The rat gracile nucleus in vitro: III. Unitary spike potentials and their conditioned inhibition

Unitary spike potentials were recorded from the gracile nucleus in vitro following stimulation of the ipsilateral dorsal column. All of the unitary spike potentials were thought to be post-synaptic in origin because of their latencies and their type of response pattern to increasing the stimulus strength from threshold. Two main classes of response pattern were observed which were analogous to those of the relay cell and interneurone recorded in vivo. A further type of response pattern was observed but its anatomical substrate was not deduced. When examining the unitary spike potentials evoked by the second of a pair of identical stimuli an increase in the latency and/or decrease in the number of spikes was observed. This inhibitory period consisted of an initial intense phase followed by a less intense and long-lasting phase. At conditioning intervals of less than 40 msec this inhibition was sensitive to bicuculline. Only 13% of the units encountered in this study were spontaneously active. The activity of these units was depressed following stimulation of the dorsal column. The same inhibitory phenomena observed to act on the field potentials in this slice also act on unitary spike potentials.