Myosin, a major component of skeletal muscle, is encoded by a multigene family. Three major isoforms of the heavy chain subunits, one slow twitch and two fast twitch, have been distinguished in adult skeletal muscles of mammals (Whalen, 1985). These isoforms are related to specific, physiologically defined fiber types and their expression is clearly nerve-dependent. In chicken and amphibia, there is also another slow myosin heavy chain isoform — slow tonic myosin, present in multiply innervated, slow contracting fibers (Pierobon-Bormioli et al., 1980). This isoform is also expressed in mammalian skeletal muscle, but only in extraocular muscles and in some intrafusal fibers of muscle spindles (Pierobon-Bormioli et al., 1980; Rowlerson et al., 1985).
KeywordsMyosin Heavy Chain Muscle Spindle Intrafusal Fiber Slow Myosin Fast Myosin
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