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In physiology, an action potential is a short-lasting event in which the electrical membrane potential of a cell rapidly rises and falls, following a consistent trajectory. Action potentials occur in several types of animal cells, called excitable cells, which include neurons, muscle cells, and endocrine cells, as well as in some plant cells.Sony VAIO VGN-SZ691N/X Battery

In neurons, they play a central role in cell-to-cell communication. 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.Sony VAIO VGN-SZ71E/B Battery

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 is often said to "fire".

Action potentials are generated by special types of voltage-gated ion channels embedded in a cell's plasma membrane.Sony VAIO VGN-SZ71VN/X Battery

These channels are shut when the membrane potential is near the resting potential of the cell, but they rapidly begin to open if the membrane potential increases to a precisely defined threshold value. 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. Sony VAIO VGN-SZ71WN/C Battery

This then causes more channels to open, producing a greater electric current, 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. Sony VAIO VGN-SZ74B/B Battery

As the sodium channels close, sodium ions can no longer enter the neuron, and they are actively transported 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. Sony VAIO VGN-SZ750N/C Battery

After an action potential has occurred, there is a transient negative shift, called the afterhyperpolarization or refractory period, due to additional potassium currents. This is the mechanism which prevents an action potential traveling back the way it just came.Sony VAIO VGN-SZ75B/B Battery

In animal cells, there are two primary types of action potentials, one type generated by voltage-gated sodium channels, the other by voltage-gated calcium channels. Sodium-based action potentials usually last for less than one millisecond, whereas calcium-based action potentials may last for 100 milliseconds or longer.Sony VAIO VGN-SZ76 Battery

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.Sony VAIO VGN-SZ770N/C Battery

Function

Nearly all cells from animals, plants and fungi function as batteries, in the sense that they maintain a voltage difference between the interior and the exterior of the cell, with the interior being the negative pole of the battery. Sony VAIO VGN-SZ77N Battery

The voltage of a cell is usually measured in millivolts (mV), or thousandths of a volt. A typical voltage for an animal cell is –70 mV—approximately one-fifteenth of a volt. Because cells are so small, voltages of this magnitude give rise to very strong electric forces within the cell membrane.Sony VAIO VGN-SZ780 Battery

In the majority of cells, the voltage changes very little over time. There are some types of cells, however, that are electrically active in the sense that their voltages fluctuate. In some of these, the voltages sometimes show very rapid up-and-down fluctuations that have a stereotyped form: These up-and-down cycles are known as action potentials.Sony VAIO VGN-SZ78N Battery

The durations of action potentials vary across a wide range. In brain cells of animals, the entire up-and-down cycle may take place in less than a thousandth of a second. In other types of cells, the cycle may last for several seconds.

The electrical properties of an animal cell are determined by the structure of the membrane that surrounds it.Sony VAIO VGN-SZ791N/X Battery

A cell membrane consists of a layer of lipid molecules with larger protein molecules embedded in it. The lipid layer is highly resistant to movement of electrically charged ions, so it functions mainly as an insulator. The large membrane-embedded molecules, in contrast, provide channels through which ions can pass across the membrane, and some of the large molecules are capable of actively moving specific types of ions from one side of the membrane to the other.Sony VAIO VGN-SZ84NS Battery

Process in a typical neuron

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. Sony VAIO VGN-SZ84PS Battery

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. Sony VAIO VGN-SZ84S Battery

As a result, some parts of the membrane of a neuron may be excitable (capable of generating action potentials), whereas others are not. The most excitable part of a neuron is usually theaxon hillock (the point where the axon leaves the cell body), but the axon and cell body are also excitable in most cases.Sony VAIO VGN-SZ84US Battery

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.Sony VAIO VGN-SZ85NS Battery

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.Sony VAIO VGN-SZ85S Battery

When an action potential is triggered, the membrane potential abruptly shoots upward, often reaching as high as +100 mV, 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;Sony VAIO VGN-SZ85US Battery

that is, 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 less than a thousandth of a second.Sony VAIO VGN-SZ94NS Battery

Many types of neurons emit action potentials constantly at rates of up to 10-100 per second; some types, however, 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. Sony VAIO VGN-SZ94PS Battery

A voltage-gated ion channel is a cluster of proteins embedded in the membrane that has three key properties:

  1. It is capable of assuming more than one conformation.
  2. At least one of the conformations creates a channel through the membrane that is permeable to specific types of ions.
  3. The transition between conformations is influenced by the membrane potential.Sony VAIO VGN-SZ94S Battery

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. Ions channels switch between conformations at unpredictable times:Sony VAIO VGN-SZ94US Battery

The membrane potential determines the rate of transitions and the probability per unit time of each type of transition.

