Trophic reactions of crayfish muscle fibers and neuromuscular synapses after denervation, tenotomy, and immobilization

Abstract Severed motor nerve terminals remain morphologically intact and functionally competent for several months following transection of the motor nerve to the claw opener muscle of crayfish ( Procambarus clarki ). However, severed motor axons are not entirely normal in that excitatory synapses produce junctional potentials that are somewhat smaller than control values. Tenotomy or immobilization often produce little change in synaptic potentials for at least 180 days. Opener muscle fibers with severed motor axons or immobilized muscle fibers show little ultrastructural change for at least 100 days, provided the nerve terminals remain intact. However, after tenotomy, muscle fibers atrophy within 20 to 30 days. This atrophy is more rapid and more severe if nerve terminals on that muscle fiber have degenerated. These atrophic changes in muscle fine structure include disorganization of myofibrils and disruption or loss of the sarcomere Z bands. In summary, our data show that drastic changes in crayfish muscle structure can proceed after tentomy without concomitant changes in function of the motor nerve terminals; and conversely, although transecting the motor nerve has some effect on motor nerve terminals, decentralization has little effect on muscle fiber structure as long as nerve terminals remain intact. These data are in agreement with the hypothesis that crustacean muscle fibers are trophically dependent on the presence of functional nerve terminals and on passive fiber tension (or resting length).