Nerve growth and ectopic synapse formation induced by muscle damage in the frog

The effect of muscle damage on nerve growth and ectopic synapse formation in the frog was investigated by bisecting the cutaneus pectoris (c.p.) muscle following implantation of the hypoglossal nerve. Axons from the stump of the implanted nerve grew considerable distances towards the ends of the cut muscle fibres where they formed synapses. In some preparations, c.p. was only partially cut and in these muscles axonal growth from the implanted nerve occurred across intact fibres to reach the cut ends in the damaged area, suggesting the local release of a growth-stimulating factor. Stimulation of the hypoglossal nerve elicited contraction of the end-plate-free half of c.p. The characteristics of these ectopic synapses were similar to those found in a previous study when the hypoglossal nerve innervated the original end-plates after denervation of c.p. The mean quantal content of the end-plate potentials (e.p.p.s) was abnormally low, with long latencies between nerve stimulation and the onset of the e.p.p.s and frequent occurrence of multiphasic e.p.p.s in response to single nerve stimulation.