Physiological properties of three muscle fibre types controlling dorsal fin movements in a flatfish, Citharichthys sordidus.

Pacific sand dabs utilize their dorsal and anal fins in different behaviours which are characterized by extremely rapid fin movements, on one hand, and essentially isometric force generation on the other. Muscle fibres controlling fin movements were examined physiologically. Direct electrical shocks to localized regions of fin muscles reveal three fibre types. The longest fibres in the muscle are very fast and functionally analogous to frog twitch fibres. The shortest fibres are extremely, slow and show properties much like frog tonic fibres. The mid-length fibres produce contractile responses which are intermediate in time course. Even the fastest muscle fibres do not generate action potentials, but instead rely on summating junction potentials to drive membrane voltage to a stable level just beyond contraction threshold (-35 to -40 mV). Twitch amplitude can be finely graded by the time that membrane depolarization exceeds this threshold level.