Wide-field neurons in somatosensory thalamus of domestic cats under barbiturate anesthesia

A sample of 392 somatosensory thalamic neurons from barbiturate-anesthetized cats, all responsive to stimulation of the contralateral forepaw (CFP), was compared with a previously described sample from chloralose-anesthetized animals with respect to (i) the functional properties of CFP-responsive neurons isolated within nucleus ventralis posterolateralis (VPL), (ii) the spatial distributions of distinguishable neuronal subsets within VPL, and (iii) the distribution of CFP-evoked single-unit discharge along the mediolateral dimension of the nucleus and over time. Eighty-two percent of the barbiturate neurons were responsive only to CFP; their receptive fields were small, usually confined to one or two digits or an area on the paw dorsum. Members of this sa subset were predominantly excited by light touch or deflection of hairs. The other 18% responded to stimulation of at least one “off-focus” limb in addition to CFP. These ～sa neurons possessed larger receptive fields (though not as large as those observed under chloralose), which were bilaterally disposed in 32% of this subset. Fifty-eight percent of the ～sa neurons were excited by light touch or hair deflection. The proportion of the barbiturate sample subject to corticofugal influences (62%) was smaller than that under chloralose (74%), with inhibitory influences predominating on both sa and ～sa neurons in contrast to an excitatory effect on ～sa neurons under chloralose. Compared with chloralose data, impulse discharge reached peak intensity earlier and overall duration of response was reduced for all subsets. Although the spike density distribution for ～sa neurons was particularly abbreviated, this subset was well represented among VPL neurons responding to stimulation of the forepaw. As they are seen in barbiturate preparations also, wide-field somatosensory thalamic neurons are not an artifact of chloralose anesthesia.     

Regional variations of somatosensory input convergence in nucleus VPL of cat thalamus

Regional differences in the time course of contralateral forepaw-evoked discharge have been observed that correlate with the adequate stimuli and receptive field sizes of neurons predominating in particular subareas within nucleus ventralis posterolateralis of chloralose-anesthetized cats. Early activity is due primarily to small-field, forepaw-specific neurons in the anterior, medial forepaw “focus,” defined here in terms of the evoked thalamic “localizing potential” and correlated unit spike discharge density. Later activity is produced largely by wide-field neurons that respond to other appendages, in addition to contralateral forepaw, in the caudolateral portion of the nucleus. Hair-sensitive neurons are clustered medially within the forepaw focus and discharge earliest, whereas touch-sensitive neurons are scattered diffusely throughout the focus and the adjacent central zone, and lag behind hair cells in onset, build-up, and decline of activity. Of those affected by cortex, most hair cells are inhibited and most touch cells are excited. The anterior-medial portion of ventralis posterolateralis is dominated by crossed lemniscal input, and shows some segregation of neurons according to modality, whereas dorsal column, spinothalamic, and perhaps other projections converge upon the caudolateral region, where there is obvious bilateral input.     

Excitation of wide-field ventralis posterolateralis neurons by contralateral thalamic stimulation

Interconnections between the nuclei ventralis posterolateralis (VPL) of the thalamus were studied using single-unit recordings from chloralose-anesthetized cats. Responses to stimulation of the contralateral VPL (cVPL) were evoked in 28.9% of the 218 neurons sampled. Those neurons responsive to cVPL stimulation were almost exclusively those with an ipsilateral component to their receptive fields, i.e., sb (receptive fields on both fore- or hind limbs) and m neurons (receptive fields on all four limbs), but not all neurons of either of these sets responded. Only 2 of 127 neurons with receptive fields on either the contralateral fore- or hind limb, sa neurons, responded to cVPL stimulation. cVPL-Responsive and -unresponsive m neurons differed little in the properties of their responses to skin stimulation and no differences were found in their distribution within the nucleus. Corticopetal neurons were found among both cVPL-responsive and -unresponsive neurons. VPL neurons were shown to exhibit coadunate behavior, like cortical neurons, but no evidence was found of modulation of m neuron excitability by local sa neurons. Responsiveness of m neurons to cVPL stimulation was blocked by application of ice or KCl crystals to the contralateral, but not the ipsilateral cortex. The possibility is considered that some responses to cVPL stimulation are antidromically conducted.     

