Medial Pterygoid Muscle Single Motor Units: Effect of Bite Angle on Recruitment

Objectives: To investigate the neural control of single motor units in the medial pterygoid muscle. To do this we measured the bite force required for the recruitment of a motor unit at various angles. The "common drive" hypothesis (DeLuca & Erim, TINS 17: 299, 1994) suggests that recruitment should be independent of the intended biting angle. Methods: Six pain free subjects (3 males and 3 females) were recruited. The insertion point of the medial pterygoid was palpated and fine wire electrodes were inserted using a 27 gauge needle to measure electrical activity. Isometric bite force was measured with specially designed beams and force transducer that allowed various angles of bite force to be applied at the same jaw position. Bite force angles could be altered in the sagittal and frontal planes. An amplitude threshold was used to select single motor unit action potentials. Sampling was initiated when the identified single motor unit started to fire. The EMG from the fine wires and the bite force were sampled at 10 kHz for 100 ms for off-line analysis. Results: Four motor units from right medial pterygoids and two from left medial pterygoids were obtained. The force at which a single motor unit was recruited was dependent on the angle of the force transducer. In the sagittal plane for example, a single motor unit may be recruited at a low force at a small angle and as the angle increased the force at which the unit was recruited also increased. Conclusions: These data are not consistent with the view that there is a common drive to the motor neurons of the medial pterygoid muscle at various angles of bite force. Supported in part by USPHS Grant DE 07106.