Motor axon sprouting and site of synapse formation in intact innervated skeletal muscle of the frog

The pattern of motor innervation to the cutaneous‐pectoris muscle of the frog is altered after injuring the motor nerve to the contralateral muscle in that muscle fibers innervated by a single motor nerve cell become polyneuronally innervated. This polyneuronal pattern of innervation was detected by single cell recordings of multiple end‐plate potentials from muscle fibers. The present study shows that the anatomical basis for this electrophysiological observation is the formation of new additional synapses by intact motor neurons on already innervated muscle fibers at new sites. Light and electron microscopical examination of muscles stained with Zinc Iodide and Osmium revealed that the sources for the new axon terminals were the intact motor axons and nerve endings that gave rise to sprouts that formed synaptic connections with muscle fibers apparently not innervated by their parent axons. Furthermore, new synapses were formed at new synaptic sites. The increased incidence of polyneuronal innervation that was detected electrophysiologically was associated, in the same muscles, with a proportional increase in the average end‐plate size estimated from the measurements of cholinesterase‐stained sites. Additional evidence that synapses were formed at new sites was that the shape of the different components of multiple end‐plate potentials in some polyneuronally innervated muscle fibers differed in their rise‐times. No such recordings were observed in polyneuronally innervated muscle fibers of normal frogs.