Response of somatosensory cerebral neurons to stimulation of dorsal and dorsolateral spinal funiculi

Cerebral responses were studied after direct stimulation of the dorsal and the dorsolateral funiculi at C4–C5; precautions were taken to prevent stimulus current spread from one funiculus to the next. Observations were made in the forepaw foci of somatosensory area I and area II, and of the postcruciate site. The surface potentials evoked by dorsolateral funicular stimulation were small and usually monophasic, and they were abolished by lesions aimed at the spinocervical tract. They became comparable in size and shape to the response evoked by dorsal funicular stimulation when stimulus current was allowed to spread into the dorsal funiculus. Using shock to the contralateral forepaw as a hunting stimulus, 283 cortical neurons were studied. All responded readily to stimulation of the dorsal funiculus, whereas only 70 could be affected by stimulation of the dorsolateral funiculus. Among these 70 neurons, few responded well to dorsolateral funicular stimulation; consecutive response latencies ranged from 6 to 40 msec for some, and the average response probability was about 0.5. Only 5% of the small-field neurons could be affected by dorsolateral funicular stimulation, whereas 72% of the wide-field neurons were affected. Though a small corticofugal reflex discharge followed each dorsolateral funicular stimulus, no corticospinal neurons were found that could be excited by this input. The possible role of the spinocervical pathway in producing these results is discussed.     

Sensory nerve crush and regeneration and the receptive fields and response properties of neurons in the primary somatosensory cerebral cortex of cats

Extracellular recordings were made of activity evoked in neurons of the forepaw focus of somatosensory cerebral cortex by electrical stimulation of each paw in control cats and cats that had undergone crush injury of all cutaneous sensory nerves to the contralateral forepaw 31 to 63 days previously. Neurons responding only to stimulation of the contralateral forepaw were classified as sa; neurons responding to stimulation of both forepaws were classified as sb; neurons responding to stimulation of both contralateral paws were classified as sc; and neurons responding to stimulation of at least three paws were classified as m. The ratio sa:sb:sc:m neurons was 46:3:0:0 in control cats and 104:15:3:26 in cats that had undergone nerve crush 1-2 months prior to study. sa neurons from experimental cats had depth distributions similar to those in controls and responded to contralateral forepaw stimulation with more spikes per discharge, longer latency, and higher threshold than sa neurons in control cats. m neurons from experimental cats were distributed deeper in the cortex than sa neurons, and, when compared to experimental sa neurons, they responded with longer latency and poorer frequency-following ability; however, the number of spikes per discharge and threshold were not significantly different. The appearance of wide-field neurons in this tissue may be explained in terms of strengthening of previously sub-threshold inputs to neurons in the somatosensory system. If the neurons in sensory cortex play a requisite role in cutaneous sensations and if changes similar to those reported here occur and persist in human cortex after nerve crush, then "complete" recovery of sensation in such patients may occur against a background of changed cortical neuronal responsiveness.     

Distribution of somatic sensory and active-movement neuronal discharge properties in the MI-SI cortical border area in the rat

The rat somatosensory (SI) cortex contains a precise map of the cutaneous periphery, yet its rostromedial edge, which includes part of the fore- and hind paw representation, has been reported to functionally overlap with the electrically excitable primary motor (MI) cortex. Thus, the MI cortex in the rat contains two subregions: (i) rostrally, the "MI-agranular" cortex (i.e., "typical" MI cortex), and (ii) caudally, the "MI-SI-granular" cortex (i.e., the MI-SI overlap). The aim of this study was to assess the degree of overlap in the physiologic properties of single neurons recorded across the MI-SI boundary zone in awake, freely moving rats. Computer techniques were used to characterize both somatosensory receptive fields (cutaneous or "passive joint-manipulation") and discharge correlates of active limb movement in these MI-agranular and MI-SI-granular subregions of the MI. "Active-movement" units were defined as those which discharged strongly in correlation with specific limb movements, but in a manner which could not be predicted by their observable somatosensory properties. Of 92 completely analyzed cells in the MI-SI-granular forepaw region, 86 exhibited specific cutaneous receptive fields on the palmar surface of the forepaw. By contrast with the similarity of these neurons' responses to passive stimulation, they varied markedly in their discharge during active limb movements. For example, many did not respond when their forepaw receptive fields touched the ground during stepping. Furthermore, 31 (of 92) neurons in this region were identified as active-movement, firing in correlation with reaching movements of the forelimb. Seven of these were completely unresponsive to any sensory stimuli, but 24 exhibited an apparent convergence of cutaneous and active-movement properties. Of 86 units recorded in the MI-agranular subregion, 46 responded strongly to passive joint manipulation, but only three responded exclusively to cutaneous stimulation. Twenty-eight (of 86) cells were defined as active-movement, discharging mainly in correlation with forelimb reaching movements. Thus, the active-movement properties of neurons in these two subregions were quite similar, whereas the somatosensory properties were markedly different.     

