Motor unit territories in masticatory muscles appear to be smaller than territories in limb muscles, and this would suggest a more localized organization of motor control in masticatory muscles. Motor unit cross-sectional areas show a wide range of values, which explains the large variability of motor unit force output. The proportion of motor unit muscle fibers containing more than one myosin heavy-chain (MHC) isoform is considerably larger in masticatory muscles than in limb and trunk muscles. This explains the continuous range of contraction speeds found in masticatory muscle motor units. Hence, in masticatory muscles, a finer gradation of force and contraction speeds is possible than in limb and in trunk muscles. The proportion of slow-type motor units is relatively large in deep and anterior masticatory muscle regions, whereas more fast-type units are more common in the superficial and posterior muscle regions. Muscle portions with a high proportion of slow-type motor units are better equipped for a finer control of muscle force and a larger resistance to fatigue during chewing and biting than muscle portions with a high proportion of fast units. For the force modulation, masticatory muscles rely mostly on recruitment gradation at low force levels and on rate gradation at high force levels. Henneman's principle of an orderly recruitment of motor units has also been reported for various masticatory muscles. The presence of localized motor unit territories and task-specific motor unit activity facilitates differential control of separate muscle portions. This gives the masticatory muscles the capacity of producing a large diversity of mechanical actions. In this review, the properties of masticatory muscle motor units are discussed.

The masseter muscle of the rabbit has a complex architectural design. Restricted motor unit territories in the muscle provide an anatomic basis for accurate control of the force vector through selective activation. In addition, the muscle shows regional differences in fiber type composition. The main objective of the present study was to measure the force vectors of single motor units within the rabbit masseter muscle by a direct mechanical approach to test the hypothesis that: (1) motor units within the masseter muscle are capable of generating different force vectors; and (2) different motor unit types are distributed heterogeneously throughout the muscle. We used a force transducer, capable of measuring both the magnitude and the position of the line of action of a force in a single plane. Motor units in the masseter muscle showed a large range of twitch contraction times and force magnitudes. There was also a large variation in the direction and moment arm of the lines of action. The variation of the lines of action was (almost) as large as the range of fiber directions found inside the muscle. Largest forces, with relatively slow contraction velocities, were produced by motor units in the anterior masseter. Smaller forces and fastest twitch contractions were produced by motor units in the posterior deep masseter. In addition, motor units in the anterior masseter showed more variability in force production than in the posterior masseter. Our results support the idea that the masseter muscle is divided into functionally different parts.

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 degree) 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.

Masticatory muscles contain a large variety of motor units with different physiological and morphological properties. In this study, we tested the hypothesis that a relationship exists between the mechanical and myo-electric properties of single motor units in the masseter muscle of the rabbit. It was expected that faster-contracting motor units, which usually have a relatively large number of fibers with large diameters, should have faster action potentials with larger amplitudes than slower motor units. Single motor units were stimulated. A two-dimensional force transducer registered mechanical parameters of the units. EMG electrodes were used to determine amplitude and frequency parameters of the action potentials of the same units. The results showed that faster-contracting motor units indeed produced action potentials with higher conduction velocities. However, faster motor units had no significant larger amplitude of the action potential. Small but significant positive correlations were found between the tetanic peak force and the amplitude of the action potentials. Little difference was found among the various frequency and amplitude parameters, respectively, making them equally suitable to describe the action potential. Surprisingly, a negative correlation between the amplitude and frequency parameters of the action potential was found, which may result from variability in arrival times of action potentials at the electrode site. Regional differences in the frequency parameters were found between the anterior and posterior parts of the superficial masseter.

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Action potentials of rabbit masseter motor units (n = 42) were registered at different jaw angles to examine whether the shape of the action potential is related to length of muscle fibers in motor units and depends on the intramuscular location of the motor unit. Twitches were elicited by stimulating motoneurons in the trigeminal motor nucleus. During jaw opening (0-21 degrees), the duration of the action potentials increased by about 10%. Anteriorly located motor units showed an increase in duration larger than that of more posteriorly located units, which was probably due to a larger stretching of the more anteriorly located units.