Structure and neuromuscular physiology of a newly discovered muscle in the walking legs of the lobster Homarus americanus

The structure and neuromuscular physiology of a newly discovered muscle (Wales et al., '70) in the walking legs of Homarus americanus was investigated. This muscle is quite small and is located at the posterior surface of the carpopodite near the distal end of the stretcher muscle. The new muscle is innervated by one excitatory and one inhibitory axon. The excitor appears to be separate from all other axons in the leg, while the inhibitor is shared with the stretcher and closer muscles. The new muscle can be considered to be of the slow or tonic type of crustacean muscle from both a structural and functional standpoint. Structural features determined for the new muscle are: long sarcomeres, thick, uneven Z lines, poorly defined H zones, and a high thin to thick filament ratio. Contractions of the new muscle are graded in nature and the rate and magnitude of tension development depends on the frequency of nerve stimulation. Upon stimulation of its excitatory axon the new muscle produces first slight rotation and then weak bending of the leg at the carpo‐propodite joint. Similarities between the new muscle and the rotator muscle found in decapod Natantia and Stomatopoda are discussed. Thus far Homarus is the only genus of the decapod Reptantia known to possess the new muscle.