Variability of peripheral representation in ventrobasal thalamic nuclei of the cat: Effects of chloralose treatment

The effects of the intravenous administration of α-chloralose on the static and dynamic properties of 20 highly specific somatosensory neurons isolated within the forepaw focus of the thalamic ventralis posterolateralis (VPL) nucleus of locally anesthetized cats were studied. Observations were made (extracellular recording) before, during, and after injection. Nine units of the sample were identified as thalamocortical relay cells. Spontaneous activity, activity during tonic afferent drive maintained by weak peripheral stimuli, responsiveness to stimulation of the dorsal column and dorsolateral funiculus, and location and extent of peripheral receptive fields were examined. In none of the sample units did the drug bring to light aspecific properties such as those previously described in sizable proportions of the cell populations sampled from the VPL nucleus of cats anesthetized with chloralose or subjected to surgical interventions which lower the amount of the ascending or corticofugal influx to the thalamus. After the treatment, eight units (six of which were identified as thalamocortical relay cells) exhibited continuous or (in two cases) discontinuous enlargements of their peripheral receptive fields, which, however, remained rather small compared to the aspecific fields. The effects are tentatively explained by hypothesizing the “unmasking” of a convergent specific input, directed only to a fraction of thalamocortical relay cells and normally blocked by afferent or corticofugal inhibition. It is suggested that the unmasked input could be mediated by the spinocervicothalamic pathway.     

Reticular thalamic responses to nociceptive inputs in anesthetized rats

The present study compares nociceptive responses of neurons in the reticular thalamic nucleus (RT) to those of the ventroposterior lateral nucleus (VPL). Extracellular single-unit activities of cells in the RT and VPL were recorded in anesthetized rats. Only units with identified tactile receptive fields in the forepaw or hindpaw were studied. In the first series of experiments, RT and VPL responses to pinching with a small artery clamp were tested with the rats under pentobarbital, urethane, ketamine, or halothane anesthesia. Under all types of anesthesia, many RT units were inhibited. Second, the specificity of the nociceptive response was tested by pinching and noxious heating of the unit's tactile receptive field. Of the 39 VPL units tested, 20 were excited by both types of noxious stimuli. In sharp contrast, of the 30 RT units tested, none were excited and 17 were inhibited. In a third series of experiments, low-intensity and beam-diffused CO(2) laser irradiation was used to activate peripheral nociceptive afferents. Wide-dynamic-range VPL units responded with short- and long-latency excitations. In contrast, RT units had short-latency excitation followed by long-latency inhibition. Nociceptive input inhibited RT units in less than 500 ms. We conclude that a significant portion of RT neurons were polysynaptically inhibited by nociceptive inputs. Since all the cells tested were excited by light tactile inputs, the somatosensory RT may serve in the role of a modality gate, which modifies (i.e. inhibits) tactile inputs while letting noxious inputs pass.     

大鼠丘脑腹后外侧核神经元触觉反应的特性(英文)

目的为确定大鼠丘脑腹后外侧核是否也发生触觉信息的会聚,本文研究了大鼠丘脑腹后外侧核神经元触觉反应的特性。方法采用在体细胞外单位放电记录技术,记录麻醉状态下大鼠丘脑腹后外侧核触觉神经元的放电活动。结果在记录的156个神经元中,140个神经元(89.7%)具有单一、连续的小感受野,16个神经元(10.3%)具有两个不连续的感受野。部分神经元对作用于感受野不同部位的相同触觉刺激产生不同的放电形式。此外,还观察到4.5%触觉神经元(n=7),它们仅对感受野内的运动性触觉刺激敏感,而对点状触觉刺激不敏感。结论以上结果表明,在大鼠丘脑腹后外侧核,大多数神经元具有皮肤感受器的时空和模式特性,而少数神经元表现出不同外周感受器相互作用的反应。由此可见,大鼠丘脑腹后外侧核中存在触觉信息的会聚。     

A comparison of receptive field properties of vibrissa neurons between the rat thalamic reticular and ventro-basal nuclei

Response properties of vibrissa-responding neurons in the somatosensory part of the rat thalamic reticular nucleus (S-TR) and ventro-basal complex (VB) were studied. Receptive field size was approximately the same between S-TR and VB neurons, i.e. most of the neurons were driven from only single vibrissa. On the other hand, there was a noticeable difference in direction sensitivity. VB neurons generally had a preference for a particular direction of vibrissa deflection; but most of the S-TR neurons responded equally well to all directions. In addition to the neurons showing excitatory responses, there were the small number of VB neurons which had exclusively inhibitory receptive fields. Response latencies of S-TR neurons to electrical stimulation of the medial lemniscus were longer by 0.9 ms on the average than those of VB neurons, indicating that the former neurons were driven monosynaptically by the latter.     

