The relations between long-latency reflexes in hand muscles, somatosensory evoked potentials and transcranial stimulation of motor tracts.

In 15 normal subjects the latency of electrically elicited long-latency reflexes (LLRs) of thenar muscles was compared with somatosensory evoked potentials (SEPs) after median nerve stimulation and with the latencies of thenar muscle potentials after transcranial stimulation (TCS) of the motor cortex. Assuming a transcortical reflex pathway the intracortical relay time for the LLR was calculated to be 10.4 +/- 1.9 msec (mean +/- S.D.) or 8.1 +/- 1.6 msec depending on the experimental conditions. The duration of the cortical relay time is not correlated with the peripheral or central conduction times, with body size or arm length. If the LLRs of hand muscles are conducted transcortically the long duration of the cortical relay time suggests a polysynaptic pathway.