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With few exceptions, afferent neurons in the various sensory systems respond to wide ranges of stimuli. In those sensory systems for which the stimulus dimensions are understood, the response functions of these neurons may be described; they are usually simple functions with one maximum, although many variations exist. In the chemical senses, the stimulus dimensions are not known, and thus the neural response functions of these neurons have never been described. The present paper presents methods to determine these response functions and the stimulus dimensions for the chemical senses. A tentative response function for taste is developed, and preliminary steps are taken toward disclosing the stimulus dimensions.

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The functional significance of reorganization in somatosensory cortex following peripheral denervation has not been thoroughly addressed. In this paper, two distinct hypotheses dealing with this issue are discussed. The first is the hypothesis of functional respecification. This influential view suggests that sets of partially deafferented cortical neurons, which respond to new peripheral inputs and acquire new receptive fields, undergo corresponding changes in perceptual meaning. Excitation of these neurons by stimulation of their novel receptive fields is thought to result in a change in referral of sensation from the original (now denervated) skin fields to the newly acquired skin fields. The second hypothesis is that of functional conservation. This equally plausible alternative is that sets of partially deprived neurons, although they respond to novel peripheral inputs, retain their original perceptual meaning. Excitation of these neurons by stimulation of their new receptive fields is thought to evoke sensation formerly mediated by those neurons, and hence is still projected to the original, now denervated skin regions or phantom. Behavioral evidence strongly suggests that cortical reorganization after peripheral denervation does not result in major functional respecification, but that the original perceptual function mediated by those neurons is preserved.

The aim of this study was to compare the intrinsic intracortical connectivities of 2 functionally distinct subdivisions of the somatosensory (Sml) forepaw cortex of the raccoon--the somatotopic glabrous skin representation and the more heterogeneous, hairy skin and claw representation of the digits. HRP was injected into one or the other functional subdivision of a particular digit subgyrus of Sml cortex in 10 adult raccoons. The distribution of HRP-labeled neurons and axon terminals in the cortex showed that intrinsic "horizontal" connections exist within and between individual cortical digit zones; the labeling tended to have an oval-shaped configuration that was longer in the mediolateral than in the anteroposterior curvilinear plane. The 2 cortical sectors were found to have different patterns of intracortical projections. The connections of the glabrous skin region of each cortical digit zone were primarily local and confined to that same digit representation. HRP-filled neurons were concentrated near the injection site and decreased in density within the banks and fundi demarcating the injected digit subgyrus; few labeled cells were found in adjoining digit zones. Longer projections to the glabrous subdivision of a particular digit area typically originated from neurons in the heterogeneous subdivision of that same digit area. In contrast, the connections of the heterogeneous region of each digit zone were much more extensive and usually included projections from nonadjacent, as well as neighboring digit zones. The density of HRP-positive neurons declined more gradually with distance from the injection site, and considerable labeling was present in the heterogeneous sectors of adjacent digit zones. The intracortical projections of both functional subdivisions were often, but not always, reciprocal, and the cells of origin tended to be distributed in clusters. The laminar distributions of labeled neurons were similar for both sectors; HRP-filled cells were concentrated more in the supragranular layers, especially in layer III; fewer were found in the infragranular layers, mainly in layer VI and rarely in layer V. These results show that the intrinsic connections of the glabrous cortical subdivisions are fairly localized, whereas those of the heterogeneous cortical subdivisions are more diffuse and highly convergent. The differing intracortical connectional patterns of the 2 sectors are consistent with their contrasting thalamocortical projection patterns and may contribute to the unique functional properties of neurons located within each sector.(ABSTRACT TRUNCATED AT 400 WORDS)

Recordings were made from neurons in primary somatosensory (SmI) forepaw cortex of rats to study the time course of changes in responses beginning immediately following denervation (ligation) of a single digit. Before denervation, neuronal receptive fields (RFs) defined by tactile stimulation varied in size from small regions of one digit to larger areas covering several digits and palmar pads. With electrical stimulation, most neurons responded best to one (on-focus) digit and less to other (off-focus) digits; on-focus stimulation yielded more spikes per stimulus and shorter spike latencies (Lmin) than did off-focus stimulation. After ligation of the on-focus digit, most neurons showed increased responsiveness to stimulating one or several off-focus digits and palmar regions of the forepaw: (1) tactile stimulation showed that the RFs of all but one neuron expanded to include previously "ineffective" skin regions, such as digits or palmar pads adjoining the original RF; (2) electrical stimulation usually evoked stronger responses from neighboring off-focus digits and sometimes elicited novel responses from previously ineffective digits--seven of ten neurons showed increases in spikes per stimulus, which tended to approach stable values within 60-90 min after denervation; three of ten neurons showed decreases in Lmin with time, but most revealed no significant changes. These results suggest that dynamic response properties, as well as RFs, of SmI cortical neurons can be modified rapidly by blocking afferent input from dominant on-focus skin regions. RFs expand and novel responses appear, with concomitant increases in response magnitude and, in some cases, decreases in response latency over time. These findings seem to reflect a rapid increase in synaptic efficacy of weak or previously ineffective inputs from cutaneous afferent nerve fibers.