Highly active neuromuscular system in developing lobsters with programmed obsolescence

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Abstract
The primary locomotory apparatus in the three larval stages of the lobster, Homarus americanus, are paddlelike structures on the thoracic appendages called exopodites, which beat almost continuously. Consequently their power and return‐stroke muscles are examples of highly active but short‐lived neuromuscular systems. The muscles, which are well vascularized, are of the fast type with 2‐3‐μm sarcomere lengths and 6 thin filaments surrounding a thick one. The most striking feature, however, is the large volume of mitochondria making up 40‐50% of the fiber. They appear as simple cylinders packed several layers deep along the periphery of the fiber and as large, multibranched forms distributed throughout the fiber and subdividing it into smaller units. The motor innervation to the return‐stroke muscle is via 3 excitatory axons, which generate large junctional potentials and twitch contractions. The muscle is densely populated with large neuromuscular synapses, most of which have a well‐defined active site or dense bar denoting the site of transmitter release. Altogether this motor system is specialized for prolonged activity. Atrophy of the neuromuscular system occurs by the late larval third stage. The muscle fibers lose their identity, fuse, and become vacuolated. The myofibrils condense and erode and the mitochondria are lost. Atrophy of motor innervation is gradual with individual axons dropping out. The largest axon providing most of the innervation is the first to degenerate. Early degenerative changes affect the axon and neuromuscular terminals but not the synaptic contacts, dense bars, and vesicles, which appear intact. Continued atrophy in the postlarval fourth stage reduces the exopodites to vestiges. Thus the returnstroke muscle of the larval exopodites in which muscle fiber and motoneurons are identifiable permits study of the interaction between a neuron and its target muscle undergoing programmed obsolescence.