Neuronal control of extrajunctional acetylcholine receptor-channels induced by injury in frog skeletal muscle fibres.

(1) Frog Sartorius muscles were transected to form two muscles: a neural segment (tibial part) containing all the endplates, and an aneural segment (pelvic part). The acetylcholine receptor-channels which appear at both new ends of the muscle fibres were studied by spectral analysis of membrane current fluctuations elicited by acetylcholine (ACh). (2) During the first two weeks after muscle transection, the ACh-receptors at the new muscle-tendon junction of the neural segment opened channels which had a long open time, similar to that of extrajunctional ACh-receptor channels induced by denervation of non-transected muscles. Subsequently, the open time of the new channels became shorter, and, about three weeks post-transection, it resembled that of channels at innervated endplates of normal muscles. The characteristics of the end-plate channels were not appreciably altered by the transection. (3) Similar results were observed when the muscle was crushed instead of being transected; and the change from slow to fast channel kinetics occurred earlier. (4) Denervation of neural segments prevented the change from slow to fast channel kinetics. The channels in the new muscle-tendon junction of the aneural segment remained slow up to three months post-transection. (5) These results indicate that the motor nerve controls the properties of the ACh-receptors induced by muscle transection.