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Fiber-type Composition of the Human Jaw Muscles—(Part 1) Origin and Functional Significance of Fiber-type Diversity

Department of Functional Anatomy, Academic Center for Dentistry Amsterdam, Meibergdreef 15, 1105 AZ Amsterdam, The Netherlands;

View larger version (52K): [in a new window]   Figure 1. Diagram showing the level of organization within a skeletal muscle fiber, from single fiber to myofibrils, sarcomeres, actin and myosin myofilaments, and myosin molecule. The myosin molecule is composed of 4 light-chain (MyLC) molecules (2 essential and 2 regulatory) and 2 myosin heavy-chain (MyHC) subunits.

View larger version (104K): [in a new window]   Figure 2. Example of an area showing the antero-deep portion of the masseter incubated with antibodies against MyHC-I (A), MyHC-cardiac (B), MyHC-IIA (C), MyHC-fetal (E), and MyHC-IIA and -IIX isoforms (F). Lower magnification of cross-sections in (A-F) is shown in (D). Drawing (G) shows some of the fiber types: (1) MyHC type I, (2) MyHC type IIX, (5) MyHC type IIA, (6) MyHC type I+IIA, (7) MyHC type fetal+I, and (8) MyHC type fetal+cardiac +I.

View larger version (124K): [in a new window]   Figure 3. Example of a cross-section of a fiber bundle from the anterior belly of the digastric muscle (A-D) and the electrophoresis results (E,F). (B-D) Magnifications of the indicated area in A after incubation with antibodies against MyHC-I (B), MyHC-IIA (C), and MyHC-IIA+IIX (D). Bar = 100 µm. (E) MyHC composition of this bundle as determined by gel electrophoresis. Only the MyHCs are shown. The plot obtained from laser densitometry (F) shows the integrated optical density of this particular fiber bundle.

View larger version (13K): [in a new window]   Figure 4. Cumulative muscle force vs. number of recruited motor units. Based on Kernell (1992). The recruitment of motor units follows the size principle. According to this principle, the smallest (slow, fatigue-resistant) motor units are recruited first; these units produce relatively small forces. Therefore, for a small muscle force to be produced, a relatively large number of motor units is recruited. For muscle force to be increased, the larger (faster, less fatigable) units are recruited; these units produce larger forces, and thus, a relatively small number of motor units is recruited. As a consequence of the size principle, a finer modulation of muscle force is possible at lower than at higher muscle forces.