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.Sony VAIO VGN-SZ95NS Battery

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 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.Sony VAIO VGN-SZ95S Battery

Several types of channels that are capable of producing the positive feedback necessary to generate an action potential 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.Sony VAIO VGN-SZ95US Battery

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. Sony VAIO VGN-TZ121 Battery

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".)Sony VAIO VGN-TZ13 Battery

An NaV channel has three possible states, known asdeactivated, 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. Sony VAIO VGN-TZ130N/B Battery

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.Sony VAIO VGN-TZ131 Battery

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 cycledeactivated?activated?inactivated?deactivated. Sony VAIO VGN-TZ132/N Battery

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 theinactivated state is refractory until it has transitioned back to the deactivated state.Sony VAIO VGN-TZ132N Battery

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. Sony VAIO VGN-TZ150N/B Battery

Hodgkin and Huxley approached the problem by developing a set of differential equations for the parameters that govern the ion channel states, known as the Hodgkin-Huxley equations. These equations have been extensively modified by later research, but form the starting point for most theoretical studies of action potential biophysics.Sony VAIO VGN-TZ150N/N Battery

As the membrane potential is increased, sodium ion channels open, allowing the entry of sodium ions into the cell. This is followed by the opening of potassium ion channels that permit the exit of potassium ions from the cell. The inward flow of sodium ions increases the concentration of positively-charged cations in the cell and causes depolarization, Sony VAIO VGN-TZ160CB Battery

where the potential of the cell is higher than the cell's resting potential. The sodium channels close at the peak of the action potential, while potassium continues to leave the cell. The efflux of potassium ions decreases the membrane potential or hyperpolarizes the cell.Sony VAIO VGN-TZ160N/B Battery

For small voltage increases from rest, the potassium current exceeds the sodium current and the voltage returns to its normal resting value, typically ?70 mV. However, if the voltage increases past a critical threshold, typically 15 mV higher than the resting value, the sodium current dominates. Sony VAIO VGN-TZ16GN/B Battery

This results in a runaway condition whereby the positive feedback from the sodium current activates even more sodium channels. Thus, the cell "fires," producing an action potential.

Currents produced by the opening of voltage-gated channels in the course of an action potential are typically significantly larger than the initial stimulating current.Sony VAIO VGN-TZ16N Battery

Thus, the amplitude, duration, and shape of the action potential are determined largely by the properties of the excitable membrane and not the amplitude or duration of the stimulus. This all-or-nothing property of the action potential sets it apart from graded potentials such as receptor potentials, electrotonic potentials, and synaptic potentials, Sony VAIO VGN-TZ16N/B Battery

which scale with the magnitude of the stimulus. A variety of action potential types exist in many cell types and cell compartments as determined by the types of voltage-gated channels, leak channels, channel distributions, ionic concentrations, membrane capacitance, temperature, and other factors.Sony VAIO VGN-TZ17/N Battery

The principal ions involved in an action potential are sodium and potassium cations; sodium ions enter the cell, and potassium ions leave, restoring equilibrium. Relatively few ions need to cross the membrane for the membrane voltage to change drastically. The ions exchanged during an action potential, therefore, make a negligible change in the interior and exterior ionic concentrations.Sony VAIO VGN-TZ170N/B Battery

The few ions that do cross are pumped out again by the continuous action of the sodium–potassium pump, which, with other ion transporters, maintains the normal ratio of ion concentrations across the membrane. Calcium cations and chloride anions are involved in a few types of action potentials, such as the cardiac action potential and the action potential in the single-cell algaAcetabularia, respectively.Sony VAIO VGN-TZ170N/N Battery

