SHORT-TERM PLASTICITY IN PRIMARY SOMATOSENSORY CORTEX OF THE RAT - RAPID CHANGES IN MAGNITUDES AND LATENCIES OF NEURONAL RESPONSES FOLLOWING DIGIT DENERVATION

Recordings were made from neurons in primary somatosensory (Sml) forep aw cortex of rats to study the time course of changes in responses beg inning immediately following denervation (ligation) of a single digit. Before denervation, neuronal receptive fields (RFs) defined by tactil e stimulation varied in size from small regions of one digit to larger areas covering several digits and palmar pads. With electrical stimul ation, most neurons responded best to one (on-focus) digit and less to other (off-focus) digits; on-focus stimulation yielded more spikes pe r stimulus and shorter spike latencies (L(min)) than did off-focus sti mulation. After ligation of the on-focus digit, most neurons showed in creased 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 ''ineffe ctive'' skin regions, such as digits or palmar pads adjoining the orig inal RF; (2) electrical stimulation usually evoked stronger responses from neighboring off-focus digits and sometimes elicited novel respons es from previously ineffective digits - seven of ten neurons showed in creases in spikes per stimulus, which tended to approach stable values within 60-90 min after denervation; three of ten neurons showed decre ases in L(min) with time, but most revealed no significant changes. Th ese results suggest that dynamic response properties, as well as RFs, of SmI cortical neurons can be modified rapidly by blocking afferent i nput from dominant on-focus skin regions. RFs expand and novel respons es appear, with concomitant increases in response magnitude and, in so me cases, decreases in response latency over time. These findings seem to reflect a rapid increase in synaptic efficacy of weak or previousl y ineffective inputs from cutaneous afferent nerve fibers.