Fine structure of the swimming paddle opener muscle and its innervation in the blue crab, Callinectes sapidus

grantor: University of TorontoThe fifth pereiopod of the blue crab is a specialized swimming limb, the dactyl of which acts as a rowing paddle. Abduction of this paddle is brought about by contractions of the opener muscle that has assumed a specialized role as a fairly equal antagonist to the closer muscle (the adductor of the dactyl). Electron microscopic examinations of the central region of the opener muscle revealed typically slow fibres with long sarcomere lengths and a high actin to myosin ratio. Muscle fibres were highly innervated with a majority having multiple innervation sites each with an inhibitory and excitatory terminal profile. This large amount of innervation may play a role in ensuring fatigue resistance and fine control for the specialized muscle. Inhibitory terminals originated synapses that were two thirds neuromuscular, inhibiting the muscle directly, and one third axo-axonal, inhibiting the excitatory axon itself. This is an unusually high degree of presynaptic inhibition possibly associated with the fine control of the paddle limb. The overwhelming majority of synapses formed by the excitatory terminal were neuromuscular, in keeping with its primary function of making the muscle contract. A very unusual synapse was also recorded in every serially sectioned area examined: axo-axonal synapses polarized from the excitatory terminal to the inhibitory terminal, suggesting putative presynaptic excitation of the inhibitory nerve terminals. Altogether, the innervation of the blue crab swimming paddle opener muscle indicated specializations for its function as a highly active muscle rapidly contracting for prolonged periods of time.M.Sc