Although action potentials are generated locally on patches of excitable membrane, the resulting currents can trigger action potentials on neighboring stretches of membrane, precipitating a domino-like propagation.Sony VAIO VGN-TZ17GN/B Battery

In contrast to passive spread of electric potentials (electrotonic potential), action potentials are generated anew along excitable stretches of membrane and propagate without decay. Myelinated sections of axons are not excitable and do not produce action potentials and the signal is propagated passively as electrotonic potential.Sony VAIO VGN-TZ17N Battery

Regularly spaced unmyelinated patches, called the nodes of Ranvier, generate action potentials to boost the signal. Known as saltatory conduction, this type of signal propagation provides a favorable tradeoff of signal velocity and axon diameter. Depolarization of axon terminals, in general, triggers the release of neurotransmitter into the synaptic cleft. Sony VAIO VGN-TZ18/N Battery

In addition, backpropagating action potentials have been recorded in the dendrites of pyramidal neurons, which are ubiquitous in the neocortex. These are thought to have a role in spike-timing-dependent plasticity.

Anatomy of a neuron

Several types of cells support an action potential, such as plant cells, muscle cells, and the specialized cells of the heart (in which occurs the cardiac action potential). Sony VAIO VGN-TZ180N/RC Battery

However, the main excitable cell is the neuron, which also has the simplest mechanism for the action potential.

Neurons are electrically excitable cells composed, in general, of one or more dendrites, a single soma, a single axon and one or more axon terminals. The dendrite is one of the two types of synapses, the other being the axon terminal boutons. Sony VAIO VGN-TZ185N/WC Battery

Dendrites form protrusions in response to the axon terminal boutons. These protrusions, or spines, are designed to capture the neurotransmitters released by the presynaptic neuron. They have a high concentration of ligand activated channels. It is, therefore, here where synapses from two neurons communicate with one another. Sony VAIO VGN-TZ18GN/X Battery

These spines have a thin neck connecting a bulbous protrusion to the main dendrite. This ensures that changes occurring inside the spine are less likely to affect the neighbouring spines. The dendritic spine can, therefore, with rare exception (seeLTP), act as an independent unit. The dendrites then connect onto the soma. Sony VAIO VGN-TZ18N Battery

The soma houses the nucleus, which acts as the regulator for the neuron. Unlike the spines, the surface of the soma is populated by voltage activated ion channels. These channels help transmit the signals generated by the dendrites. Emerging out from the soma is the axon hillock. Sony VAIO VGN-TZ190N/B Battery

This region is characterized by having an incredibly high concentration of voltage-activated sodium channels. In general, it is considered to be the spike initiation zone for action potentials. Multiple signals generated at the spines, and transmitted by the soma all converge here. Immediately after the axon hillock is the axon. Sony VAIO VGN-TZ190N/BC Battery

This is a thin tubular protrusion traveling away from the soma. The axon is insulated by a myelin sheath. Myelin is composed of Schwann cells that wrap themselves multiple times around the axonal segment. This forms a thick fatty layer that prevents ions from entering or escaping the axon.Sony VAIO VGN-TZ191N/XC Battery

This insulation prevents significant signal decay as well as ensuring faster signal speed. This insulation, however, has the restriction that no channels can be present on the surface of the axon. There are, therefore, regularly spaced patches of membrane, which have no insulation.Sony VAIO VGN-TZ195N/XC Battery

These nodes of ranvier can be considered to be 'mini axon hillocks', as their purpose is to boost the signal in order to prevent significant signal decay. At the furthest end, the axon loses its insulation and begins to branch into several axon terminals. These axon terminals then end in the form the second class of synapses, axon terminal buttons.Sony VAIO VGN-TZ198N/RC Battery

These buttons have voltage-activated calcium channels, which come into play when signaling other neurons.

Initiation

Before considering the propagation of action potentials along axons and their termination at the synaptic knobs, it is helpful to consider the methods by which action potentials can be initiated at the axon hillock. Sony VAIO VGN-TZ250N/B Battery

The basic requirement is that the membrane voltage at the hillock be raised above the threshold for firing.[9] There are several ways in which this depolarization can occur.

