action potential action potential an action potential occurs when the membrane potential of a specific cell location rapidly rises and falls. this depolarization then causes adjacent locations to similarly depolarize. action potentials occur in several types of animal cells, called excitable cells, which include neurons, muscle cells, and in some plant cells. certain endocrine cells such as pancreatic beta cells, and certain cells of the anterior pituitary gland are also excitable cells. as an action potential (nerve impulse) travels down an axon there is a change in electric polarity across the membrane of the axon. 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. na+ channels open at the beginning of the action potential, and na+ moves into the axon, causing depolarization. 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. the impulse travels down the axon in one direction only, to the axon terminal where it signals other neurons. 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. 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". 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 potential 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 towards zero. 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 they are then 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. 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. overview shape of a typical action potential. the membrane potential remains near a baseline level until at some point in time, it abruptly spikes upward and then rapidly falls. 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. a typical voltage across an animal cell membrane is −70 mv. this means that the interior of the cell has a negative voltage relative to the exterior. in most types of cells, the membrane potential usually stays fairly constant. some types of cells, however, are electrically active in the sense that their voltages fluctuate over time. in some types of electrically active cells, including neurons and muscle cells, the voltage fluctuations frequently take the form of a rapid upward (positive) spike followed by a rapid fall. these up-and-down cycles are known as action potentials. in some types of neurons, the entire up-and-down cycle takes place in a few thousandths of a second. in muscle cells, a typical action potential lasts about a fifth of a second. in plant cells, an action potential may last three seconds or more. the electrical properties of a cell are determined by the structure of its membrane. a cell membrane consists of a lipid bilayer of molecules in which larger protein molecules are embedded. the lipid bilayer is highly resistant to movement of electrically charged ions, so it functions as an insulator. the large membrane-embedded proteins, in contrast, provide channels through which ions can pass across the membrane. action potentials are driven by channel proteins whose configuration switches between closed and open states as a function of the voltage difference between the interior and exterior of the cell. these voltage-sensitive proteins are known as voltage-gated ion channels. process in a typical neuron approximate plot of a typical action potential shows its various phases as the action potential passes a point on a cell membrane. the membrane potential starts out at approximately −70 mv at time zero. a stimulus is applied at time = 1 ms, which raises the membrane potential above −55 mv (the threshold potential). after the stimulus is applied, the membrane potential rapidly rises to a peak potential of +40 mv at time = 2 ms. just as quickly, the potential then drops and overshoots to −90 mv at time = 3 ms, and finally the resting potential of −70 mv is reestablished at time = 5 ms. all cells in animal body tissues are electrically polarized – in other words, they maintain a voltage difference across the cell's plasma membrane, known as the membrane potential. this electrical polarization results from a complex interplay between protein structures embedded in the membrane called ion pumps and ion channels. in neurons, the types of ion channels in the membrane usually vary across different parts of the cell, giving the dendrites, axon, and cell body different electrical properties. as a result, some parts of the membrane of a neuron may be excitable (capable of generating action potentials), whereas others are not. recent studies have shown that the most excitable part of a neuron is the part after the axon hillock (the point where the axon leaves the cell body), which is called the axonal initial segment, but the axon and cell body are also excitable in most cases. each excitable patch of membrane has two important levels of membrane potential: the resting potential, which is the value the membrane potential maintains as long as nothing perturbs the cell, and a higher value called the threshold potential. at the axon hillock of a typical neuron, the resting potential is around –70 millivolts (mv) and the threshold potential is around –55 mv. synaptic inputs to a neuron cause the membrane to depolarize or hyperpolarize; that is, they cause the membrane potential to rise or fall. action potentials are triggered when enough depolarization accumulates to bring the membrane potential up to threshold. when an action potential is triggered, the membrane potential abruptly shoots upward and then equally abruptly shoots back downward, often ending below the resting level, where it remains for some period of time. the shape of the action potential is stereotyped; this means that the rise and fall usually have approximately the same amplitude and time course for all action potentials in a given cell. (exceptions are discussed later in the article). in most neurons, the entire process takes place in about a thousandth of a second. many types of neurons emit action potentials constantly at rates of up to 10–100 per second. however, some types are much quieter, and may go for minutes or longer without emitting any action potentials. biophysical basis action potentials result from the presence in a cell's membrane of special types of voltage-gated ion channels. a voltage-gated ion channel is a transmembrane protein that has three key properties: it is capable of assuming more than one conformation. at least one of the conformations creates a channel through the membrane that is permeable to specific types of ions. the transition between conformations is influenced by the membrane potential. thus, a voltage-gated ion channel tends to be open for some values of the membrane potential, and closed for others. in most cases, however, the relationship between membrane potential and channel state is probabilistic and involves a time delay. ion channels switch between conformations at unpredictable times: the membrane potential determines the rate of transitions and the probability per unit time of each type of transition. action potential propagation along an axon voltage-gated ion channels are capable of producing action potentials because they can give rise to positive feedback loops: the membrane potential controls the state of the ion channels, but the state of the ion channels controls the membrane potential. thus, in some situations, a rise in the membrane potential can cause ion channels to open, thereby causing a further rise in the membrane potential. an action potential occurs when this positive feedback cycle (hodgkin cycle) proceeds explosively. the time and amplitude trajectory of the action potential are determined by the biophysical properties of the voltage-gated ion channels that produce it. several types of channels capable of producing the positive feedback necessary to generate an action potential do exist. voltage-gated sodium channels are responsible for the fast action potentials involved in nerve conduction. slower action potentials in muscle cells and some types of neurons are generated by voltage-gated calcium channels. each of these types comes in multiple variants, with different voltage sensitivity and different temporal dynamics. the most intensively studied type of voltage-dependent ion channels comprises the sodium channels involved in fast nerve conduction. these are sometimes known as hodgkin-huxley sodium channels because they were first characterized by alan hodgkin and andrew huxley in their nobel prize-winning studies of the biophysics of the action potential, but can more conveniently be referred to as nav channels. (the "v" stands for "voltage".) an nav channel has three possible states, known as deactivated, activated, and inactivated. the channel is permeable only to sodium ions when it is in the activated state. when the membrane potential is low, the channel spends most of its time in the deactivated (closed) state. if the membrane potential is raised above a certain level, the channel shows increased probability of transitioning to the activated (open) state. the higher the membrane potential the greater the probability of activation. once a channel has activated, it will eventually transition to the inactivated (closed) state. it tends then to stay inactivated for some time, but, if the membrane potential becomes low again, the channel will eventually transition back to the deactivated state. during an action potential, most channels of this type go through a cycle deactivated→activated→inactivated→deactivated. this is only the population average behavior, however – an individual channel can in principle make any transition at any time. however, the likelihood of a channel's transitioning from the inactivated state directly to the activated state is very low: a channel in the inactivated state is refractory until it has transitioned back to the deactivated state. the outcome of all this is that the kinetics of the nav channels are governed by a transition matrix whose rates are voltage-dependent in a complicated way. since these channels themselves play a major role in determining the voltage, the global dynamics of the system can be quite difficult to work out. hodgkin and huxley approached the problem by developing a set of differential equations for the parameters that govern the ion channel states, known as