Contractile responses of single fibers in lobster claw closer muscles: Correlation with structure, histochemistry, and innervation

This study incorporates contractile, structural (sarcomere length), histochemical (myofibrillar ATPase and NADH diaphorase), and physiological (fast and slow axon innervation) parameters to define fiber types in lobster muscle. Single fibers were examined in the closer muscle of adult cutter and crusher claws and of developing larval claws of the lobster Homarus americanus. Fibers with short sarcomeres ( < 4 μm) had rise times of 50–160 msec for attaining maximum tension which either was maintained for the period of stimulation (fatigue‐resistant) or declined (fatiguable). Fibers with long sarcomeres ( > 6 μm) had rise times of 90–800 msec for reaching maximum tension, which either was subsequently held at this level or continued to increase. The contractile properties and sarcomere length of single fibers were correlated with their enzymatic properties such that fast‐contracting, short‐sarcomere fibers had high myofibrillar ATPase activity and low oxidative capacity compared to their slow‐contracting, long‐sarcomere counterparts. The slow fibers were also further differentiated into subcategories based on contractile and histochemical properties. Fibers with the fastest rise times tended to be innervated primarily by the fast axon, and those with the slowest rise times were supplied solely by the slow axon. The organization of the massive closer muscle into discrete bundles in which fibers have uniform contractile, structural, and physiological properties leads to functional subdivision within the muscle. The contractile properties of developing fibers were similar to the adult except that larval fibers did not possess very slow rise times.