Sensory-interference in rat primary somatosensory cortical neurons

To study sensory interaction in the primary somatosensory cortex (SI), we registered 221 neurons in the SI of pentobarbital-anaesthetized Wistar rats. Tactile stimulation was applied in the receptive field of the SI neuron with an electronically controlled probe (20 ms duration). Tactile stimulation elicited 2.33 +/- 0.13 spikes per stimulus in SI neurons. Simultaneous application of paintbrush tickles of the contralateral limb usually decreased tactile responses (1.59 +/- 0.11 spikes per stimulus). This effect was considered a 'sensory-interference'. Light flashes applied at random did not modify tactile response. Applying atropine (1 mm), a muscarinic receptor antagonist, and bicuculline (1 mm), a GABAA receptor antagonist, to the SI cortex blocked the sensory-interference effect, while application of mecamylamine (10 mm), a nicotinic cholinergic receptor antagonist, did not affect sensory-interference. Results reveal sensory interactions in SI cortex that control tactile responses, and suggest the participation of the basal forebrain in the sensory-interference effect.     

Cholinergic modulation of sensory interference in rat primary somatosensory cortical neurons

Sensory interaction was studied using extracellular recordings from 275 neurons in the primary somatosensory (SI) cortex of pentobarbital-anesthetized rats. Tactile stimulation was applied to the receptive field using a 1 mm diameter probe that indented the skin for 20 ms, at 0.5 Hz, (test stimulus). Tactile test responses of SI neurons decreased during simultaneous application of a gentle tickling (distracter stimuli) continuously for 60 s on a separate receptive field located in the same or the contralateral hindlimb (ipsi- or contralateral distraction). This decrease in neural response produced by distracter stimuli was interpreted as "sensory interference". Sensory interference was observed in 66% and 61% of recorded SI neurons when ipsi- or contralateral distracters were applied, respectively and was blocked by a novel stimulus obtained by increasing the stimulation frequency of the test tactile stimuli from 0.5 to 2 Hz. The number of neurons showing sensory interference in response to a contralateral distracter was not modified after corpus callosum transection, suggesting that interhemispheric connections are not crucial for sensory interference. In contrast, the number of neurons showing sensory interference decreased in animals with 192 IgG-saporin basal forebrain lesions that decreased the number of cortical cholinergic fibers. This finding indicates that cholinergic afferents from the basal forebrain are fundamental to sensory interference and suggests that the associative cortices - basal forebrain - sensory cortices network may be implicated in sensory interference.     

From innervation density to tactile acuity: 1. Spatial representation

We tested the hypothesis that the population receptive field representation (a superposition of the excitatory receptive field areas of cells responding to a tactile stimulus) provides spatial information sufficient to mediate one measure of static tactile acuity. In psychophysical tests, two-point discrimination thresholds on the hindlimbs of adult cats varied as a function of stimulus location and orientation, as they do in humans. A statistical model of the excitatory low threshold mechanoreceptive fields of spinocervical, postsynaptic dorsal column and spinothalamic tract neurons was used to simulate the population receptive field representations in this neural population of the one- and two-point stimuli used in the psychophysical experiments. The simulated and observed thresholds were highly correlated. Simulated and observed thresholds' relations to physiological and anatomical variables such as stimulus location and orientation, receptive field size and shape, map scale, and innervation density were strikingly similar. Simulated and observed threshold variations with receptive field size and map scale obeyed simple relationships predicted by the signal detection model, and were statistically indistinguishable from each other. The population receptive field representation therefore contains information sufficient for this discrimination.     

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.     

Functional organization of receptive fields in the cat somatosensory cortex II: Second representation of the forepaw in the ansate region

The receptive fields (RF) of the neurons in the coronal region of the first somatosensory cortex (SI) were studied in a preparation of unanesthetized paralyzed cats.The majority of units responded to simple light mechanical stimuli to the hairy or glabrous skin. There were units with slightly larger RFs and more complex properties, such as those preferentially responding to the moving skin stimuli with directional selectivity.The receptive fields tend to be larger in the more rostral (area 3a) or medial coronal (area 3b) region than in the caudal region (area 3b).The largest RFs intermingled among focal ones included other smaller RFs in the vicinity. Their configurations were different at different loci in the coronal region. It is suggested that the coronal SI region is organized in terms of multiple clusterings of units, each with particular receptive field characteristics. The representation of a single peripheral locus on the forepaw can thus be multiple, if it appeared in more than one of such clusterings of receptive fields.     

