Reciprocal inhibition between forearm muscles during the cortical silent period

Transcranial magnetic brain stimulation (TCS) activates inhibitory mechanisms responsible for the temporary suppression of corticomotoneuronal output (i.e. the silent period). Spinal interneurones are controlled by the output cells of the motor cortex. We studied reciprocal inhibition between forearm flexor and extensor muscles during the silent period evoked by TCS in six normal subjects. Reciprocal inhibition was studied conditioning an H reflex in the flexor carpi radialis muscle with a stimulus over the radial nerve at the spiral groove. Radial stimuli were 20 ms before (presynaptic phase)and at the same time (disynaptic phase) of the median nerve stimulus. We have seen that both the disynaptic and the presynaptic phase of reciprocal inhibition, when they were studied during the silent period, were abolished (0 ms : control 52.8+5.8%, test 109.1+7.3%; 20 ms : control 70.2+14.5%, ~ test 117.2+25.3%)(mean+SD) . Our results suggest that the functional inactivation of the corticomotoneuronal pathways switches off spinal interneurons.