Cat Primary Somatosensory Cortex Research Articles

Distribution of neurons immunoreactive for calcium-binding proteins varies across areas of cat primary somatosensory cortex

The primary somatosensory (SI) cortex in the cat contains four cytoarchitectonic areas that appear to contain separate body representations and have different functions. We tested whether functional differences among these areas are reflected in the densities of neurons containing each of three calcium-binding proteins: parvalbumin (PV), calbindin (CB), and calretinin (CR). Colocalization experiments revealed that CR was localized in a population of neurons distinct from those containing PV or CB. The general laminar distributions of the three calcium-binding proteins were similar to those described in other species and cortical areas, but there were significant density differences in layers II and III across SI. The density of PV-immunoreactive neurons was higher in areas 3b and 1 than in areas 3a and 2. CB-immunoreactive neurons were found in higher densities in anterior SI than in posterior SI, and the pattern of CR-immunoreactive neurons was reciprocal to that of CB, with significantly higher densities in posterior regions of SI. Since the firing characteristics of nonpyramidal neurons appear to be related to their calcium-binding protein content, differences in regional distributions of these neurons in layers II and III may contribute to functional differences between the cytoarchitectonic areas of SI cortex.

Correlating cytoarchitecture and function in cat primary somatosensory cortex: the challenge of individual differences

Principles of organization for the primary somatosensory cortex are generalizations derived by examining data obtained in different individuals. The manner in which these data are combined influences the conclusions derived. We found the line representing the widest anteroposterior distance across the sigmoid gyrus to be a useful reference in the cat somatosensory cortex for combining and comparing electrophysiological and cytoarchitectonic data from different individuals when we constructed cytoarchitectonic and functional maps of the bank of the medial ansate sulcus; maps prepared from combined data sets had boundaries similar to those found among individuals. Nevertheless, we argue that, for reasons inherent to the nature of the cerebral hemispheres and cortical maps, such references will never allow combinations of data capable of defining a unique high resolution prototypical map of individual body parts; the somatotopic order of body representations is, as are certain other attributes of somatosensory cortex, idiosyncratic. The genetic, developmental and use-dependent reasons for this situation are discussed.

Cytoarchitecture and responsiveness of the medial ansate region of the cat primary somatosensory cortex.

To understand its relationship to somatosensory areas in other species, we studied the rostral bank of the medial ansate sulcus in adult cats. Neurons in the shoulder and upper part of the sulcal wall responded to low-threshold cutaneous stimuli much like neurons on the crown of the gyrus, whereas neurons in some deeper portions of the sulcus required more intense but innocuous somatic stimuli. Because we found much of the body surface re-represented in this area, we suggest that, besides the representation in area 3b, there is another cutaneous representation of the hindlimb and trunk located on the gyral crown near the medial end of the medial ansate sulcus and of the forelimb and trunk within the medial ansate sulcus. Posterior to this second cutaneous representation, many parts of the body were also represented in regions activated by more intense stimuli and having a different cytoarchitecture, suggesting that they were part of another body representation. Area 3b and the shoulder of the gyrus were distinguished by relatively intense acetylcholinesterase staining of layers III and IV. In the wall of the sulcus, all layers except layer I were uniformly stained to a point where electrophysiological recordings showed the cortex to be unresponsive, whereupon the outer two-thirds of layer I became very pale. Neurons activated by afferents from knee joints were found only in a small area; we did not find a mediolateral band serving joint afferents as is reported in primates. These data suggest that cat somatosensory cortex differs in some ways from primates but that it contains multiple representations of the body, as do most other mammals.

Responses of rapidly adapting neurons in cat primary somatosensory cortex to constant-velocity mechanical stimulation