Somatic submodality distribution within the second somatosensory (SII), 7b,retroinsular, postauditory,and granular insular cortical areas of M. fascicularis

Somatic response properties were determined for over 1,300 neurons isolated within and near the lateral sulci of unanesthetized and unparalyzed cynomolgus monkeys. Somatic stimuli unequivocally activated the majority of units studied in SII (93%) and in cortical fields surrounding SII: area 7b (65%), the retroinsular field (74%), and the granular insula (76%). No activation other than somatic was seen for SII neurons, and noxious somatic stimulation was rarely required. The SII units almost always responded in a rapidly adapting manner to hair or skin stimulation, but not both; however, the submodality distribution seen in SII varied as a function of peripheral receptor locations. Two small zones within SII contained neurons that responded only if the animal actively interacted with the stimulus. In contrast, one-half of the sample of neurons from area 7b unequivocally responded only to somatic stimulation. Although many neurons in the lateral parts of area 7b were vigorously activated by innocuous tactile stimulation, others demonstrated little association with an identifiable somatic submodality, had sluggish responses, required complex, noxious, visual or other non-somatic stimuli for activation, and had labile response properties and receptive fields. Indeed, the responses of some area 7b neurons suggested a possible relationship with the animal's attention towards or anticipation of a noxious or a novel somatic stimulus. Neurons within the retroinsular cortex (Ri), which receives projections from the posterior nucleus (PO), primarily responded to light tactile stimulation of rapidly adapting skin receptors; less than 3% responded to moderate or high threshold mechanical stimulation. The sensitivity to tactile stimulation in Ri closely resembled the responses of SII neurons. Neurons in the granular insula (Ig) often responded to gentle hair deflection within receptive fields covering large areas of the body. Ig and area 7b were the principle loci within the lateral sulcus that contained neurons responding to noxious stimulation. Owing to the great similarity in the somatic response properties within these areas in the awake and unparalyzed animal, the designation of cortical areas could only be made after correlating the recording sites with connectional and cytoarchitectonic analyses in the same animal. Consequently, previous physiological studies may have attributed to SII some of the response characteristics of neurons in neighboring areas.     

Afferent properties of periarcuate neurons in macaque monkeys. II. Visual responses

The visual responses of single neurons of the periarcuate cortex have been studied in the macaque monkey. Two sets of neurons responding to visual stimuli have been found. The first set, located rostral to the arcuate sulcus, was formed by units that could be activated by stimuli presented far from the animal. These neurons had large receptive fields and were neither orientation nor direction selective. The second set, found predominantly caudal to the arcuate sulcus, was formed by units that were maximally or even exclusively activated by stimuli presented in the space immediately around the animal. These neurons were bimodal, responding also to somatosensory stimuli.According to the location of their visual responding regions the bimodal neurons were subdivided into pericutaneous (54%) and distant peripersonal neurons (46%). The former responded best to stimuli presented a few centimeters from the skin, the latter to stimuli within the animal's reaching distance. The visual responding regions were spatially related to the tactile fields.It is argued that neurons with a receptive field consisting of several responding areas, some in one sensory modality, some in another, have a praxic function and that they are involved in organizing sequences of movements.     

Excitability and functional organization of cutaneous tactile units of the bullfrog (R. catesbeiana).

Frog skin touch receptors are discrete structures which appear as dome-shaped translucent elevations of the epidermis. These domes are particularly responsive to direct touch. Tactile stimulation of adjacent skin never caused impulse activity, except when the skin distortion disturbed the domes. On stimulation at threshold intensity, the response of all units studied always consisted of a single impulse whether the stimulus energy was electrical or mechanical. While most tactile units discharged one impulse upon application of mechanical stimulation, a few also discharged one impulse upon stimulus release (on-off response). No after discharge was seen at stimulus strengths of three or four times threshold or after repetitive stimulation at 1,000 pps. The range of distribution of active domes/receptive field was 1--14. Generally the most sensitive domes appeared to be in or near center field. Twice as many of the tactile units responded more readily to cold than to warmth. The impulse frequency of units responding to thermal stimulation ranged from 3-11/sec. The conduction velocity of tactile units measured was within a range of 9--25 m/sec, with a mean of 12 m/sec. The results suggest that the following: 1) the existence of discrete structures which respond to tactile stimuli; a phylogenetic relationship might be postulated; 2) cold/touch receptors which appear to be present in greater quantity in the frog's skin than are warm/touch receptors, 3) touch units which should be considered primarily as mechanoreceptors with a possible secondary function as thermoreceptors.     

