Electrical potential actively generated by excitable cells. In nerve cells, the action potential is generated by a transient (less than 1 ms) increase in Na+ and K+ conductances, which brings the membrane potential to the equilibrium potential of Na+. Immediately afterwards, the membrane repolarizes and becomes more negative than before, generating an afterhyperpolarization. In unmyelinated axons, the action potential propagates along the length of the axon through local depolarization of each neighboring patch of membrane. In myelinated axons, action potential is generated only in the Ranvier nodes and jumps rapidly between nodes increasing markedly the propagation speed.