Some types of cells, however, are electrically active in the sense that their voltages fluctuate over time. In most types of cells, the membrane potential usually stays fairly constant.

This means that the interior of the cell has a negative voltage relative to the exterior. A typical voltage across an animal cell membrane is −70 mV. Nearly all cell membranes in animals, plants and fungi maintain a voltage difference between the exterior and interior of the cell, called the membrane potential. The membrane potential remains near a baseline level until at some point in time, it abruptly spikes upward and then rapidly falls. 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. In some types of neurons, slow calcium spikes provide the driving force for a long burst of rapidly emitted sodium spikes. Sodium-based action potentials usually last for under one millisecond, but calcium-based action potentials may last for 100 milliseconds or longer. One type is generated by voltage-gated sodium channels, the other by voltage-gated calcium channels. In animal cells, there are two primary types of action potentials. After an action potential has occurred, there is a transient negative shift, called the afterhyperpolarization. Potassium channels are then activated, and there is an outward current of potassium ions, returning the electrochemical gradient to the resting state. As the sodium channels close, sodium ions can no longer enter the neuron, and they are then actively transported back out of the plasma membrane. The rapid influx of sodium ions causes the polarity of the plasma membrane to reverse, and the ion channels then rapidly inactivate. The process proceeds explosively until all of the available ion channels are open, resulting in a large upswing in the membrane potential. This then causes more channels to open, producing a greater electric current across the cell membrane and so on. 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 towards zero. 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 potential increases to a precisely defined threshold voltage, depolarising the transmembrane potential. A neuron that emits an action potential, or nerve impulse, is often said to 'fire'.Īction potentials are generated by special types of voltage-gated ion channels embedded in a cell's plasma membrane. 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'. In beta cells of the pancreas, they provoke release of insulin. In muscle cells, for example, an action potential is the first step in the chain of events leading to contraction. In other types of cells, their main function is to activate intracellular processes. In neurons, action potentials play a central role in cell–cell communication by providing for-or with regard to saltatory conduction, assisting-the propagation of signals along the neuron's axon toward 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. Certain endocrine cells such as pancreatic beta cells, and certain cells of the anterior pituitary gland are also excitable cells. Action potentials occur in several types of animal cells, called excitable cells, which include neurons, muscle cells, and in some plant cells. This depolarization then causes adjacent locations to similarly depolarize.

The impulse travels down the axon in one direction only, to the axon terminal where it signals other neurons.Īn action potential occurs when the membrane potential of a specific cell rapidly rises and falls. Repolarization occurs when K + channels open and K + moves out of the axon, creating a change in electric polarity between the outside of the cell and the inside. Na + channels open at the beginning of the action potential, and Na + moves into the axon, causing depolarization. In response to a signal from another neuron, sodium- (Na +) and potassium- (K +)–gated ion channels open and close as the membrane reaches its threshold potential. As an action potential (nerve impulse) travels down an axon there is a change in electric polarity across the membrane of the axon.

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