Early integration of bilateral touch in the primary somatosensory cortex

Animal, as well as behavioural and neuroimaging studies in humans have documented integration of bilateral tactile information at the level of primary somatosensory cortex (SI). However, it is still debated whether integration in SI occurs early or late during tactile processing, and whether it is somatotopically organized. To address both the spatial and temporal aspects of bilateral tactile processing we used magnetoencephalography in a tactile repetition‐suppression paradigm. We examined somatosensory evoked‐responses produced by probe stimuli preceded by an adaptor, as a function of the relative position of adaptor and probe (probe always at the left index finger; adaptor at the index or middle finger of the left or right hand) and as a function of the delay between adaptor and probe (0, 25, or 125 ms). Percentage of response‐amplitude suppression was computed by comparing paired (adaptor + probe) with single stimulations of adaptor and probe. Results show that response suppression varies differentially in SI and SII as a function of both spatial and temporal features of the stimuli. Remarkably, repetition suppression of SI activity emerged early in time, regardless of whether the adaptor stimulus was presented on the same and the opposite body side with respect to the probe. These novel findings support the notion of an early and somatotopically organized inter‐hemispheric integration of tactile information in SI. Hum Brain Mapp 36:1506–1523, 2015. © 2014 Wiley Periodicals, Inc.     

Activation of the somatosensory cortex during Abeta-fiber mediated hyperalgesia. A MSI study

OBJECTIVE: To investigate the neural activation in the primary somatosensory cortex (SI) that is induced by capsaicin-evoked secondary Abeta-fiber-mediated hyperalgesia with magnetic source imaging (MSI) in healthy humans. BACKGROUND: Dynamic mechanical hyperalgesia, i.e. pain to innocuous light touching, is a symptom of painful neuropathies. Animal experiments suggest that alterations in central pain processing occur so that tactile stimuli conveyed in Abeta low threshold mechanoreceptive afferents become capable of activating central pain signalling neurons. A similar state of central sensitization can be experimentally produced with capsaicin. METHODS: In six individuals the somatosensory evoked magnetic fields (SEFs) induced by non-painful electrical stimulation of Abeta-afferents at the forearm skin were recorded. Capsaicin was injected adjacent to the stimulation site to induce secondary dynamic Abeta-hyperalgesia. Thereafter, the SEFs induced by the identical electrical stimulus applied within the secondary hyperalgesic skin were analyzed. The electrical stimulus was subsequently perceived as painful without changing the stimulus intensity and location. Latencies, anatomical source location and amplitudes of SEFs during both conditions were compared. RESULTS: Non-painful electrical stimulation of Abeta-afferents induced SEFs in SI at latencies between 20 and 150 ms. Stimulation of Abeta-afferents within the capsaicin-induced secondary hyperalgesic skin induced SEFs at identical latencies and locations as compared with the stimulation of Abeta-afferents within normal skin. The amplitudes, i.e., the magnetic dipole strengths of the SEFs were higher during Abeta-hyperalgesia. CONCLUSIONS: Acute application of capsaicin produces an increase in the excitability of central neurons, e.g., in SI. This might be due to sensitization of central neurons so that normally innocuous stimuli activate pain signalling neurons or cortical neurons might increase their receptive fields.     

