Fig. 1.   Schematic representation of the linkage of neurotransmitters to cell development. During specified critical periods, neurotransmitter receptors and their associated signaling cascades can elicit changes in gene expression that influence differentiation. Depending on the context in which stimulation occurs, the same neurotransmitter, operating through the same receptors, can promote cell replication, can elicit a switch from replication to differentiation, can promote or arrest cell growth, can evoke apoptosis or can program the genes that determine the future responsiveness of the cell to external stimulation (cell learning).