1. The responses of rapidly adapting (RA) neurons to constant-velocity ramp stimulation were studied in the forepaw representation of primary somatosensory cortex (SI) of anesthetized cats. Single probe stimuli were used to indent the skin or to move hair parallel to the skin surface. The velocity of the moving stimulus probe was varied to determine the rate sensitivity of the neurons. 2. The cortical RA neurons were classified into four categories identified as G1/F1, Gint/Fint, G2/F2, and complex classes. The primary bases for classification in the present experiments were the pattern of response during ramp stimulation, velocity threshold, and directional sensitivity. 3. Of the RA neurons recorded in SI, 84% (49/58) could be assigned to one of the three response classes with little ambiguity. The remaining neurons showed more complex responses. The form of the complex responses suggested that they arose from a combination of inputs of different response classes. Some of these appeared to arise from a combination of different RA input classes, whereas others had components that resembled responses previously described for C mechanoreceptors. 4. Increased ramp velocity resulted in increased average firing frequency in 87% of the RA neurons. This relationship, which could be fitted with a power function, varied with response class. G1/F1 neurons were more sensitive to stimulus rate than G2/F2 neurons. Significant differences between response classes also were seen in the relationship between ramp velocity and their number of evoked action potentials and in their spontaneous firing rates. 5. The results demonstrate that a discrete SI neuron population is sensitive to the rate of stimulus movement. This observation is consistent with psychophysical studies reporting effects of stimulus indentation rates on perception of single probe stimuli. The appearance of complex responses in a small proportion of SI neurons provides evidence of convergence in somatosensory pathways to SI.

Distribution and proportions of GABA-immunoreactive neurons in cat primary somatosensory cortex.

Certain receptive field properties of cortical neurons depend upon inhibitory, GABAergic inputs. In the somatosensory cortex, iontophoresis of bicuculline, a GABAA receptor blocker, results in enlargement of receptive fields. However, bicuculline's effectiveness in changing receptive field size varies with the neuron's adaptation characteristics, location within a particular submodality region, and laminar location. To test whether regional differences in the effectiveness of bicuculline are correlated with the distribution of cortical GABAergic neurons, we determined the numbers and proportions of GABA-immunoreactive [GABA(+)] neurons within cat primary somatosensory cortex. The laminar distribution of GABA(+) neurons was similar across all four cytoarchitectonic areas of primary somatosensory cortex, with layer II containing the highest areal density of GABA(+) neurons. Numerical proportions of GABA(+) neurons in the total neuron population were similar in areas 3b and 2 (29.8% and 22.6%, respectively). Laminar distributions of the proportions of GABA(+) neurons were also similar in these two areas; in both areas, layer I contained the highest proportion of GABA(+) neurons. The laminar distributions of GABA(+) neuron densities as well as GABA(+) neuron proportions differed from the reported laminar distribution of bicuculline effects on receptive field size. Moreover, within area 3b, these measures showed no evident patterns that might correspond to rapidly adapting and slowly adapting submodality regions.

Regional distribution of GABAA receptor binding sites in cat somatosensory and motor cortex.

Inhibition in primary sensory cortex plays a role in neuronal responses to peripheral stimuli. For many neurons in cat primary somatosensory cortex, blockade of GABAA receptors by bicuculline results in receptive field enlargement. The magnitude of this effect varies with the neuron's adaptation characteristics and its location in particular laminae and submodality regions. To test whether these variations are correlated with the distribution of GABAA receptors, we analyzed [3H]muscimol binding in cat primary somatosensory and motor cortical areas. The highest levels of binding were in layers I-III, and the lowest levels were in layers V-VI. In somatosensory cortical areas, layer IV was distinguished by higher levels of binding than in adjacent layers. Within layer IV, levels of binding were significantly higher in posterior area 3b than in anterior area 3b. These differences may correspond to the rapidly adapting and slowly adapting submodality regions which have been described in this area. The laminar distribution of [3H]muscimol binding differed from that of [3H]flunitrazepam, and neither resembled the distribution of the magnitude of bicuculline's effects on receptive field size. The laminar distribution of [3H]muscimol binding was highly correlated with the areal density of GABA-immunoreactive neurons described in a companion study.

Corticocortical connections of cat primary somatosensory cortex.

The organization of corticocortical connections in the representation of the forepaw in cat primary somatosensory cortex (SI) was studied following injections of various tracers into different cortical cytoarchitectonic areas. Small injections of horseradish peroxidase, wheat germ agglutinin-conjugated HRP, Phaseolus vulgaris leukoagglutinin, or fast blue were placed into the representation of the forepaw in areas 3b, 1, or 2. The positions of labeled neurons in SI and the surrounding cortical areas were plotted on flattened surface reconstructions to determine the organization of the corticocortical connections within SI. A strong, reciprocal projection linked the two forepaw representations which have been described in area 3b and the part of area 2 which lies in the anterior bank of the lateral ansate sulcus (see Iwamura and Tanaka 1978a, b). Dense projections also linked these areas with SII, as previously reported (Burton and Kopf 1984a). Additional projections to area 3b arose primarily from areas 3a and 1. Projections to area 2 were more widespread than those to area 3b, and arose from all other areas of SI as well as from areas 4 and 5a. All injections into SI tended to label groups of neurons which lay in mediolateral strips. Corticocortical projection neurons which were most heavily labeled by SI injections were pyramidal cells in layer III. Additional projections from area 2 to 3b, area 5a to 2, and SII to areas 2 and 3b arose from layer VI as well. Although neurons of layers III and VI were always the most densely labeled, large injections into SI labeled neurons in layers II and V as well.