Dynamics

Action potentials are most commonly initiated by excitatory postsynaptic potentials from a presynaptic neuron.Sony VAIO VGN-TZ250N/N Battery

Typically, neurotransmitter molecules are released by the presynaptic neuron. These neurotransmitters then bind to receptors on the postsynaptic cell. This binding opens various types of ion channels. This opening has the further effect of changing the local permeability of the cell membrane and, thus, the membrane potential.Sony VAIO VGN-TZ250N/P Battery

If the binding increases the voltage (depolarizes the membrane), the synapse is excitatory. If, however, the binding decreases the voltage (hyperpolarizes the membrane), it is inhibitory. Whether the voltage is decreased or increased, the change propagates passively to nearby regions of the membrane (as described by the cable equation and its refinements).Sony VAIO VGN-TZ27/N Battery

Typically, the voltage stimulus decays exponentially with the distance from the synapse and with time from the binding of the neurotransmitter. Some fraction of an excitatory voltage may reach the axon hillock and may (in rare cases) depolarize the membrane enough to provoke a new action potential. Sony VAIO VGN-TZ270N/B Battery

More typically, the excitatory potentials from several synapses must work together at nearly the same time to provoke a new action potential. Their joint efforts can be thwarted, however, by the counteracting inhibitory postsynaptic potentials.

Neurotransmission can also occur through electrical synapses.Sony VAIO VGN-TZ27N Battery

Due to the direct connection between excitable cells in the form of gap junctions, an action potential can be transmitted directly from one cell to the next in either direction. The free flow of ions between cells enables rapid non-chemical-mediated transmission. Rectifying channels ensure that action potentials move only in one direction through an electrical synapse.Sony VAIO VGN-TZ28/N Battery

Electrical synapses are found in all nervous systems, including the human brain, although they are a distinct minority.

"All-or-none" principle

The amplitude of an action potential is independent of the amount of current that produced it. In other words, larger currents do not create larger action potentials. Sony VAIO VGN-TZ285N/RC Battery

Therefore, action potentials are said to be all-or-none (or boolean), since either they occur fully or they do not occur at all. Instead, the frequencyof action potentials is what encodes for the intensity of a stimulus. This is in contrast to receptor potentials, whose amplitudes are dependent on the intensity of a stimulus.Sony VAIO VGN-TZ28N Battery

Sensory neurons

In sensory neurons, an external signal such as pressure, temperature, light, or sound is coupled with the opening and closing of ion channels, which in turn alter the ionic permeabilities of the membrane and its voltage. Sony VAIO VGN-TZ290EAB Battery

These voltage changes can again be excitatory (depolarizing) or inhibitory (hyperpolarizing) and, in some sensory neurons, their combined effects can depolarize the axon hillock enough to provoke action potentials. Examples in humans include the olfactory receptor neuron and Meissner's corpuscle, which are critical for the sense of smell and touch, respectively.Sony VAIO VGN-TZ290EAN Battery

However, not all sensory neurons convert their external signals into action potentials; some do not even have an axon! Instead, they may convert the signal into the release of a neurotransmitter, or into continuous graded potentials, either of which may stimulate subsequent neuron(s) into firing an action potential. Sony VAIO VGN-TZ290EAP Battery

For illustration, in the human ear, hair cells convert the incoming sound into the opening and closing of mechanically gated ion channels, which may cause neurotransmitter molecules to be released. In similar manner, in the humanretina, the initial photoreceptor cells and the next two layers of cells (bipolar cells and horizontal cells) do not produce action potentials;Sony VAIO VGN-TZ295N/XC Battery

only some amacrine cells and the third layer, the ganglion cells, produce action potentials, which then travel up the optic nerve.

Pacemaker potentials

In sensory neurons, action potentials result from an external stimulus. However, some excitable cells require no such stimulus to fire: Sony VAIO VGN-TZ298N/XC Battery

They spontaneously depolarize their axon hillock and fire action potentials at a regular rate, like an internal clock.[16]The voltage traces of such cells are known as pacemaker potentials.[17] The cardiac pacemaker cells of the sinoatrial node in theheart provide a good example.Sony VAIO VGN-TZ33/B Battery

Although such pacemaker potentials have a natural rhythm, it can be adjusted by external stimuli; for instance, heart rate can be altered by pharmaceuticals as well as signals from the sympathetic and parasympatheticnerves. The external stimuli do not cause the cell's repetitive firing, but merely alter its timing.Sony VAIO VGN-TZ33/N Battery

In some cases, the regulation of frequency can be more complex, leading to patterns of action potentials, such as bursting.