大鼠丘脑腹后外侧核神经元触觉反应的特性

目的为确定大鼠丘脑腹后外侧核是否也发生触觉信息的会聚,本文研究了大鼠丘脑腹后外侧核神经元触觉反应的特性。方法采用在体细胞外单位放电记录技术,记录麻醉状态下大鼠丘脑腹后外侧核触觉神经元的放电活动。结果在记录的156个神经元中,140个神经元（89．7％）具有单一、连续的小感受野,16个神经元（10．3％）具有两个不连续的感受野。部分神经元对作用于感受野不同部位的相同触觉刺激产生不同的放电形式。此外,还观察到4．5％触觉神经元（n=7）,它们仅对感受野内的运动性触觉刺激敏感,而对点状触觉刺激不敏感。结论以上结果表明,在大鼠丘脑腹后外侧核,大多数神经元具有皮肤感受器的时空和模式特性,而少数神经元表现出不同外周感受器相互作用的反应。由此可见,大鼠丘脑腹后外侧核中存在触觉信息的会聚。     

Projections from the renal nerve to the cat''s lateral somatosensory thalamus

The representation of the kidney in the lateral somatosensory thalamus was mapped using electrical stimulation of the renal nerve in pentobarbitone-anesthetized cats. Ninety-five of 197 thalamic neurons studied responded to renal nerve stimulation. The responsive neurons were located in the periphery of the ventral posterolateral nucleus (42%, VPL_p) and the neighboring dorsal and lateral aspects of the posterior complex (58%; POd and POl). No visceroceptive neurons were found within VPL proper. The mean response latency of the thalamic neurons to electrical nerve stimulation was 9.5 ± 2.6 ms (mean ± S.D.), suggesting an involvement of A δ, and possibly A β fibers in the primary afferent pathway. The visceroceptive neurons were further characterized with innocuous mechanical stimulation of the body surface, and for 94 of the 95 neurons a somatic receptive field could be determined. Of these, 35% were located on the lower back and belly, i.e., the dermatomes of the lower thoracic and upper lumbar spinal projection areas of the renal nerve. 52% of the somatic receptive fields were located on the contralateral foot, thigh, tail, or hind leg (lower lumbar, sacral and coccygeal dermatomes) and 13% covered the arm and upper body (upper thoracic and lower cervical dermatomes). Comparison between the thalamic representations of the renal and pelvic nerves showed that both covered comparable areas adjacent and around, but not within VPL proper. It is concluded that VPL_p, POd and POl play a role in processing visceral, possibly including nociceptive, information from the kidney of the cat.     

Responses of thalamic and hypothalamic neurons to scrotal warming in rats: Non-specific responses?

Activities of thalamic and hypothalamic neurons in response to scrotal temperature change were investigated in urethanized (1.2-1.5 g/kg) rats with special attention to changes in cortical electroencephalogram (EEG). Somatosensory relay neurons were identified electrophysiologically in the ventrobasal complex (VB) of the thalamus. These neurons had tactile receptive fields in areas outside the scrotum. Forty out of 44 of these neurons responded to scrotal warming by increase in firing rate. The responses occurred abruptly at threshold temperatures ranging from 31 to 40 degrees C (switching response) with simultaneous changes in EEG from high to low voltages (desynchronization). In both the thalamus and the hypothalamus, neurons excited or inhibited by scrotal warming were also excited or inhibited, respectively, by noxious stimulation that produced EEG desynchronization. Neurons showing no response to scrotal warming were not affected by noxious stimulation. In deeply anesthetized (2.5 g/kg urethane) rats, VB relay neurons responded to tactile stimulation of their receptive fields, but scrotal warming produced no change in either EEG or activities of thalamic and hypothalamic neurons. These facts suggest that the responses of thalamic and hypothalamic neurons to scrotal warming may be 'non-specific'. Most thalamic and hypothalamic neurons showing switching responses did not appear to mediate specific information concerning scrotal skin temperature.     