Patterns of connectivity in the cat sensory‐motor cortex: A light and electron microscope analysis of the projection arising from area 3a

Through its complex network of interconnections, each of the functionally specialized subdivisions in the cat primary somatosensory cortex may provide the motor cortex with different input and thus play a unique role in motor behavior. Areas 2 and 3a receive separate peripheral information. Cells in both regions project directly to the cat motor cortex but are thought to target different populations of neurons. In this study the morphology and distribution patterns of the area 3a projection to the motor cortex were compared to previous findings on the projection from area 2. Also, details of the projection from area 3a to area 2 were studied. Injections of Phaseolus vulgaris leucoagglutinin were made into area 3a and fixed brain tissue was processed for immunohistochemical staining of this anterograde tracer. Tissue was examined with the light microscope to determine the patterns of the 3a projections to area 2 and motor cortex, area 4. Axons arising from cells in area 3a terminated in multiple column-like clusters in both motor cortex and area 2. The small number of cells labeled at the injection sites suggested that multiple foci of the 3a fibers are formed by collateral axonal branches of the same neurons. The topography of the projection from area 3a to the motor cortex was more clearly defined than that from area 3a to area 2. Electron microscope analysis determined the laminar distribution and types of synapses formed between area 3a efferents and their target neurons. A high proportion of synapses was found in layer III in both target regions. However, unlike in area 4, labeled terminals were sparse in laminae I and II of area 2. Axospinous synapses were slightly more numerous than axodendritic synapses, but both were distributed similarly throughout the thickness of the cortex. In area 2 the axospinous synapses accounted for 63% of synapses and in area 4 for 57% of synapses. No axosomatic synapses were detected.

Benzodiazepines and synaptic processing in the spatial domain within the cat's primary somatosensory cortex

In the primary somatosensory cortex of cats, the size of the receptive fields (RFs) of cutaneously responsive neurones is under the control of gamma-aminobutyric acid (GABA) mediated inhibition when the cells are situated in rapidly adapting (RA) background regions. Cells located in slowly adapting (SA) or low-velocity rapidly adapting (LVRA) background regions do not appear to be affected by GABA significantly in the spatial domain, although other response properties such as threshold and firing pattern are under the influence of bicuculline methiodide (BMI) sensitive processes. The GABA receptor is one component of the oligomeric complex that includes the benzodiazepine (Bzd) binding site, the barbiturate recognition site, and the Cl- ionophore. Owing to current debates about the possible existence of endogenous ligands of Bzd receptors, we have examined whether Bzd agonists, in addition to GABA and BMI, have RF-modulating actions on RA S1 neurones and have assessed the effectiveness of the Bzd antagonist, Ro 15-1788, in this experimental paradigm. Ro 15-1788 is an imidazobenzodiazepine that acts as a specific competitive antagonist of Bzds by exerting high-affinity interactions with that Bzd receptor through which anticonvulsant effects of flurazepam (flu) and diazepam are expressed. This has been shown previously in neurochemical, behavioral, neurological, and pharmacological studies. Ro 15-1788 has little or no affinity for nonneuronal binding sites in the CNS. Ro 15-1788 binding does not displace GABA from its own binding site but does compete for all major Bzd ligands that act as pharmacological agonists and inverse agonists of the Bzd receptor through which anticonvulsant and convulsant effects are expressed. Bzd agonists elevated the threshold for somatic activation, depressed spontaneous activity, and decreased RF size. One exception in this regard was midazolam, which sometimes decreased somatic thresholds and increased spontaneous discharges. These latter effects were reversed at higher doses of the agonist. BMI returned RFs to control sizes when the drug was administered concurrently with Bzd agonists, or it caused RFs to assume greater than normal sizes, depending on the strength of current ejecting the antagonist. Ro 15-1788 given alone decreased response thresholds, increased spontaneous firing, and sometimes enlarged RFs. This antagonist also reversed the RF size-decreasing action of flu, diazepam, and midazolam. Quantitative analyses of air-puffer responses evoked from low-threshold, S1 cells revealed that Bzds do not selectively attenuate spatial summation, but that they act preferentially in the surround, or in the peripheral, regions of cutaneous excitatory RFs.(ABSTRACT TRUNCATED AT 400 WORDS)