Phases

The course of the action potential can be divided into five parts: the rising phase, the peak phase, the falling phase, the undershoot phase, and the refractory period. Sony VAIO VGN-TZ33/W Battery

During the rising phase the membrane potential depolarizes (becomes more positive). The point at which depolarization stops is called the peak phase. At this stage, the membrane potential reaches a maximum. Subsequent to this, there is a falling phase. During this stage the membrane potential hyperpolarizes (becomes more negative).Sony VAIO VGN-TZ350N/B Battery

The undershoot phase is the point during which the membrane potential becomes temporarily more negatively charged than when at rest. Finally, the time during which a subsequent action potential is impossible or difficult to fire is called the refractory period, which may overlap with the other phases.Sony VAIO VGN-TZ350N/N Battery

The course of the action potential is determined by two coupled effects. First, voltage-sensitive ion channels open and close in response to changes in themembrane voltage Vm. This changes the membrane's permeability to those ions. Second, according to the Goldman equation,Sony VAIO VGN-TZ350N/P Battery

this change in permeability changes in the equilibrium potential Em, and, thus, the membrane voltage Vm.[23] Thus, the membrane potential affects the permeability, which then further affects the membrane potential. This sets up the possibility for positive feedback, which is a key part of the rising phase of the action potential.Sony VAIO VGN-TZ370N/B Battery

A complicating factor is that a single ion channel may have multiple internal "gates" that respond to changes in Vm in opposite ways, or at different rates. For example, although raising Vmopens most gates in the voltage-sensitive sodium channel, it also closes the channel's "inactivation gate", albeit more slowly.Sony VAIO VGN-TZ37N/G Battery

Hence, when Vm is raised suddenly, the sodium channels open initially, but then close due to the slower inactivation.

The voltages and currents of the action potential in all of its phases were modeled accurately by Alan Lloyd Hodgkin and Andrew Huxley in 1952, for which they were awarded the Nobel Prize in Physiology or Medicine in 1963.Sony VAIO VGN-TZ37N/P Battery

However, their model considers only two types of voltage-sensitive ion channels, and makes several assumptions about them, e.g., that their internal gates open and close independently of one another. In reality, there are many types of ion channels,[28]and they do not always open and close independently.Sony VAIO VGN-TZ37N/R Battery

Stimulation and rising phase

A typical action potential begins at the axon hillock with a sufficiently strong depolarization, e.g., a stimulus that increases Vm. This depolarization is often caused by the injection of extra sodium cations into the cell;Sony VAIO VGN-TZ37N/X Battery

these cations can come from a wide variety of sources, such as chemical synapses, sensory neurons orpacemaker potentials.

For a neuron at rest, there is a high concentration of sodium and chlorine ions in the extracellular fluid compared to the intracellular fluid while there is a high concentration of potassium ions in the intracellular fluid compared to the extracellular fluid. Sony VAIO VGN-TZ38N/X Battery

This concentration gradient along with potassium leak channelspresent on the membrane of the neuron causes an efflux of potassium ions making the resting potential close to EK? –75 mV. The depolarization opens both the sodium and potassium channels in the membrane, allowing the ions to flow into and out of the axon, respectively.Sony VAIO VGN-TZ398U/XC Battery

If the depolarization is small (say, increasing Vmfrom ?70 mV to ?60 mV), the outward potassium current overwhelms the inward sodium current and the membrane repolarizes back to its normal resting potential around ?70 mV. However, if the depolarization is large enough, the inward sodium current increases more than the outward potassium current and a runaway condition (positive feedback) results:Sony VAIO VGN-TZ50B Battery

the more inward current there is, the more Vm increases, which in turn further increases the inward current.[4] A sufficiently strong depolarization (increase in Vm) causes the voltage-sensitive sodium channels to open; the increasing permeability to sodium drives Vm closer to the sodium equilibrium voltage ENa? +55 mV. Sony VAIO VGN-TZ90HS Battery

