Percutaneous stimulation of human corticospinal tract: a single-fibre EMG study of individual motor unit responses.

The technique of electrical stimulation of the brain through intact scalp was applied to activate the corticospinal tract (CST) of healthy human volunteers. Single motoneurone responses were picked up by a special (single fibre EMG) needle electrode. Our finding of relatively small latency variation of consecutive single motor unit responses seems to support the concept of predominantly monosynaptic transmission at the spinal level. The alternation between several discrete latencies seems to reflect a complexly shaped spinal motoneurone excitatory postsynaptic potential (EPSP), which could be due to either alternating direct/indirect (via other cerebral neurones) activation of the corticospinal tract (CST) neurones, or alternating mono- or oligosynaptic transmission at the spinal level, or multiple CST impulses in response to a single stimulus.