Morphology and Physiology of Masticatory Muscle Motor Units

Abstract

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.

Burst characteristics of daily jaw muscle activity in juvenile rabbits

“…Although this might be true, the detailed characterization of muscle activity presented in the present study shows that high duty times can also be generated by large numbers of short bursts (see temporalis) and thus could be related to large percentages of fast fiber types. The differences in burst length distribution between the deep and superficial masseter are in line with reported differences in histochemical and physiological properties between these muscle regions, indicating that the deep masseter contains more fast type units than the superficial masseter (English et al, 1999;van Eijden and Turkawski, 2001).…”

Uniform strain in broad muscles: active and passive effects of the twisted tendon of the spotted ratfish Hydrolagus colliei

“…In an untwisted system, the degree of stretching may vary considerably across the muscle due to the distance between insertion points, as points further from the jaw joint undergo greater excursions during jaw opening (Goto et al, 2001;Herring et al, 1979;Pappas et al, 2002;van Eijden and Turkawski, 2001). Anterior fibers in the hypothetical untwisted system of the H. colliei AMA-␣ are predicted to undergo strains that are 50% larger than posterior fibers (relative to their respective resting lengths) at maximum gape (Fig.·4).…”

“…masseter) have been shown to be heterogeneous with respect to various architectural/functional aspects including fiber architecture and angle (Goto et al, 2001;Turkawski and van Eijden, 2001;van Eijden and Turkawski, 2001;van Eijden et al, 2002). For example, in pigs, rabbits and rats anterior masseter sarcomeres are shorter than posterior ones when the jaw is closed (Herring et al, 1979); as the jaw is opened, fibers across the muscle will tend to strain to similar positions in their length-tension curves due to the differences in their resting sarcomere lengths.…”

“…Apparently, these solutions to moment arm variation are not mutually exclusive and broad muscles may display a complex mosaic of heterogeneities van Eijden and Turkawski, 2001;van Eijden et al, 2002). For example, the mammalian latissumus dorsi exhibits the twisted tendon but also functional compartmentalization.…”

Modulation of mandibular loading and bite force in mammals during mastication

“…In mammalian masticatory muscles, as in the locomotor muscles, force modulation at low force amplitudes appears to be predominantly via muscle fiber recruitment rather than rate modulation (Goldberg and Derfler, 1977;Hannam and McMillan, 1994;Scutter and Türker, 1998). Mammalian masticatory muscles are not uniform in their fiber types (Anapol and Herring, 2000;Herring, 1994;Maxwell et al, 1979;Wall et al, 2006;Wall et al, 2005), and a large number of studies suggest that smaller, slower motor units are recruited before larger, faster motor units (Clark et al, 1978;Desmedt and Godaux, 1979;Goldberg and Derfler, 1977;LevTov et al, 1993;Lund et al, 1979;Miles and Türker, 1986;Miles et al, 1987;van Eijden and Turkawski, 2001;Van Wessel et al, 2005;Wall et al, 2006;Wall et al, 2005;Yemm, 1977). Thus, the evidence suggests that the generation of progressively higher bite forces during rhythmic mastication is achieved through increased recruitment of larger, faster motor units, resulting in increases in the rate of the generation of muscle force.…”