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Magnetic and Electrical Transcranial Brain Stimulation to Orofacial Muscles in Man. EMG Study of Motor Unit Responses

Recording from individual motor units (MUs) activated by magnetic or electrical stimulation applied to the intact scalp provides information of the nature by which the corticospinal tract or corticobulbar tract projections contact spinal or bulbar motoneurones in man. We studied corticobulbar influence on the orbicularis oris muscles by magnetic and anodal stimulation and compared it to the corticospinal influence of the first dorsal interosseus muscles of healthy human volunteers. we recorded single Mu responses and applied PSTH technique to determine their properties. we found relatively small latency variation of consecutive single MU responses for both muscles. Also, we found a narrow initial peak in 5 MU of the orbicularis oris muscle. This short duration indicates the presence of monosynaptic cortical projection in the lower facial motoneurones. 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.

magnetic; electrical stimulation; facial muscles

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