Fin movement after regeneration of neuromuscular connections: An investigation of myotypic specificity

Abstract

Regeneration of the brachial plexus after surgical section restores full and coordinated movement to the pectoral fins of a cichlid fish ( Astronotus ocellatus). If nerve trunks from the plexus are sectioned close to the muscles they supply, and are implanted into antagonistic muscles, neuromuscular transmission is re-established but the resultant fin movement is uncoordinated and remains so. The fish spinal cord does not seem to be able to compensate by rearrangement of internal architecture for a large number of inappropriate neuromuscular connections. Restoration of coordinated muscle action when the brachial plexus is allowed to regenerate unhindered is, therefore, most likely to depend on the selective re-establishment in the periphery of a majority of correct nerve muscle connections. Multiterminal polyneuronal innervation of skeletal muscles is common among the lower vertebrates in which muscular coordination is restored when motor nerves regenerate, whereas single end-plate innervation is common in the higher vertebrates in which regeneration of severed motor nerves leads to undifferentiated mass contraction whenever reinnervated muscle groups are called into play. I suggest, therefore, that the degree of fixity of adult spinal cord organization is similar in these two groups of vertebrates, but competitive selective reinnervation of muscle fibers can occur when several nerve sprouts form endings on one muscle cell and cannot occur if each muscle fiber will normally accept only one motor nerve ending.