A novel class of unit in the substantia gelatinosa of the spinal cat

Using platinum-tipped microelectrodes, 34 units of a kind not previously reported were found in the substantia gelatinosa of spinal, unanesthetized cats. Units were inhibited by innocuous mechanical stimuli, but after the withdrawal of such stimuli, a burst of activity lasting 5 to 20 s appeared with an initial delay (latent period) of 50 to 200 ms. In the absence of background activity, noxious stimuli had no immediate effect. However, in a period of about 20 min, units tested with noxious stimuli displayed a gradual increase in regular background firing and in the occurrence of unevoked bursts. Noxious stimuli were excitatory when applied to the units with this background activity. An approximately 1-s period of diminished activity followed withdrawal of noxious stimuli. Consecutive identical stimuli produced bursts of activity with considerable variability; latency and burst length fluctuated. Also the probability of bursts fluctuated in units with low background activity. Poststimulus time histograms showed, under some conditions, a decrease in the average number of impulses in the first 200 ms which roughly corresponded in time course to the negative dorsal root potential. It is argued that the initial latency and the decrease in activity in the histograms are temporal prolongations of the inhibitory process produced by innocuous stimuli. Moreover, based on these data, a mechanism for the generation of the negative dorsal root potential is proposed, involving dendroaxonic synaptic transmission from the substantia gelatinosa units to primary afferent terminals.