REORGANIZATION OF THE RACCOON CUNEATE NUCLEUS AFTER PERIPHERAL DENERVATION

The effects of peripheral nerve transection on the cuneate nucleus wer e studied in anesthetized raccoons using extracellular, single-unit re cordings. The somatotopic organization of the cuneate nucleus first wa s examined in intact, control animals. The cuneate nucleus in the racc oon is organized with the digits represented in separate cell clusters . The dorsal cap region of the cuneate nucleus contains a representati on of the claws and hairy skin of the digits. Within the representatio n of the glabrous skin, neurons with rapidly adapting properties tende d to be segregated from those with slowly adapting properties. The rep resentations of the distal and proximal pads on a digit also were segr egated. Electrical stimulation of two adjacent digits' provided a deta iled description of the responses originating from the digit that cont ains the tactile receptive field (the on-focus digit) and from the adj acent (off-focus) digit. Stimulation of the on focus digit produced a short latency excitation in all 99 neurons tested, with a mean of 10.5 ms. These responses had a low threshold (426 mu A). Stimulation of an off-focus digit activated 65% of these neurons. These responses had a significantly longer latency (15.3 ms) than on-focus responses and th e threshold was more than twice as large. Two to five months after amp utation of digit 4, 97 cells were tested with stimulation of digits 3 and 5. A total of 44 were in the intact regions of the cuneate nucleus . They had small receptive fields on intact digits and their responses to electrical stimulation did not differ from the control neurons. Th e remaining 53 neurons were judged to be deafferented and in the fourt h digit region on the basis of their location with respect to intact n eurons. All but two of these cells had receptive fields that were much larger than normal, often including more than one digit and part of t he palm. When compared with the off-focus control neurons, their respo nses to electrical stimulation had lower thresholds and an increased r esponse probability and magnitude. The latencies of these cells did no t decrease, however, and were the same as the off-focus control values . The enhanced responses of the deafferented neurons to adjacent digit stimulation indicate that there is a strengthening of synapses that w ere previously ineffective. The increased proportion of neurons that c ould be activated after amputation suggests that there is also a growt h of new connections. This experiment demonstrates that reorganization in the adult somatotopic system does occur at the level of the dorsal column nuclei. As a consequence, many of the changes reported at the cortex and thalamus may be due to the changes occurring at this first synapse in the somatosensory pathway.