The mode of synaptic activation of pyramidal neurons in the cat primary somatosensory cortex: an intracellular HRP study

A total of 141 pyramidal neurons in the cat primary somatosensory cortex (SI) were recorded intracellularly under Nembutal anesthesia (7 in layer II, 43 in layer III, 8 in layer IV, 58 in layer V and 25 in layer VI). Most neurons were identified by intracellular staining with HRP, though some layer V pyramidal neurons were identified only electrophysiologically with antidromic activation of medullary pyramid (PT) or pontine nuclear (PN) stimulation. Excitatory synaptic potentials (EPSPs) were analyzed with stimulation of the superficial radial nerve (SR), the ventral posterolateral nucleus (VPL) in the thalamus and the thalamic radiation (WM). The pyramidal neurons in layers III and IV received EPSPs at the shortest latency: 9.1 +/- 2.1 ms (Mean +/- S.D.) for SR and 1.6 +/- 0.7 ms for VPL stimulation. Layer II pyramidal neurons also responded at a short latency to VPL stimulation (1.7 +/- 0.5 ms), though their mean latencies for SR-induced EPSPs were relatively longer (10.6 +/- 1.9 ms). The mean latencies were much longer in layers V and VI pyramidal neurons (10.2 +/- 2.4 ms and 2.9 +/- 1.5 ms in layer V pyramidal neurons and 9.9 +/- 2.5 ms and 2.8 +/- 1.6 ms in layer VI pyramidal ones, respectively for SR and VPL stimulation). The comparison of the latencies between VPL and WM stimulation indicates that most layer III-IV pyramidal neurons and some pyramidal cells in layers II, V and VI received monosynaptic inputs from VPL. These findings are consistent with morphological data on the laminar distribution of thalamocortical fibers, i.e., thalamocortical fibers terminate mainly in the deeper part of layers III and IV with some collaterals in layers V, VI and II-I. The time-sequences of the latencies of VPL-EPSPs indicate that corticocortical and/or transcallosal neurons (pyramidal neurons in layers II and III) fire first and are followed by firing of the output neurons projecting to the subcortical structures (pyramidal neurons in layers V and VI).

The distribution of intrinsic cortical axons in area 3b of cat primary somatosensory cortex.

The morphology of single neurons in area 3b of cat primary somatosensory (SI) cortex was examined after horseradish peroxidase (HRP) injections. Neurons were labeled either by intracellular injection of HRP following intracellular recording or by small extracellular iontophoretic HRP injections. Both pyramidal and nonpyramidal neurons were labeled and reconstructed from serial sections. Their axons had local, interlaminar and interareal patterns of termination. Most neurons formed local axonal fields around their cell bodies and dendrites. Pyramidal neurons in cortical layer IV sent axons up into layers II and III, neurons in layers II and III sent axons down to layer V, and layer V neurons sent axons to layer VI as well as back to the upper layers. Layer VI neurons sent axons back to the upper cortical layers in a unique bowl-shaped pattern. The horizontal distribution of axons of pyramidal cells in layer III was extremely widespread. Axons of layer III neurons in area 3b terminated within 3b and area 1, but not in other areas of SI. Layer III neurons in area 1 distributed axon collaterals to all fields of SI as well as projecting a main axon to motor cortex. In general, the axon collaterals of area 3b pyramidal cells outside layer III remained confined to area 3b. Most of the nonpyramidal neurons labeled were basket cells in layers III and VI. These neurons formed dense axonal fields around their cell bodies, and none of their axons could be followed into the underlying white matter. The results of the present study demonstrate that area 3b somatosensory cortical neurons and their axons are vertically organized in a manner similar to that reported for other sensory cortical areas. They also show that widespread horizontal connections are formed by pyramidal neurons of layer III, and that these horizontal axons can travel for great distances in the cortical grey matter.