Reactions of the neurons of the reticular and ventral anterior nuclei of the optic thalamus to afferent stimulation of different modalities]

Responses of 146 reticular (R) and 98 ventral anterior (VA) thalamic neurons to electrical stimulation of pads, to light flashes and sound clicks were studied in cats immobilized with d-tubocurarine or myorelaxine. The contralateral forepaw was the most effective receptive field: 24.9% of R and 31.3% of VA investigated neurons responded to its stimulation. Only 4.4% of R and 2.4% of VA neurons responded to the click. Almost all responding neurons reacted to different kind of the applied stimulation by phasic or tonic excitation. Inhibition of background activity was observed after the pads stimulation only in 2.6-4.3% of R and in 1.7%-2.1% of VA neurons. The latency of phasic responses in most neurons ranged: to electrical stimulation of the contralateral forepaw from 6 to 64 ms, to the contralateral hindpaw -- from 11 to 43 ms, to light -- 10-60 ms, and to the click -- 8-60 ms. 75.1-95.6% of R and 68.7-97.6% of VA neurons did not respond at all to different kinds of peripheral stimulation. Of a sample of cells tested to all inputs 25% of R and 47% of VA neurons responded to stimulation of more than one paw; 16% of R and 22% of VA neurons revealed convergence of volleys of different modality. The functional role of this convergence consists in inhibition (more seldom facilitation) of the neuronal response to a testing signal following 40-70 ms after a conditioning one.     

Modulatory influences of red nucleus stimulation on the somatosensory responses of cat trigeminal subnucleus oralis neurons

Little is known of the effect of red nucleus (RN) stimulation on somatosensory neurons despite its known anatomic projections to somatosensory relay nuclei. The effect of RN stimulation on the somatosensory responses of trigeminal subnucleus oralis (Vo) neurons was investigated in chloralose- or barbiturate-anesthetized cats. Arrays of bipolar stimulating electrodes were inserted into the contralateral and ipsilateral RN and the contralateral thalamus. Extracellular single-unit recordings were obtained in Vo with tungsten microelectrodes. Neurons in Vo were excited to just suprathreshold by electrical stimulation within their receptive fields. Red nucleus influences were studied by applying 100-ms, 500-Hz conditioning trains to the contralateral or ipsilateral RN 130 ms prior to the peripheral test stimulus. The effect of RN stimulation was also tested on mechanically evoked responses of Vo cells. The somatosensory responses of most cells (70/73) were inhibited after RN stimulation. Some of these cells (15/70) could be antidromically activated from the contralateral thalamus. Stimulation of the RN resulted in excitation followed by inhibition in nine Vo cells. The results suggest that the RN may modulate transmission of somatosensory information through Vo.     

Characterization of responses of primary somatosensory cerebral cortex neurons to noxious visceral stimulation in the rat

In pentobarbital-anesthetized rats, responses of single neurons in primary somatosensory cortex (SI) to graded noxious visceral (colorectal distention, CRD) and cutaneous stimulation were recorded. One-hundred fifteen SI neurons were identified on the basis of spontaneous activity, 66 of which responded to CRD. CRD resulted in facilitation of neuronal activity in 33% and inhibition of activity in 52% of these cells. Fifteen percent had mixed facilitated/inhibited responses to varying CRD pressures. Cutaneous receptive fields were identified in 71% of CRD-responsive neurons, with low-threshold or wide dynamic range responses in most cases. Nearly all cutaneous receptive fields were small contralateral sites. Responses to CRD were independent of neuronal depth within the cortex. These data support a role of primary somatosensory cerebral cortical neurons in visceral nociception.     

Response properties of distinct neuronal subsets in hindlimb sensorimotor cerebral cortex of the domestic cat

