The reflex responses of single motor units in human lower lip muscles to mechanical stimulation

Single motor unit activity was recorded from human lower lip muscles while subjects maintained a firing rate of 20 ips using audio and visual feedback. Each motor unit was classified as belonging to one of three muscles, orbicularis oris inferior (OOI), mentalis (MENT), or depressor labii inferior (DLI). Reflex responses were elicited in single motor units by applying small mechanical displacements (0.5 mm) to the corner of the mouth. Reflex responses were analyzed by constructing peristimulus histograms. Of 130 units analyzed, 123 showed statistically significant changes in their probability of firing in the 50 ms period following the stimulus. Reflex responses were typically polymodal with both excitatory and suppression components present. The excitatory responses occurred at latencies consistent with previous descriptions of the perioral reflexes and were labelled E1 (means = 15.9 ms) and E2 (means = 35.6 ms). Reflex suppression responses (S) were also consistently observed. There was a tendency for the pattern and/or relative magnitudes of reflex response components to depend on the muscle classification of the motor unit; E1 responses predominating in OOI units, E2 responses in MENT units, and S responses in DLI units. This finding provides preliminary evidence for muscle-specific projections between the trigeminal sensory system and facial nucleus motoneurones in humans.