MECHANISMS UNDERLYING SPINAL MOTOR-NEURON EXCITABILITY DURING THE CUTANEOUS SILENT PERIOD IN HUMANS

The transient suppression of muscle contraction during the cutaneous s ilent period (CSP) could be produced either through postsynaptic inhib ition of motoneurons or through presynaptic inhibition of the excitato ry inputs to motoneurons that sustain voluntary contraction. We sought to delineate the mechanisms underlying the CSP in hand muscles by mea suring changes in H-reflexes and motor-evoked potentials (MEPs) produc ed by transcranial magnetic stimulation (TMS) during the CSP in 10 hea lthy volunteers. H-reflexes and MEPs both measure the excitability of the motoneuron pool and activate similar subpopulations of motoneurons through different pathways. Inhibition of H-reflexes and MEPs of simi lar size was maximal at the midpoint of the CSP and gradually returned to baseline. The similar time course of recovery suggests that the H- reflex and MEP are affected by inhibition at a common site, most likel y postsynaptic inhibition of the motoneurons. (C) 1998 John Wiley & So ns, Inc.