Neurotrophic relations.

Increasing evidence for the existence of neurotrophic (non-impulse) mechanisms, especially in nerve-muscle cell relations, has been discussed. Studies on axoplasmic transport, release of agents (other than transmitter) from the nerve, and possible transfer of macromolecules at the NMJ and on differentiation of impulse and non-impulse (neurotrophic) activities have advanced, but not solved, the basic questions. Progress has been slowed because often less than adequate indicators of neurotrophic functions have been used and because only a single neurotrophic agent was generally assumed. Neurotrophic actions are best understood as components of multiple regulation in the context of general intercellular relations. The analysis of neurotrophic regulations will become clear only after chemical definition of the neurotrophic agents. Until then, study of the differentiation and interaction of neuronal impulse and non-impulse activities is, and will remain, an important problem for an understanding of the plasticity of the NMJ and muscle.