The nervous system collects, coordinates, integrates, and disseminates diverse types of information regarding both the external and internal environments. Processing of this information leads to appropriate physiological and/or behavioral responses. In order for stimuli to produce accurate descriptions of the environment, a variety of neural codes and operations arose during evolution that allow information exchange within and between neurons (for review, see Perkel and Bullock, 1968; Gerstner et al., 1997). One mechanism, the action potential (AP), arose early during evolution and is essential for rapid signaling in the nervous system.

In this article, we focus on how embryonic neurons acquire the ability to fire APs. AP generation represents a significant challenge because of the requirement for function of several different membrane proteins. Further, there are several examples of neurons that fire APs with developmentally regulated properties. Consequently, the roles of APs in emerging nervous systems are not static and depend upon developmental stage. We review mechanisms that lead to the developmental regulation of excitability. We conclude by identifying key issues that remain unresolved and warrant being the focus of future study (see Neuronal Migration, Axon Pathfinding, A Tale of Two CPGs: Phylogenetically Polymorphic Networks).

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