Convergent inputs to single neurons in two different subdivisions of somatosensory forepaw digit cortex of the raccoon

Abstract

The aim of this study was to compare the physiological properties of single neurons in the glabrous (G) and heterogeneous (H) subdivisions of primary somatosensory digit 3 cortex of adult raccoons. Extracellular recordings were obtained from 50 G neurons whose receptive fields (RFs) were confined to the glabrous skin of a digit, and 41 H neurons whose RFs were located on hairy skin, claws, or mixtures of skin types. Both electrical and mechanical stimulation of the digits were used to assess excitatory neuronal responsiveness. The two sets of neurons, which had nearly identical depth distributions, differed considerably in their input convergence: (i) the percentage of neurons (%N) responding to electrical or mechanical stimulation of each off-focus digit and (ii) the number of digits from which individual cells could be driven were significantly greater for H neurons. Those G and H cells which could be excited by off-focus inputs were examined for probability of response (P), number of spikes per response (S/R), and latency of response (L) to digit stimulation. Surprisingly, for input from any one digit, there were no significant differences in these response properties between the two sets of neurons. However, inputs from different (on-focus versus off-focus) digits varied significantly and revealed patterns of response properties that were qualitatively similar for both G and H neurons. Specifically, %N and P decreased while L increased symmetrically with distance of each off-focus digit from the central on-focus digit 3, reflecting corresponding variations in the synaptic accessibility and conduction time of off-focus excitatory inputs. In contrast, S/R values were very similar for all digits, suggesting that the synaptic strength of off-focus inputs is regulated independently of accessibility. Finally, preliminary findings indicated that denervation of the third digit caused a decrease in off-focus response latencies, while the normal latency profile across digits was retained. This suggests that the previously existing pattern of off-focus inputs to G and H neurons provides a template for denervation-induced cortical reorganization, whereby the synaptic efficacy of off-focus inputs is increased by disinhibition or facilitation.