THE DORSAL COLUMN NUCLEI 



421 



Fig. 2. Evoked and spontaneous cuneate unit activity inhibited by stimulation of 

 the contralateral motor cortex. Peripheral shock artifacts are marked with dot. 

 A {left) — Response of unit to maximal ipsilateral forepaw stimulation. Z = 7-48 

 msec; 7fcl = A-Q; Jf = 250 /sec. Lower sweep shows the 40 msec following 

 peripheral stimulation. b1 and 3. Response to near threshold peripheral stimulus 

 intensity before and after a conditioning interaction. T ^ 7-49 msec; sjd = 2-85. 

 b2. Inhibition of peripherally evoked response by 84 msec train of 312/sec 

 shocks to the contralateral motor cortex beginning 106 msec before the testing 

 input. L = 7-47 msec; JJc^ = 1-09. c and d. Continuous film of same interac- 

 tion, showing effect on spontaneous activity, e. Conditioning pulse durations 

 increased from 0-1 msec to 0-5 msec resulting in longer suppression of the 

 spontaneous activity. Lower tips of spikes in c, d and E are indicated by dots. 



that between peripheral inputs. It is sufficiently powerful, however, to block 

 the neuron discharge produced by a tactual stimulator or by maintained 

 pressure. 



The direction of the changes of response latency and the number of spikes 

 in each discharge during interaction are consistent with the interpretation of 

 an inhibition as defined in another Chapter in this volume (p. 410). Recourse 

 to this argument is not always necessary, however, for inhibition is readily 

 demonstrated in units excited monosynaptically from the skin; an occlusive 

 type of interaction is not possible in these instances. 



When the brain stem has been transected at the level of the inferior olivary 

 nucleus so that only the pyramidal tracts remain intact (Fig. 5), then both 

 cortical excitation and cortical inhibition of cuneate neurons can be obtained; 



