Discharge patterns accompanying sustained activation of motor units in speech musculature

Abstract

Data is presented from speech and nonspeech muscles partially confirming the hypothesis that steadily firing motor units show two different discharge patterns depending on firing rate. The transition zone, characterized by rates intermediate between those showing the two patterns, represents a basic property of motor units which may allow inferences about other neurophysiological properties, and mechanical properties of the unit. Transition zones of speech muscles innervated by cranial nerves (Anterior belly of Digastric Genioglossus, Orbicularis Oris, Depressor Labii Inferioris) were relatively similar and occurred at mean interspike intervals ranging mostly from 40–65 msec. These zones were similar to that of a “nonspeech” cranial muscle (masseter) but at smaller intervals than in limb muscles, innervated by spinal nerve (Biceps 80 msec, Rectus Femoris 85–95 msec). When two units fired together, the “gain” of the unit recruited second (i.e., the relative change of firing rate with input change) was virtually always greater. This difference was at least partly due to the relative responsiveness of the controlling motoneurons themselves, rather than to differences in amount of input to them. [Work supported by NSF.]