Intracellular analysis of directional sensitivity of tectal neurons of the frog

The directional sensitivity of tectal neurons of the frog was examined by means of in vivo whole cell recording technique. Three kinds of stimulus were applied; (1) diffuse light 'on-off', (2) moving dark spot and (3) light spot given at one dimensional grid points. The first stimulus revealed whether or not retinal 'on-off' (R3) or 'off' (R4) fibers contribute to the response. As reported earlier, the following patterns were found for both light 'on' and light 'off': EPSPs only, IPSPs only or a combination of EPSPs and IPSPs. Four directionally sensitive neurons and three non-directionally sensitive neurons were found using the second stimulus. Using the third stimulus, responses at up to 11 positions separated by 2 degrees or 4 degrees were recorded. By measuring the amplitudes of 'on' and 'off' responses at different times, spatio-temporal receptive fields were composed. Two types of directional sensitivity were found. The response of the first type was composed of exclusively excitatory potentials, but the second type was composed of a combination of excitatory and inhibitory potentials. The spatio-temporal receptive field of the second type showed spatially separated excitatory and inhibitory regions with constant latencies. Such simple spatio-temporal receptive field organization was not found for directional sensitive neurons of the cat visual cortex. The spatio-temporal receptive field organization of the second type of directionally sensitive neuron in the present study is in agreement with striated receptive field found in some of the T5 neurons classified by extracellular unit recording [Frog Neurobiology (1976) 297].     

Effects of menthol on peripheral nerve and cortical unit responses to thermal stimulation of the oral cavity in the rat

Neurophysiological responses were recorded in individual fibers of the lingual and chorda tympani nerves and in single cortical neurons in the rat in response to a battery of tactile, thermal and chemical stimuli applied to the oral cavity. Two categories of thermally sensitive units were identified. Chorda tympani fibers and one type of cortical unit ('Type I') were activated by cold water stimulation but were unaffected by warm water or menthol. In contrast, lingual fibers and a different category of cortical units ('Type II') were extremely sensitive to menthol exposure. These units were cold water sensitive, however, this sensitivity was suppressed following menthol presentation.     

Mechanical vibratory stimulation of feline forepaw skin induces long-lasting potentiation in the secondary somatosensory cortex

We investigated the long-lasting effects of mechanical vibratory stimulation of the skin on the excitability of feline cortical neurons in the forelimb areas of the primary (SI) and secondary (SII) somatosensory cortices. Conditioning mechanical stimuli were 300 bursts of 10 pulses at 200 Hz delivered with a 10-s interburst interval from a mechanical stimulator. Test field potentials and unit discharges were evoked by electrical stimulation to the ventral posterolateral thalamic nucleus (VPL) or by single mechanical stimuli applied to the skin. In SII, the mechanical burst stimulation to the skin increased the amplitudes of field potentials and the frequency of unit discharges elicited by single mechanical stimuli applied to the skin. The vibratory conditioning stimulus also produced a similar potentiation of the VPL-evoked field potentials (126-139% increase in amplitude, P < 0.05) with an associated increase in firing rates of extracellularly recorded neuronal activity (117%, P < 0.001). These potentiations persisted through the entire experimental period of 120 min. The translaminar current source density analysis calculated from the VPL-evoked field potentials increased to 127% of the control value (P < 0.01). In contrast, in SI we observed no significant changes in the field potential amplitudes or in the currents generated in superficial layers (91-117%). Taken together with the previous finding that tetanic electrical stimulation of VPL induces long-lasting potentiation of the VPL-evoked cortical responses in SII but not of those in SI, the present results suggest that SII has a large capacity for the rapid functional plasticity involved in the learning that occurs during repeated tactile experiences.     

Visual receptive fields in the lateral suprasylvian area (Clare-Bishop area) of the cat.