Three-dimensional computer-aided reconstruction of non-pyramidal neurons in the cat primary somatosensory cortex (SI)

Three-dimensional computer-aided reconstruction of non-pyramidal neurons in the cat primary somatosensory cortex (SI)

Short latency activation of local circuit neurons in the cat somatosensory cortex

Intracellular horseradish peroxidase (HRP) study was performed in the cat primary somatosensory cortex (SI) under Nembutal anesthesia. Response properties of neurons were analysed with stimulation of the peripheral nerve (superficial radial nerve; SR) and thalamic ventrobasal nucleus (VB). A total number of 23 cells (15 in layers III and IV, 6 in layer V and 2 in layer VI) were identified morphologically as local circuit neurons with intracellular HRP staining. The latencies of SR-induced (7.7–8.5 ms) as well as VB-induced excitatory postsynaptic potentials (EPSPs) (1.3–1.5 ms) were significantly shorter than those of pyramidal neurons (9.1–10.6 ms for SR and 1.6–2.8 ms VB EPSPs). Morphological features of identified local circuit neurons are all, except one in layer VI, aspiny and presumed to be inhibitory in nature. The present study indicates that presumed inhibitory interneurons in the cat SI could be activated first by thalamic inputs among cortical neurons and set to inhibit the output cells for the sharp contrast in the sensory processing.

Organization and synaptic relationships of the projection from the primary sensory to the primary motor cortex in the cat.

It is known from previous studies that fibers originating from cells in area 2 of the cat primary somatosensory cortex project topographically to area 4 of the motor cortex and that they terminate preferentially in the caudal region of the cruciate sulcus. We examined this pathway to determine more precisely the distribution pattern of fibers and the laminar arrangement of axon terminals in the motor cortex. The recently developed technique of PHA-L staining enabled us to label anterogradely the axons that form this projection. Iontophoretic injections of PHA-L were made into the rostral bank of the ansate sulcus (area 2). After 7 days the cats were perfused and the tissue was processed immunohistochemically to stain the PHA-L filled fibers. Light microscopic examination revealed that a small cluster of cells in the sensory cortex gave rise to multiple foci of labeled axons in area 4. The labeled fibers formed columnlike arrays, which were located for the most part in the posterior bank of the cruciate sulcus and were separated by irregular intervals of cortex devoid of labeled fibers. Clusters of labeled fibers were also found in the anterior region of the cruciate sulcus in some of the animals. The dimensions of the labeled areas and the small number of cells that gave rise to each group of fibers suggested that axonal branches of cells within the injection site formed the multiple foci. Variations in the immunohistochemical staining enabled us to study the laminar distribution of sensory cortex axon terminals with the electron microscope. Whereas some PHA-L labeled terminals were found in the deep cortical layers, the majority (82%) were spread throughout layers I-III. Differences in the laminar distribution of sensory cortex afferents that formed axodendritic or axospinous synapses were noted. Synapses formed with dendritic shafts were relatively sparse (28%) and were confined to the superficial layers. Some of the more numerous axospinous synapses, which accounted for 72% of identified synapses, were found in layers V and VI, although most were in layers I-III. The distribution pattern of terminals showed little variation between columns in different areas of the motor cortex, including that in the anterior cruciate region. The pattern of termination of the sensory to motor cortex projection is discussed in relation to the physiological characteristics of this pathway.

Baclofen and γ-aminobutyric acid differentially suppress the cutaneous responsiveness of primary somatosensory cortical neurones

Extracellular recordings from neurones in the cat's primary somatosensory cortex (S1) have been made with carbon fibre-filled multibarrel pipettes used for microiontophoresis. γ-Aminobutyric acid (GABA) and baclofen elevated the thresholds to tactile and airpuffer stimulation, reduced the sizes of cutaneous receptive fields (RFs), depressed spontaneous activity and decreasd the magnitudes of thalamically evoked responses. However, the manner in which baclofen produced its alterations in response properties could be differentiated from that of GABA in several respects. Specially, responses which reflected spatially integrated driving of cutaneous RFs across peripheral and central regions were suppressed more readily by GABA than by baclofen. Furthermore, baclofen was observed to exert suppressions of responses which were evoked from the peripheral regions of cutaneous RFs more effectively than it could those responses which were evoked from the central regions. GABA affected central and peripheral RF subregions relatively indiscriminately. These results suggest that not only bicuculine-sensitive processes, but also those activated by baclofen, are involved in controlling the sensitivity of S1 cortical neurones to afferent stimuli.

