Mechanical properties of single motor units in the rabbit masseter muscle as a function of jaw position

Positions and contractile properties of rabbit masseter motor units were investigated at different jaw gapes. Twitch responses were measured at gapes ranging from dental occlusion (0 degrees) to maximum opening (21 degrees), in steps of 3 degrees. The twitches were elicited by stimulating motoneurons extracellularly in the trigeminal motor nucleus. The units appeared to produce a large variety of force vectors. On average motor units in the deep parts of the masseter produced considerably less twitch force (average: 25-30 mN) than those in the superficial parts (average: 45-50 mN) and anteriorly located motor units were slower than posteriorly located units. With an increase of jaw angle, twitches became slower, reflected by an increase (30%) of the twitch contraction time. Most motor units had a parabolic-like active jaw angle-force relationship. A large variation in the shape of the curves was found. The average optimum jaw angle was reached at 12 degrees jaw opening. In general, force output was relatively low (20-60% of maximum force) at occlusion and relatively high (60-100% of maximum force) at maximal-jaw opening. Anteriorly and posteriorly located motor units differed significantly in their angle-force curves. Anteriorly located motor units produced less relative force at occlusion, showed a steeper increase of force with an increase of jaw angle, reached maximum force at larger jaw angles and produced larger forces at maximum jaw opening. The larger force changes in the more anterior units are probably related to their longer distance from the axis of jaw rotation. The large variability of motor unit properties and angle-force curves suggests that a fine gradation of both force magnitude and direction is possible within the masseter and that the angle-force curve of the whole muscle or of whole muscle parts is broader than that of individual motor units. This broadening may be considered as a mechanism to sustain active muscle force throughout a large movement range.