Stages of neurotransmitter development in autonomic neurons.

The ontogeny of neurotransmitters in autonomic neurons proceeds through the successive stages of early expression, definitive expression, modulation, and regulation, extending from embryonic life to maturity. Although different extracellular signals influence development at different stages, a number of signals that influence development continue to govern transmitter function during maturity. The sequential ontogenetic stages parallel the progressive restriction of mutability of phenotypic expression; however, some degree of neuronal mutability appears to persist through maturity.