Fiber composition and innervation patterns of the limb closer muscle in the lobster Homarus americanus

The closer muscle in the first walking leg of the lobster, Homarus americanus, is composed of mostly (78%) tonic fibers with low levels of myofibrillar adenosine triphosphatase (ATPase) activity. The remainder are phasic fibers, which have high ATPase activity and are restricted to a dorsal bundle. Based on histochemical demonstration of reduced nicotinamide dinucleotide (NADH) diaphorase, phasic fibers have a lower oxidative capacity than tonic fibers. The tonic fibers have mitochondria criss-crossing the entire cross-sectional profile or restricted to the periphery. Fiber types based on myosin ATPase activity correlated closely with those based on resting sarcomere and A-band lengths on the dorsal surface of the closer muscle. Thus phasic fibers with high ATPase activity had short (6 μm) sarcomeres. Innervation patterns of the dorsal fibers revealed all possible combinations between the two types of muscle fibers (phasic and tonic) and the two types of axons (fast and slow) except phasic fibers innervated only by slow axons. The distribution of fast and slow axons therefore serves to broaden the contractile performance of the closer muscle.