Preliminaries to the study of single motor unit activity in speech musculature

Abstract

Study of single motor units in speech musculature appears to be the most direct approach to the understanding of neurophysiological processes underlying speech production. A number of hypotheses as to the processes controlling single motor unit activity are discussed, including the following. Motor units possess thresholds for recruitment into a muscle contraction that are directly related to motoneuron (and motor unit) size. Motor units in different muscles and in the same muscle possess characteristic firing frequencies which are inversely related to the duration of afterhyperpolarization of their motoneurons. Characteristic firing frequencies increase in the order: limb and trunk musculature, speech (or cranial) musculature, extraocular musculature. Higher firing frequencies appear to be associated with a greater number of possible states of contraction and an ability to change states more rapidly. Many motor units possess wide ranges of frequency of firing divided into two sub-ranges: the primary range of relatively low frequency firing, and the secondary range in which firing increases more for a given increase in input current. It seems unlikely that the tonic-phasic distinction is reflected in a dichotomy of patterns of firing frequency in units within the same muscle. Some initial studies of single motor unit activity in speech musculature are reported and discussed in terms of these hypotheses and a number of questions for further study are presented.