Evidence of direct connection of corticobulbar fibers to orofacial muscles in man: electromyographic study of individual motor unit responses.

Abstract

We studied corticobulbar influence on the orbicularis oris (OR) muscles by anodal and magnetic transcranial stimulation and compared it to the corticospinal influence on first dorsal interosseous muscles of healthy human volunteers. We recorded single motor unit (MU) responses and applied peristimulus time histogram (PSTH) technique to determine their properties. Peaks in the PSTHs are assumed to coincide with multiple excitatory postsynaptic potentials (EPSPs) induced at the motoneurons 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 projection in the lower facial motoneurons. The EPSP amplitudes, estimated on the basis of initial peak, were used as a measure of the synaptic transmission efficiency. Our results support the concept of predominantly monosynaptic transmission at the spinal level.