Regular alternation of fiber types in the transversus abdominis muscle of the garter snake.

The snake transversus abdominis muscle is an extremely simple segmentally repeating muscle containing 80 to 100 muscle fibers in a single-fiber-thick sheet. This muscle exhibits a striking pattern of muscle fiber types: twitch fibers alternate with tonic fibers and, among the twitch fibers, slower and faster contracting subtypes also alternate. Thus, in many regions of the muscle the pattern of fiber types is: faster twitch, tonic, slower twitch, tonic, faster twitch, tonic, and so on. The existence of a spatial pattern of fiber types, perhaps discernible in this muscle because of the muscle's extreme geometrical simplicity, provides good evidence for an intrinsic component to muscle fiber differentiation.