The increasing voltage in turn causes even more sodium channels to open, which pushes Vm still further towards ENa. This positive feedback continues until the sodium channels are fully open and Vm is close to ENa.[9] The sharp rise in Vm and sodium permeability correspond to the rising phase of the action potential.Sony VAIO VGN-TZ90NS Battery

The critical threshold voltage for this runaway condition is usually around ?45 mV, but it depends on the recent activity of the axon. A membrane that has just fired an action potential cannot fire another one immediately, since the ion channels have not returned to their usual state. Sony VAIO VGN-TZ90S Battery

The period during which no new action potential can be fired is called the absolute refractory period. At longer times, after some but not all of the ion channels have recovered, the axon can be stimulated to produce another action potential, but only with a much stronger depolarization, e.g., ?30 mV.Sony VAIO VPC CW2MFX/PU Battery

The period during which action potentials are unusually difficult to provoke is called the relative refractory period.

Peak and falling phase

The positive feedback of the rising phase slows and comes to a halt as the sodium ion channels become maximally open. Sony VAIO VPC S11V9E/B Battery

At the peak of the action potential, the sodium permeability is maximized and the membrane voltage Vm is nearly equal to the sodium equilibrium voltage ENa. However, the same raised voltage that opened the sodium channels initially also slowly shuts them off, by closing their pores; the sodium channels become inactivated.Sony VAIO VPCB119GJ/B Battery

This lowers the membrane's permeability to sodium relative to potassium, driving the membrane voltage back towards the resting value. At the same time, the raised voltage opens voltage-sensitive potassium channels; the increase in the membrane's potassium permeability drives Vm towards EK.Sony VAIO VPCB11AGJ Battery

Combined, these changes in sodium and potassium permeability cause Vm to drop quickly, repolarizing the membrane and producing the "falling phase" of the action potential.

Afterhyperpolarization

The raised voltage opened many more potassium channels than usual, and some of these do not close right away when the membrane returns to its normal resting voltage. Sony VAIO VPCB11AVJ Battery

In addition, further potassium channels open in response to the influx of calcium ions during the action potential. The potassium permeability of the membrane is transiently unusually high, driving the membrane voltage Vm even closer to the potassium equilibrium voltage EK.Sony VAIO VPCB11V9E Battery

Hence, there is an undershoot or hyperpolarization, termed an afterhyperpolarization in technical language, that persists until the membrane potassium permeability returns to its usual value.

Refractory period

Each action potential is followed by a refractory period, which can be divided into an absolute refractory period, Sony VAIO VPCB11X9E Battery

during which it is impossible to evoke another action potential, and then a relative refractory period, during which a stronger-than-usual stimulus is required. These two refractory periods are caused by changes in the state of sodium and potassium channel molecules. Sony VAIO VPCCW18FJ/P Battery

When closing after an action potential, sodium channels enter an "inactivated" state, in which they cannot be made to open regardless of the membrane potential—this gives rise to the absolute refractory period. Even after a sufficient number of sodium channels have transitioned back to their resting state, Sony VAIO VPCCW18FJ/R Battery

op battery">it frequently happens that a fraction of potassium channels remains open, making it difficult for the membrane potential to depolarize, and thereby giving rise to the relative refractory period. Because the density and subtypes of potassium channels may differ greatly between different types of neurons, the duration of the relative refractory period is highly variable.Sony VAIO VPCCW18FJ/W Battery

The absolute refractory period is largely responsible for the unidirectional propagation of action potentials along axons. At any given moment, the patch of axon behind the actively spiking part is refractory, but the patch in front, not having been activated recently, is capable of being stimulated by the depolarization from the action potential.Sony VAIO VPCCW19FJ/W Battery

Propagation

The action potential generated at the axon hillock propagates as a wave along the axon. The currents flowing inwards at a point on the axon during an action potential spread out along the axon, and depolarize the adjacent sections of its membrane.Sony VAIO VPCCW1AFJ Battery

If sufficiently strong, this depolarization provokes a similar action potential at the neighboring membrane patches. This basic mechanism was demonstrated by Alan Lloyd Hodgkin in 1937. After crushing or cooling nerve segments and thus blocking the action potentials, he showed that an action potential arriving on one side of the block could provoke another action potential on the other, provided that the blocked segment was sufficiently short. Sony VAIO VPCCW1AHJ Battery