Response properties and morphological features of nonpyramidal neurons in the cat primary somatosensory cortex(SI): An intracellular HRP study

Response properties and morphological features of nonpyramidal neurons in the cat primary somatosensory cortex(SI): An intracellular HRP study

Acetylcholine permits long-term enhancement of neuronal responsiveness in cat primary somatosensory cortex

Acetylcholine (ACh) was administered iontophoretically to single neurons in cat somatosensory cortex. Using extracellular recording techniques, neuronal responsiveness was determined at regular intervals from the number of action potentials produced either by iontophoretically applied glutamate or by tactile stimulation of the cutaneous receptive field. The responses were altered in only 21% ( 1361) of the neurons following the application of ACh alone. In contrast, 75% ( 6688) of the neurons displayed altered responses during administration of ACh simultaneously with either iontophoretically administered glutamate or with tactile stimulation of the receptive field. Forty-seven percent ( 2962) of the responses potentiated in the presence of ACh remained enhanced for periods lasting from 8 min to over 1 h. The responsiveness of cortical neurons to afferent inputs changes during the reorganization of somatotopic maps that occurs after deafferentation, and perhaps during some forms of learning. As ACh has been implicated in some of these processes, it may be that the changes in responsiveness observed here following iontophoretically applied ACh are similar to those which facilitate the acquisition of neuronal responses to altered or novel afferent inputs.

A functional role of GABA/benzodiazepine receptors in cat primary somatosensory cortex

A functional role of GABA/benzodiazepine receptors in cat primary somatosensory cortex

Identification of two populations of corticothalamic neurons in cat primary somatosensory cortex.

Extracellular and intracellular recordings of corticothalamic (CT) cells were performed in the primary somatosensory cortex of the cat. CT neurons were antidromically activated by electrically stimulating the ventroposterior lateral (VPL) nucleus of the thalamus and were classified into two types according to their physiological properties. Type 1 had no spontaneous activity and no identifiable somatic receptive field. Type 2 fired action potentials spontaneously and responded to mechanical stimulation of the skin or underlying tissues. Axonal conduction velocities were slower for type 1 cells and their cell bodies were located slightly deeper in the cortex than those of type 2 cells. Both types of CT neurons exhibited inhibitory postsynaptic potentials in response to VPL stimulation but an early synaptic excitation and rebound discharge was observed almost exclusively in type 2 cells. These results suggest that only type 2 CT cells can modify the activity of thalamic neurons through a corticothalamic feedback loop.

Functional role of GABA in cat primary somatosensory cortex: shaping receptive fields of cortical neurons.

Extracellular recordings of 209 neurons were obtained with carbon fiber-containing multibarrel micropipettes. The cells were isolated in the primary somatosensory cortex of cats anesthetized with barbiturate and classified according to the nature of their response to natural stimuli, the nature of the surrounding multiunit responses to the same stimuli, the response to thalamic stimulation, and their depth in the cortex. To study factors controlling the excitability of somatosensory neurons, their receptive fields were examined in the presence of iontophoretically administered gamma-aminobutyric acid (GABA), glutamate, and bicuculline methiodide (BMI). Even when the neurons were depolarized to perithreshold levels with glutamate, or when local inhibitory influences mediated by GABA were antagonized by BMI, the apparent specificity for one class of afferent input was maintained. Neurons responding to stimulation of either cutaneous or deep receptors maintained their modality specificity, and neurons in cutaneous rapidly adapting regions never took on slowly adapting properties. When ejected at currents that did not elicit action potentials, glutamate lowered the threshold for activation by cutaneous stimuli but did not enlarge the receptive field. With larger ejecting currents, the neurons developed an on-going discharge, but even at these higher doses, glutamate did not produce an increase in the receptive-field size. Some neurons in regions of cortex exhibiting slowly adapting multiunit responses were relatively insensitive to glutamate. These cells required four to five times more glutamate to evoke discharges than did most neurons. Other cells, previously unresponsive to somatic stimuli, could be shown to possess distinct cutaneous receptive fields when either glutamate or BMI was ejected in their vicinity. Iontophoretically administered BMI altered the firing pattern of somatosensory neurons, causing them to discharge in bursts of 3-15 impulses. BMI enlarged the receptive-field size of neurons in regions displaying rapidly adapting multiunit background discharges but not in those regions with slowly adapting multiunit discharges. This differential effect of BMI, suggesting that GABA controls receptive-field size in rapidly adapting regions, also indicates that neurons in rapidly adapting regions differ pharmacologically from those in other submodality regions. In all cortical regions, BMI blocked the poststimulus inhibitory period that normally followed thalamic stimulation.(ABSTRACT TRUNCATED AT 400 WORDS)