Recruitment order, contractile characteristics, and firing patterns of motor units in the temporalis muscle of monkeys

Abstract

Action potentials of single motor units were recorded in the temporalis muscle of rhesus monkeys trained to hold static forces on a bite bar. The average relationship between mandibular force and motor unit firing rate was determined for many motor units using a computer program, and from this relationship the threshold force at which a motor unit began to fire was determined. Several aspects of motor unit firing rate were examined in relation to recruitment threshold. Computer averaging was used to determine twitch tension and twitch contraction times for many motor units. Motor units recruited at low forces had longer contraction times and produced smaller twitch tensions than higher-threshold motor units. Motor units with small action potentials were nearly always recruited at lower force levels than those with larger action potentials. These results indicate that the motor units of the monkey temporalis muscle were recruited in an orderly fashion, which is in accord with the predictions of the “size principle”.