Muscle degeneration following remote nerve injury

Muscle depends upon innervation and contraction to maintain a differentiated state. Denervation can therefore induce muscle atrophy. In grasshoppers, muscle degeneration can also be triggered by the severing of a leg during autotomy. In this case, the muscles that degenerate are neither damaged nor denervated. This phenomenon suggests the existence of transneuronal mechanisms that influence muscle survival. To characterize this autotomy-induced process, we studied the degeneration of a thoracic tergotrochanteral depressor muscle (M#133b,c) subsequent to the shedding of a hindlimb in the grasshoppers Barytettix psolus and Barytettix humphreysii. Both histochemical and electrophysiological methods were used to follow muscle degeneration 1, 3, 5, 10, and 15 days postautotomy. Muscle fibers began to show denervation-like electrophysiological changes (i.e., depolarized resting membrane potentials and postinhibitory rebound) as soon as 3 days postautotomy. By 10 days, significant muscle degeneration was evident and electrophysiological changes were found in all animals tested. Muscle anatomical degeneration was not induced by synaptic transmission failure, because neuromuscular transmission was maintained in most fibers. The rate of muscle degeneration was not constant. Between 1 and 10 days, mean fiber cross-sectional area did not change on the autotomized side, although this is normally a time of muscle growth. However after 10 days, cross-sectional area became drastically reduced and the number of muscle fibers within M#133b,c was decreased. The variability in rate of fiber degeneration was not dependent upon fiber type, since M#133b,c only contains fast-type fibers.