Task-related behavior of motor units in different regions of the human masseter muscle.

Muscle activity patterns appear to vary regionally in the human masseter. However, studies of motor-unit (MU) behaviour in the masseter have been hampered by the absence of a reliable technique for locating needle-electrode recording sites. Here, voluntary MU behaviour patterns were examined in verified regions of the muscle. Activity was recorded from 50 stereotactically mapped masseter MUs. Initially, the task specificity of each MU was determined. Then for each task, the lowest sustainable firing frequency (LSFF) was reached by slow increases and decreases in voluntary firing rate, followed by sustained firing at the lowest possible rate. Pulse-discrimination and digital sampling of consecutive interspike intervals were used to measure the LSFF for each task to which the MU contributed. All MUs fired continuously during the performance of 2-6 separate tooth-contact and postural tasks. There were significant differences between LSFFs for the tasks performed by 47 units. Masseter MU task profiles appear to vary regionally, and are dependent on jaw position, the bite point along the tooth row, and the direction of effort. Descending neural drive to masseter MUs thus seems to be highly task dependent, even when the unit firing rate is controlled voluntarily at its LSFF.