoNT‐A decreased the proportion of neurons expressing slowly adapting mechanically gated currents linked to mechanical pain transduction. Inhibition of mechanotransduction provides a novel locus of action of BoNT‐A, further understanding of which may extend its use as an analgesic agent. Ann Neurol 2014;75:591–596     

Tactile sensory function in the forearm of the monotreme Tachyglossus aculeatus

Peripheral tactile neural mechanisms in the forepaw of the echidna (Tachyglossus aculeatus, from the order Monotremata) were investigated to establish the extent of correspondence or divergence that has emerged over the widely different evolutionary paths taken by monotreme and placental mammals. Electrophysiological recordings were made in anesthetized echidnas from 29 single tactile sensory nerve fibers isolated in fine strands of the median or ulnar nerves of the forearm. Controlled tactile stimuli were applied to the forepaw glabrous skin to classify fibers, initially, into two broad divisions, according to their responses to static skin displacement. One displayed slowly adapting (SA) response properties, and the other showed a selective sensitivity to the dynamic components of the skin displacement. The SA class was made up of low-threshold SA fibers and other less sensitive SA fibers, and the purely dynamically sensitive tactile fibers could be subdivided according to vibrotactile sensitivity and receptive field characteristics into a rapidly adapting (RA) class, sensitive to low-frequency (< or =50-Hz) vibration, that resembled a corresponding RA class in placental species, and another class, sensitive to a broader range of vibrotactile frequencies (approximately 50-300 Hz), that may represent a monotreme equivalent of the Pacinian corpuscle (PC)-related fiber class of placental mammals. The differential tactile sensitivity of the three principal fiber classes and their individual coding characteristics, determined by quantitative stimulus-response analysis, indicate, first, that this triad of fiber classes can subserve high-acuity tactile signalling from the echidna footpad and, second, that peripheral tactile sensory mechanisms are highly conserved across evolutionarily divergent mammalian orders.     

Latency to detection of first pain

The latency to detection of heat stimuli applied to the distal forearm and thenar eminence was measured in 3 subjects in order to determine whether short latency responses correlated with perception of first pain. Only one temperature was used in a given run and stimuli ranged from 39 to 51 °C. In addition, subjects were interviewed at the end of each run regarding the quality of sensations experienced. In one series of experiments the quality of the first sensation evoked by each stimulus rather than latency was recorded. The median response latency decreased exponentially from 1100 ms to 400 ms for the distal arm and 1100 ms to 700 ms for the hand. The higher temperatures elicited a double pain sensation on the arm, but not on the glabrous hand. Warmth was always the first sensation felt on the hand. It is concluded that short latencies (less than 450 ms) reliably denote the presence of first pain, and that at least some portion of the primary afferents that signal first pain must have conduction velocities greater than 6m/s.     

Pain from excitation of identified muscle nociceptors in humans

The technique of intraneural microstimulation (INMS) combined with microneurography was used to excite and to record impulse activity in identified afferent peroneal nerve fibers from skeletal muscle of human volunteers. Microelectrode position was minutely adjusted within the impaled nerve fascicle until a reproducible sensation of deep pain projected to the limb was obtained during INMS. During INMS trains of 5–10 s in duration and at threshold for sensation, volunteers perceived a well defined area of deep pain projected to muscle. Psychophysical judgements of the magnitude of pain increased with increasing rates of INMS between 5 and 25 Hz. Also, the area of the painful projected field (PF) evoked during trains of INMS of various duration but constant intensity and rate typically expanded with duration of INMS. The intraneural microelectrode was alternatively used to record neural activity originating from primary muscle afferents. Eight slowly adapting units with moderate to high mechanical threshold were identified by applying pressure within or adjacent to the painful PF. Conduction velocities ranged from 0.9 to 6.0 m/s, and fibers were classed as Group III or Group IV. Capsaicin (0.01%) injected into the RF of two slowly conducting muscle afferents (one Group III and one Group IV) produced spontaneous discharge of each fiber and caused intense cramping pain, suggesting that the units recorded were nociceptive. Our results endorse the concept that the primary sensory apparatus that encodes the sensation of cramping muscle pain in humans is served by mechanical nociceptors with slowly conducting nerve fibers. Results also reveal that muscle pain can be precisely localized, although the human cortical function of locognosia for muscle pain becomes blunted as a function of duration of the stimulus.     

Responses of pyramidal tract neurons in the postcentral cortex to tactile inputs

Pyramidal tract neurons were recorded from postcentral cortex of awake monkeys and their responses to step indentation and vibratory stimulus were studied. The majority of them exhibited slowly adapting response to the indentation stimulus but failed to show phase-locked response to 50-200 Hz vibrations. The response properties appeared to be in contrast to those of non-pyramidal tract neurons whose responses were largely quickly adapting.