Functional properties of single motor units in the human medial pterygoid muscle: Thresholds

Little is known regarding the functional properties of single motor units (SMUs) in the medial pterygoid muscle (MPt) during jaw movements.

The aims are (a) to report the thresholds of onset of MPt SMUs during 4 goal-directed jaw movement tasks, and (b) to determine whether the threshold of onset of SMU activation varies with the velocity of jaw movement and the location within the muscle.

Intra-muscular electrodes were inserted in the right MPt of 18 participants performing ipsilateral (right), contralateral, protrusive and opening-closing jaw movements recorded at 2 velocities. Task phases were as follows: BEFORE, OUT, HOLDING, RETURN and AFTER. SMU onset thresholds were determined from the displacement (mm) of the lower mid-incisor point. Electrode location within 4 arbitrary muscle divisions was determined with computer tomography. Statistical tests: Spearman's correlations, Kruskal-Wallis tests; significance accepted at P < .05.

A significant inverse relation occurred between velocity and threshold for the RETURN of the ipsilateral movement (n = 62 SMU thresholds), while a significant positive relation occurred for the OUT of the contralateral movement (n = 208); there were no significant associations for the protrusive (n = 131) and opening-closing (n = 58) tasks. Significant threshold differences occurred across the 4 muscle divisions only during the OUT of the contralateral and protrusive movements. Some evidence was provided for gender differences in MPt SMU properties.

The absence of a significant inverse relation between velocity and SMU threshold for most recorded movements suggests the MPt acts as a stabilizer of the jaw in horizontal and opening-closing jaw movements.