<P> In physiology, an action potential occurs when the membrane potential of a specific axon location rapidly rises and falls: this depolarisation then causes adjacent locations to similarly depolarise . Action potentials occur in several types of animal cells, called excitable cells, which include neurons, muscle cells, endocrine cells, and in some plant cells . </P> <P> In neurons, action potentials play a central role in cell - to - cell communication by providing for--or, with regard to saltatory conduction, assisting--the propagation of signals along the neuron's axon towards synaptic boutons situated at the ends of an axon; these signals can then connect with other neurons at synapses, or to motor cells or glands . In other types of cells, their main function is to activate intracellular processes . In muscle cells, for example, an action potential is the first step in the chain of events leading to contraction . In beta cells of the pancreas, they provoke release of insulin . Action potentials in neurons are also known as "nerve impulses" or "spikes", and the temporal sequence of action potentials generated by a neuron is called its "spike train". A neuron that emits an action potential, or nerve impulse, is often said to "fire". </P>

When is an action potential produced in a muscle fiber