Recruitment and discharge patterns of single motor units during speech production Free

Study of single motor unit (SMU) activity during rapid isotonic contractions, such as those characterizing speech, provide a closer look at the inacessible central mechanisms responsible for the encoding of the speech motor program. This study represents a quantification of the discharge and recruitment patterns of SMU's in the anterior belly of digastric, a muscle active in bringing about jaw lowering. Using a highly selective intramuscular electrode and computerized analysis procedures the firing rate, recruitment intervals, and various temporal intervals between SMU activity and articulatory events were obtained. Results to be discussed include: (1) greater isotonic versus isometric discharge rates for the same motor unit; (2) higher maximum discharge rates for later and larger recruited motor units; (3) relative invariance of recruitment order; (4) temporal sequence of SMU activation in relation to initiation of jaw lowering and speech audio signal: and (5) relationship of SMU activity to articulatory displacement and velocity. Implications of these results towards a further understanding of the encoding mechanisms of speech will be discussed.