Mammalian skeletal muscle cells are composed of repeated sarcomeric units containing thick and thin filaments of myosin and actin, respectively. Excitation of the myosin ATPase enzyme is possible only with presence of Mg-ATP and Ca2+. Skeletal muscle fibres may be classified into several types according to the isoform of myosin they contain. Nine isoforms of myosin heavy chain are known to exist in mammalian skeletal muscle including type I, IIA, IIB, IIX, IIM, α, neonatal, embryonic, and extra-ocular. Healthy adult human limb skeletal muscle contains type I, IIA, IIB, and IIX myosin heavy chains. The jaw-closing muscles of most carnivores and primates have tissue-specific expression of the type IIM or ‘type II masticatory’ myosin heavy chain. Adult human jaw-closing muscles, however, do not contain IIM myosin. Rather, they express type I, IIA, IIX (as in human limb muscle), and myosins typically expressed in developing or cardiac muscle. The morphology of human jaw-closing muscle fibres is also unusual in that the type II fibres are of smaller diameter that type I fibres, except in cases of increased function and hypertrophy.
This paper describes the relationship of fibre types and motor unit function to changes in human occlusion and masticatory activity.
