Synapse formation in intact innervated cutaneous-pectoris muscles of the frog following denervation of the opposite muscle.

1. Denervation of one cutaneous-pectoris muscle of the frog induces the formation of new synapses in the intact innervated muscle on the opposite side. After crushing the motor nerve to the left muscle the incidence of polyneuronal innervation in the right intact muscle increased from an average normal value of 16% to an average value of 27% (Rotshenker & McMahan, 1976).2. The formation of the new synapses in the intact muscle is independent of the presence of denervated muscle fibres or degenerating axons peripheral to the site of axotomy. After removing the left cutaneous-pectoris muscle, the proportion of polyneuronally innervated muscle fibres in the right intact muscle increased to an average value of 34%.3. The number of new synapses formed in one muscle is dependent upon the type of the lesion to the motor nerve to the opposite muscle; 40% of the muscle fibres on the right side were found to be polyneuronally innervated after transecting the motor nerve on the opposite side, as compared to 27% after crushing it.4. The delay with which new synapses are formed on the unoperated side is dependent upon the distance from the spinal cord of the axotomy. New synapses were detected 4-8 weeks after cutting the opposite nerve close to the muscle. Placing the site of axotomy close to the spinal cord shortened the delay and new synapses were detected as early as 9 days after the operation.5. The stimulus for the formation of new synapses by an intact nerve is ineffective if the injured nerve on the contralateral side originates from distant segments of the spinal cord. The pattern of innervation in cutaneous-pectoris muscles was not altered following denervation of distant muscles in the hind limb.6. These results suggest that the signal for sprouting and synapse formation may arise in the damaged nerve cells, central to the site of axotomy, and then be communicated transneuronally within the spinal cord to the intact motoneurones on the opposite side.