Preliminary observations on the response of perioral single motor units to mechanical stretch

This research represents part of a series of investigations into the function of mechanoreceptor feedback in lip motor control for speech. In the present study, precise mechanical stretches were applied to subjects' lips while they held the firing frequency of a given single motor unit relatively constant using audio and visual feedback. Preliminary data indicate that mechanical stimulation produces a range of short and long latency excitatory and inhibitory effects in perioral motor units. Of particular interest are short latency inhibitory effects in the depressor labii inferior and platysma muscles. These short latency inhibitory effects suggest that the central organization of the perioral sensorimotor system may possess a form of reciprocal innervation. The operation of a circuit is described which permits improved experimental control over the timing of stretch application relative to moter unit discharge. Initial data collected with this circuit suggest that it may be effectively used to assess the time course of subthreshold excitability changes associated with individual motor units during their repeated discharge.