Single units were recorded from the visual area of the lateral suprasylvian gyrus (LSSA or Clare-Bishop area) in 20 unanesthetized cats. Most LSSA units were poorly responsive to stationary visual stimuli, but they responded vigorously to moving visual stimuli. Their receptive fields appeared to be constituted of a large activating region (discharge area) often surrounded by inhibitory flanks. Relating unit behavior to changes of stimulus length, the LSSA neurons could be subdivided into 5 categories. The first category (22 out of 95 units tested, 23.16%) consisted of units showing summation inside the discharge area. Expanding the stimulus outside the discharge area did not affect the response. The second category (7.37%) was formed by units which showed summation inside the discharge area and inhibition when the stimulus was extended outside the discharge area. The third category (21.05%) consisted of units largely insensitive to the stimulus length inside the discharge area, but surrounded by inhibitory flanks. The fourth category (41.05%) consisted of units which showed inhibition of the response when the stimulus, well inside the discharge area, became longer than a certain optimal lenght. They were surrounded by inhibitory flanks. The fifth category (7.37%) was formed by units insensitive to variations of the stimulus length inside as well as outside the discharge area. Almost all units, independent of their category, were directionally specific, that is their response could be decreased 50% or more by varying the direction of movement away from that which gave the maximal response (preferred direction). Typically the response was halved when the stimulus was moved +/- 50 degrees from the preferred direction. Among the directionally specific units, 71% showed the minimal response 180 degrees away from the preferred direction (direction specificity curve type 1), 20% had the minimal response 90 degrees from the preferred direction (direction specificity curve type 2); the remaining could not be classified in this respect. Of LSSA units, 87% (all those of type 1 and many of those of type 2) were directionally selective, that is their response to movement in the preferred direction was at least double that in the opposite direction. The LSSA units usually preferred stimuli moving at rather high speeds. The optimal speed for 71% of units was 20 degrees/sec or greater. Almost all units responded over a wide range of speeds, many of them from 5-10 degrees/sec to over 100 degrees/sec. Most neurons had a low spontaneous activity and some of them remained completely silent for seconds.     

Bilateral and multimodal sensory interactions of single cells in the pigeon's midbrain

Standard microelectrode recording techniques were employed to monitor single unit activity in the pigeon's nucleus intercollicularis and medial substantia grisea et fibrosa periventricularis in response to visual, tactile and auditory stimuli. Approximately 40% of the units were driven exclusively by visual stimuli, 8% by tactile stimuli, 47% by both visual and tactile stimuli and a very small percentage by auditory stimuli. Visual receptive fields were generally excitatory in the contralateral eye and suppressive in the ipsilateral eye. Most units were movement selective and some demonstrated direction sensitivity, summation and habituation. Units were generally insensitive to stimulus shape or contrast reversal. Somatosensory receptive fields were located on both sides of the body and were either excitatory or suppressive or both. Ipsilateral visual and somatosensory bimodal inputs were most often of the same sign while ipsilateral visual and contralateral somatosensory bimodal inputs tended to be of opposite sign. Visual and somatosensory receptive field locations of bimodal units tended to be in register.     

SI nociceptive neurons participate in the encoding process by which monkeys perceive the intensity of noxious thermal stimulation

The activity of primary somatosensory (SI) cortical nociceptive neurons was recorded while the monkeys performed a psychophysical task in which they detected small increases in skin temperature superimposed on noxious levels of thermal stimulation. The detection latency to these stimuli, expressed as detection speed, was used as a measure of the perceived intensity of sensation. Two-thirds of the neurons that responded to noxious thermal stimulation increased their discharge in response to graded increases in stimulus intensity. The remaining neurons responded to noxious thermal stimulation, but did not grade their response with the intensity of the stimulus. The response of SI nociceptive neurons that encode the intensity of noxious thermal stimulation was significantly correlated with the monkey's detection speed. We conclude that SI nociceptive neurons are involved in the encoding process by which monkeys perceive the intensity of noxious thermal stimulation.     

Activation of the somatosensory cortex during Aβ-fiber mediated hyperalgesia: a MSI study

Objective: To investigate the neural activation in the primary somatosensory cortex (SI) that is induced by capsaicin-evoked secondary Aβ-fiber-mediated hyperalgesia with magnetic source imaging (MSI) in healthy humans. Background: Dynamic mechanical hyperalgesia, i.e. pain to innocuous light touching, is a symptom of painful neuropathies. Animal experiments suggest that alterations in central pain processing occur so that tactile stimuli conveyed in Aβ low threshold mechanoreceptive afferents become capable of activating central pain signalling neurons. A similar state of central sensitization can be experimentally produced with capsaicin. Methods: In six individuals the somatosensory evoked magnetic fields (SEFs) induced by non-painful electrical stimulation of Aβ-afferents at the forearm skin were recorded. Capsaicin was injected adjacent to the stimulation site to induce secondary dynamic Aβ-hyperalgesia. Thereafter, the SEFs induced by the identical electrical stimulus applied within the secondary hyperalgesic skin were analyzed. The electrical stimulus was subsequently perceived as painful without changing the stimulus intensity and location. Latencies, anatomical source location and amplitudes of SEFs during both conditions were compared. Results: Non-painful electrical stimulation of Aβ-afferents induced SEFs in SI at latencies between 20 and 150 ms. Stimulation of Aβ-afferents within the capsaicin-induced secondary hyperalgesic skin induced SEFs at identical latencies and locations as compared with the stimulation of Aβ-afferents within normal skin. The amplitudes, i.e., the magnetic dipole strengths of the SEFs were higher during Aβ-hyperalgesia. Conclusions: Acute application of capsaicin produces an increase in the excitability of central neurons, e.g., in SI. This might be due to sensitization of central neurons so that normally innocuous stimuli activate pain signalling neurons or cortical neurons might increase their receptive fields.     

