422 



S. J. JABBUR AND A. L. TOWE 



C-T INTERVAL IN MSEC. 



Fig. 3. Time course of the inhibitory interaction. Conditioning stimulus was 

 single 0-6 msec duration shock applied to the contralateral motor cortex at 

 various times before the peripheral testing stimulus. The time course is measured 

 as a decrease in probability of discharge of the first spike of the test discharge; 

 other measures yield a similar time course. 



the inhibitory effect, however, is greatly weakened. When, on the otiier hand, 

 only the pyramidal tract is transected at the same level, leaving the rest of 

 the brain stem intact, the excitatory effect can no longer be demonstrated, 

 but the inhibition still occurs, although it has been weakened. The behavior 

 of units driven by cortical stimulation suggests that the pyramidal tract fibers 

 excite them directly. The corticofugal inhibitory influence is less direct. 

 Perhaps the pyramidal and other projections from the cortex to the bulbar 



Fig. 4. Comparison of efficacy of contralateral and ipsilateral motor cortex in 

 inhibiting cuneate unit. A, c, d, f. Response to near threshold jpsilateral fore- 

 paw stimulation. Peripheral shock artifacts are marked with dot. L = 6 00 msec; 

 JJci = 1-8; probability of discharge, p = 10. b. Complete inhibition of test 

 response by stimulation of contralateral cortex, e. Incomplete inhibition of test 

 response by similar stimulation of the ipsilateral cortex. L = 20-75 msec (latency 

 of third spike, when present in unconditioned response was about 19 msec); 

 JJd == 100; response probability at illustrated C-T interval of 53 msec was 

 0-40. Note suppression of spontaneous activity during interaction. 



