Task dependence of human masseter motor unit reflex behaviour

Motor unit (MU) firing frequency is an important determinant of reflex inhibition in the human jaw muscles. Masseter MUs may be driven steadily by various intraoral tasks, but their lowest sustainable firing frequency varies according to task. In this study we examined the effect of task on masseter MU reflex behaviour under controlled conditions, in which the prestimulus MU firing frequency and stimulation were constrained. All MUs tested were inhibited by a non-noxious electrical stimulus applied to the oral mucosa, but there were significant differences in the magnitude of single MU inhibition depending on the task employed to drive the MUs. It appears that single masseter MU reflex behaviour can alter according to task, even when the prestimulus excitation of the masseter motoneuron pool is apparently constant. This suggests that masseter MU reflex behaviour may be modulated by task-related peripheral afferent input.