<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> <P> Action potentials are generated by special types of voltage - gated ion channels embedded in a cell's plasma membrane . These channels are shut when the membrane potential is near the (negative) resting potential of the cell, but they rapidly begin to open if the membrane increases to a precisely defined threshold voltage, depolarising the transmembrane potential . When the channels open, they allow an inward flow of sodium ions, which changes the electrochemical gradient, which in turn produces a further rise in the membrane potential . This then causes more channels to open, producing a greater electric current across the cell membrane, and so on . The process proceeds explosively until all of the available ion channels are open, resulting in a large upswing in the membrane potential . The rapid influx of sodium ions causes the polarity of the plasma membrane to reverse, and the ion channels then rapidly inactivate . As the sodium channels close, sodium ions can no longer enter the neuron, and then they are actively transported back out of the plasma membrane . Potassium channels are then activated, and there is an outward current of potassium ions, returning the electrochemical gradient to the resting state . After an action potential has occurred, there is a transient negative shift, called the afterhyperpolarization . </P> <P> In animal cells, there are two primary types of action potentials . One type is generated by voltage - gated sodium channels, the other by voltage - gated calcium channels . Sodium - based action potentials usually last for under one millisecond, but calcium - based action potentials may last for 100 milliseconds or longer . In some types of neurons, slow calcium spikes provide the driving force for a long burst of rapidly emitted sodium spikes . In cardiac muscle cells, on the other hand, an initial fast sodium spike provides a "primer" to provoke the rapid onset of a calcium spike, which then produces muscle contraction . </P>

The first step of an action potential is
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