Extracellular recordings were obtained from 387 neurons within the hindpaw focus of sensorimotor cerebral cortex of cats anesthetized with chloralose. Histological study showed that the microelectrode penetrations were confined to cytoarchitectonic field 3b. Neurons were classified according to their responsiveness to supramaximal electrical shocks delivered 1/sec to the central footpad of each of the four limbs. About two-thirds of the sample consisted of small-field (sa) neurons, which responded to stimulation of the on-focus contralateral hindpaw but to no other paw; about one-third of the neurons were wide-field (m) neurons, responsive to shocking each of the four paws. In addition, five neurons could be driven by stimulation of either hindpaw but neither forepaw (sb neurons), and one neuron responded to shocking either contralateral paw but not the ipsilateral paws (sc neuron). The sa neurons were found throughout the depth of the cortex, but were concentrated in layers III and IV; the m neurons were distributed deeper in the cortex. Each neuronal subset exhibited behavior which was characteristic of sa neurons and m neurons studied elsewhere in sensorimotor cortex. Following an electrical stimulus to the contralateral hindpaw, the sa neurons discharged earlier than the m neurons; however, the m neurons fired with more spikes per discharge, had lower electrical stimulus thresholds, and were able to respond faithfully to higher iterative stimulus rates. The sa neurons were touch-and/or hair-sensitive, whereas the m neurons were primarily hair- or hair and touch-sensitive. Furthermore, both subsets showed marked variations in mean response latency with depth. The pattern of response latencies of the m neurons, produced by stimulation of each paw, differed considerably from what would be expected from the relative conduction distances involved. Many sa neurons could be inhibited by stimulation of the three off-focus paws, especially the forelimbs. The results indicate that the primary somatosensory (SmI) cortex hindpaw focus of field 3b is more similar in character to the primary motor (MsI) cortex forepaw focus of field 4γ than to the forepaw focus of field 3b. It is argued that the different cytoarchitectonic fields within sensorimotor cortex are not homogeneous with respect to either modality or receptive field size. Rather, distinct neuronal subsets seem to vary in proportion along “continuous” gradients through the different cytoarchitectonic fields.     

Intracellular staining of physiologically identified neurons and axons in the somatosensory thalamus of the cat

Neurons and axons responding to somesthetic stimulation in the thalamic ventrobasal complex (VB) were characterized electrophysiologically by intracellular recording and then individually injected with horseradish peroxidase. Two types of thalamocortical relay neuron were identified, primarily on the basis of dendritic morphology and axon diameter. Types with cutaneous or deep receptive fields were found in each class. Neither type had collateral axons in VB but each gave branches to the thalamic reticular nucleus (RTN). Small putative interneurons in VB and RTN neurons with somatosensory receptive fields were also injected. The RTN neurons had profusely branched widely ramifying axons in VB and adjoining nuclei. Injected medial lemniscal axons in VB had a range of receptive field properties and conduction velocities and ended in elongated anteroposterior domains with one or more dense concentrations of terminal boutons of varying size and with varying numbers of boutons.     

大鼠丘脑中央下核及其邻近结构神经元对皮肤伤害性刺激的反应

利用细胞个记录技术，在麻醉大鼠丘脑中央下核（Ｓｍ）及其邻近结构内，共记录到１９２个神经元７８％的神经元对皮肤伤害性刺激反应，其中，大多数（１１８）被兴奋，２８个被抑制，４个对某些部位的刺激兴奋，而对另一些部位的刺激抑制，发现某些神经元有很长的后发放效应，另一些则在兴奋之后跟随着较长的抑制期，少数的反应仅在刺激给与和撤除的瞬间发生，感受野大且呈双侧分部，８０％对机械刺激反应的神经元也对伤害性热刺激发     

Afferent properties of periarcuate neurons in macaque monkeys. I. Somatosensory responses

The afferent properties of single neurons of the periarcuate cortex have been studied in the macaque monkey. Most of the recorded neurons responded to stimuli in one or two sensory modalities and, accordingly, they were classified as somatosensory, visual or bimodal (visual and somatosensory) neurons. Visual neurons were located rostral to the arcuate sulcus, whereas the somatosensory and the bimodal neurons were found predominantly caudal to this sulcus.Somatosensory neurons (n = 102) and bimodal neurons (n = 69) had identical somatic afferent properties. They were subdivided into ‘tactile’ neurons, ‘joint’ neurons and ‘tactile and joint’ neurons. ‘Tactile’ neurons (70%) had their receptive fields formed either by one or by two or more spatially separated responding areas. The parts of the body most represented were the hands and the mouth. ‘Joint’ neurons (10%) were activated by the rotation of one or, more often, of two or more articulations. The movement of the hand towards the mouth was the most frequently represented movement. ‘Tactile and joint’ neurons (20%) responded to both tactile and joint stimulation having receptive field locations and properties like those of the other two classes of neurons. Some ‘joint’ and ‘tactile and joint’ neurons had summing properties, i.e. their response to tactile or joint stimulation was conditional upon a simultaneous stimulation of another articulation.The data are interpreted as evidence in favor of the existence of an area in the agranular cortex that organizes the mouth and the hand to mouth movements.