Motor-unit territory in the human masseter muscle.

Motor-unit territories in human masseter are reportedly focal and related to putative subvolumes of muscle. However, in the absence of a reliable method of locating needle-electrode recording sites within the muscle in three dimensions and due to inherent weaknesses in electromyographic recording techniques, the limits of motor-unit territory in the masseter may have been underestimated. Single motor-unit responses were recorded as time-locked events from 32 paired-needle recording sites throughout the masseter muscles of three subjects. Recording sites were located stereotactically with an optical system, magnetic resonance imaging, and a common reference, then displayed graphically in three dimensions. The mean linear separation of the paired recording sites was 8.8 +/- 3.4 mm. The putative territories had a preferred orientation in the antero-posterior axis. Motor-unit territories were larger than described previously and appeared to be related to anatomical compartments. The restriction of these territories to discrete regions of the muscle provides an anatomical substrate for selective regional motor control of the human masseter muscle.