Discharge patterns accompanying sustained activation of motor units in speech musculature

Abstract

Discharge patterns were studied in motor units (MUs) of facial, mandibular and lingual musculature during periods of steady discharge produced by three subjects, using a motor unit training paradigm. Recordings were made with concentric bipolar electrodes at relatively low force levels. An attempt was made to test hypotheses derived from Kernell's studies of cat hindlimb motoneurones which show that neurons with shorter afterhyperpolarization (AHP) durations are recruited later and have greater sensitivity to input changes. It was assumed that AHP durations of the motoneurones of steadily discharging human MUs correspond to their mean interspike intervals at a transition point beyond which any decrease in mean ISI results in negative correlations between successive ISIs. Transitions to negative correlations were observed in all muscles, at mean ISIs ranging from 35–55 ms. Clear transitions were also observed in some individual MUs, but not in enough to determine whether later recruited MUs possessed shorter AHP durations. Later recruited units did possess greater sensitivity to input changes. Variability of MUs between and within muscles was also evaluated.