Two types of tonic fibers in lobster muscle based on enzyme histochemistry

Tonic fibers are differentiated into two types based on enzyme histochemistry in the claw opener and closer muscle of the lobster Homarus americanus. Some of the fibers have a much lower myofibrillar ATPase activity and much higher oxidative capacity (indicated by NADH diaphorase) than the remainder. These slower‐contracting, more fatigue‐resistant fibers are regionally distributed in the claw muscles. The occurrence of two types of tonic fibers particularly in the opener muscle, which is innervated by a single slow excitor axon, shows that the oxidative properties of these fibers are not controlled simply by the presence of this motoneuron as previously proposed.