Cotransmission.

Cotransmission, defined here as the control of a single target cell by two or more substances released from one neuron in response to the same neuronal event, does occur in experimental situations. It has not been shown to occur in the normal operation of an animal, but the likelihood that it does is great. There are many examples of potential transmitters coexisting in one neuron, suggesting that cotransmission might be widespread in the peripheral nervous system. But many transmissions still seem to be mediated by a single transmitter. In such cases, coreleased substances might act on other targets or modulate the receptors for the main transmitter. But the possibility also exists that some colocalized "transmitters" have no function in transmission. It is increasingly difficult to retain a simple classification of neuronal types based on transmitter substances. However, there are indications that some combinations of colocalized substances are "preferred" and that certain combinations typify the innervation of a particular target tissue.