Somatosensory evoked potentials (SEPs) and cortical single unit responses elicited by mechanical tactile stimuli in awake monkeys

The origins of surface recorded evoked potentials have been investigated by combining recordings of single unit responses and somatosensory evoked potentials (SEPs) from the postcentral gyrus of 4 alert macaque monkeys. Responses were elicited by mechanical tactile stimuli (airpuffs) which selectively activate rapidly adapting cutaneous mechanoreceptors, and permit patterned stimulation of a restricted area of skin. Epidurally recorded SEPs consisted of an early positive complex, beginning 8-10 msec after airpuff onset, with two prominent positive peaks (P15 and P25), succeeded by a large negative potential (N43) lasting 30 msec, and a late slow positivity (P70). SEPs, while consistent in wave form, varied slightly between monkeys. The amplitude of the early positive complex was enhanced by increasing the number of stimulated points, or by placing the airpuffs in the receptive fields of cortical neurons located beneath the SEP recording electrode. SEP amplitude was depressed when preceded 20-40 msec earlier by a conditioning stimulus to the same skin area. Single unit responses in areas 3b and 1 of primary somatosensory (SI) cortex consisted of a burst of impulses, beginning 11-12 msec after the airpuff onset, and lasting another 15-20 msec. Peak unitary activity occurred at 12-15 msec, corresponding to the P15 wave in the SEP. No peak in SI unit responses occurred in conjunction with the P25 wave. Although SI neurons fired at lower rates during P25, the lack of any peak in SI unit responses suggests that activity in other cortical areas, such as SII cortex, contributes to this wave. Most unit activity in SI cortex ceased by the onset of N43, and was replaced by a period of profound response depression, in which unit responses to additional tactile stimuli were reduced. We propose that the N43 wave reflects IPSPs in cortical neurons previously depolarized and excited by the airpuff stimulus. Late positive potentials (P70) in the SEP had no apparent counterpart in SI unit activity, suggesting generation at other cortical loci.     

Morphological characteristics and electrophysiological responses of visceral nociceptive neurons in somatosensory cerebral cortex of cat

The morphology of intracellular neurobiotin-labeled cortical neurons activated by noxious visceral stimuli in cat was reported. The electrophysiologic properties of 851 neurons and structure of 52 neurons of representative area of the greater splanchnic nerve (GSN) in the primary somatosensory cortex were described. The neurons responding to stimulation of GSN with strong intensity were referred to as viscero-nociceptive neurons (VNNs). Patterns of the responses of VNNs in SI area were excitatory, inhibitory and mixed. According to latency of discharge clusters in response, the VNNs could be classified into two kinds. The neurons only showing long latency were called specific VNNs; those having discharge clusters with both long and short latency induced by stimulus were nonspecific VNNs. Following acquisition of electrophysiological data, neurobiotin was injected into some cells by electrophoresis and their structure and location in the cerebral cortex were studied. The results provided some findings about morphological and electrophysiological characteristics of VNNs in SI area. It is thought that VNNs have more complicated morphology in the dendritic construction and axon distribution than nonvisceral nociceptive neurons (NVNNs). Most of the excitatory VNNs showed pyramidal shape and most of the inhibitory VNNs had stellate shape in camera lucida drawing. The architecture of the nonspecific VNNs might be different from specific VNNs in dendritic distribution.