EVIDENCE OF DIRECT CONNECTION OF CORTICOBULBAR FIBERS TO OROFACIAL MUSCLES IN MAN - ELECTROMYOGRAPHIC STUDY OF INDIVIDUAL MOTOR UNIT RESPONSES

We studied corticobulbar influence on the orbicularis oris (OR) muscle s by anodal and magnetic transcranial stimulation and compared it to t he corticospinal influence on first dorsal interosseous muscles of hea lthy human volunteers. We recorded single motor unit (MU) responses an d applied peristimulus time histogram (PSTH) technique to determine th eir properties. Peaks in the PSTHs are assumed to coincide with multip le excitatory postsynaptic potentials (EPSPs) induced at the motoneuro ns by corticospinal input. We found relatively small latency variation of consecutive single MU responses for both muscles. Additionally, we found a narrow initial peak in five OR MUs after anodal stimulation. This short duration indicates the presence of monosynaptic cortical pr ojection in the lower facial motoneurons. The EPSP amplitudes, estimat ed on the basis of initial peak, were used as a measure of the synapti c transmission efficiency. Our results support the concept of predomin antly monosynaptic transmission at the spinal level. (C) 1998 John Wil ey & Sons, Inc.