Once an action potential has occurred at a patch of membrane, the membrane patch needs time to recover before it can fire again. At the molecular level, thisabsolute refractory period corresponds to the time required for the voltage-activated sodium channels to recover from inactivation, i.e., to return to their closed state.Sony VAIO VPCCW1S1E Battery

There are many types of voltage-activated potassium channels in neurons, some of them inactivate fast (A-type currents) and some of them inactivate slowly or not inactivate at all; this variability guarantees that there will be always an available source of current for repolarization, even if some of the potassium channels are inactivated because of preceding depolarization. Sony VAIO VPCCW1S1E/B Battery

On the other hand, all neuronal voltage-activated sodium channels inactivate within several millisecond during strong depolarization, thus making following depolarization impossible until a substantial fraction of sodium channels is not returned to their closed state.Sony VAIO VPCCW1S1E/L Battery

Although it limits the frequency of firing, the absolute refractory period ensures that the action potential moves in only one direction along an axon.[35] The currents flowing in due to an action potential spread out in both directions along the axon.[40] However, only the unfired part of the axon can respond with an action potential;Sony VAIO VPCCW1S1E/P Battery

the part that has just fired is unresponsive until the action potential is safely out of range and cannot restimulate that part. In the usualorthodromic conduction, the action potential propagates from the axon hillock towards the synaptic knobs (the axonal termini); propagation in the opposite direction—known as antidromic conduction—is very rare.Sony VAIO VPCCW1S1E/R Battery

However, if a laboratory axon is stimulated in its middle, both halves of the axon are "fresh", i.e., unfired; then two action potentials will be generated, one traveling towards the axon hillock and the other traveling towards the synaptic knobs.

Myelin and saltatory conduction

In order to enable fast and efficient transduction of electrical signals in the nervous system, certain neuronal axons are covered with myelin sheaths.Sony VAIO VPCCW1S1E/W Battery

Myelin is a multilamellar membrane that enwraps the axon in segments separated by intervals known as nodes of Ranvier, is produced by specialized cells, Schwann cells exclusively in the peripheral nervous system, and by oligodendrocytesexclusively in the central nervous system.Sony VAIO VPCCW21FX/B Battery

Myelin sheath reduces membrane capacitance and increases membrane resistance in the inter-node intervals, thus allowing a fast, saltatory movement of action potentials from node to node. Myelination is found mainly in vertebrates, but an analogous system has been discovered in a few invertebrates, such as some species ofshrimp.Sony VAIO VPCCW21FX/L Battery

Not all neurons in vertebrates are myelinated; for example, axons of the neurons comprising autonomous (vegetative) nervous system are not myelinated in general.

Myelin prevents ions from entering or leaving the axon along myelinated segments. As a general rule, myelination increases theconduction velocity of action potentials and makes them more energy-efficient.Sony VAIO VPCCW21FX/R Battery

Whether saltatory or not, the mean conduction velocity of an action potential ranges from 1 m/s to over 100 m/s, and, in general, increases with axonal diameter.

Action potentials cannot propagate through the membrane in myelinated segments of the axon.Sony VAIO VPCCW21FX/W Battery

However, the current is carried by the cytoplasm, which is sufficient to depolarize the first or second subsequent node of Ranvier. Instead, the ionic current from an action potential at one node of Ranvier provokes another action potential at the next node; this apparent "hopping" of the action potential from node to node is known as saltatory conduction. Sony VAIO VPCCW26EC Battery

Although the mechanism of saltatory conduction was suggested in 1925 by Ralph Lillie, the first experimental evidence for saltatory conduction came from Ichiji Tasaki[48] and Taiji Takeuchi[49]and from Andrew Huxley and Robert Stämpfli.[50] By contrast, in unmyelinated axons, the action potential provokes another in the membrane immediately adjacent, and moves continuously down the axon like a wave.Sony VAIO VPCCW26FX/B Battery

