The voltage-gated channels of Ca2! action potentials: Generalization

potential propagated by axons. There are two other types of action potentials: (i) the Na!/Ca2!-dependent action potential present in axon terminals or heart muscle cells (Figure 4.2d), for example, where it is responsible for Ca2! entry and an increase of intracellular Ca2! concentration, a necessary prerequisite for neurotransmitter release (secretion) or muscle fibre contraction; and (ii) the Ca2!-dependent action potential (in which Na ions do not participate) in dendrites of Purkinje cells of the cerebellum (see Figure 17.9) and in endocrine cells (Figure 5.1a). In Purkinje cell dendrites, it depolarizes the membrane and thus modulates neuronal integration; in endocrine cells it provides a Ca2! entry to trigger hormone secretion.