Flexibility in motor-unit firing pattern in the human temporal and masseter muscles related to type of activation and location

Morphological studies have shown that each human jaw muscle and its sub-portions have a characteristic fibre-type pattern, suggesting special functional tasks. The firing pattern of 68 low-threshold motor units in different portions of the temporal and masseter muscles in 5 healthy subjects were determined by single-fibre EMG during voluntary isometric muscle contraction. The findings showed that each unit may have different activity patterns for different types of activation, which is not so for limb muscles. Thus, a given motor unit discharged with a different regularity in firing rate during retraction of the mandible than during clenching of the jaws in the intercuspal position, probably related to a changed central and peripheral influence on the motoneurone pool. No activity of single units was detected in the mandibular rest position. The results underline the complexity of the human stomatognathic system and the importance of combined morphological and physiological studies.