Myelin has two important advantages: fast conduction speed and energy efficiency. For axons larger than a minimum diameter (roughly 1 micrometre), myelination increases the conduction velocity of an action potential, typically tenfold.[53] Conversely, for a given conduction velocity, myelinated fibers are smaller than their unmyelinated counterparts. Sony VAIO VPCCW28EC Battery

For example, action potentials move at roughly the same speed (25 m/s) in a myelinated frog axon and an unmyelinated squid giant axon, but the frog axon has a roughly 30-fold smaller diameter and 1000-fold smaller cross-sectional area. Also, since the ionic currents are confined to the nodes of Ranvier, far fewer ions "leak" across the membrane, saving metabolic energy.Sony VAIO VPCCW28FJ/P Battery

This saving is a significant selective advantage, since the human nervous system uses approximately 20% of the body's metabolic energy.

The length of axons' myelinated segments is important to the success of saltatory conduction. Sony VAIO VPCCW28FJ/R Battery

They should be as long as possible to maximize the speed of conduction, but not so long that the arriving signal is too weak to provoke an action potential at the next node of Ranvier. In nature, myelinated segments are generally long enough for the passively propagated signal to travel for at least two nodes while retaining enough amplitude to fire an action potential at the second or third node.Sony VAIO VPCCW28FJ/W Battery

Thus, the safety factor of saltatory conduction is high, allowing transmission to bypass nodes in case of injury. However, action potentials may end prematurely in certain places where the safety factor is low, even in unmyelinated neurons; a common example is the branch point of an axon, where it divides into two axons.Sony VAIO VPCCW29FJ/W Battery

Some diseases degrade myelin and impair saltatory conduction, reducing the conduction velocity of action potentials.[55] The most well-known of these is multiple sclerosis, in which the breakdown of myelin impairs coordinated movement.Sony VAIO VPCCW2AFJ Battery

Cable theory

The flow of currents within an axon can be described quantitatively by cable theory[57] and its elaborations, such as the compartmental model.[58] Cable theory was developed in 1855 by Lord Kelvin to model the transatlantic telegraph cable[59] and was shown to be relevant to neurons by Hodgkin and Rushton in 1946.Sony VAIO VPCCW2AHJ Battery

In simple cable theory, the neuron is treated as an electrically passive, perfectly cylindrical transmission cable, which can be described by apartial differential equation[57]

where V(x, t) is the voltage across the membrane at a time t and a position x along the length of the neuron, and where ? and ? are the characteristic length and time scales on which those voltages decay in response to a stimulus.Sony VAIO VPCCW2S1E Battery

Referring to the circuit diagram above, these scales can be determined from the resistances and capacitances per unit length[61]

These time and length-scales can be used to understand the dependence of the conduction velocity on the diameter of the neuron in unmyelinated fibers. Sony VAIO VPCCW2S1E/B Battery

For example, the time-scale ? increases with both the membrane resistance rm and capacitance cm. As the capacitance increases, more charge must be transferred to produce a given transmembrane voltage (by the equation Q=CV); as the resistance increases, less charge is transferred per unit time, making the equilibration slower. Sony VAIO VPCCW2S1E/L Battery

In similar manner, if the internal resistance per unit length ri is lower in one axon than in another (e.g., because the radius of the former is larger), the spatial decay length ? becomes longer and theconduction velocity of an action potential should increase. Sony VAIO VPCCW2S1E/P Battery

If the transmembrane resistance rm is increased, that lowers the average "leakage" current across the membrane, likewise causing ?to become longer, increasing the conduction velocity.

In general, action potentials that reach the synaptic knobs cause a neurotransmitter to be released into the synaptic cleft.Sony VAIO VPCCW2S1E/R Battery

Neurotransmitters are small molecules that may open ion channels in the postsynaptic cell; most axons have the same neurotransmitter at all of their termini. The arrival of the action potential opens voltage-sensitive calcium channels in the presynaptic membrane; Sony VAIO VPCCW2S1E/W Battery

the influx of calcium causes vesicles filled with neurotransmitter to migrate to the cell's surface and release their contents into the synaptic cleft.[63] This complex process is inhibited by the neurotoxins tetanospasmin and botulinum toxin, which are responsible for tetanus and botulism, respectively.Sony VAIO VPCCW2S5C CN1 Battery

Some synapses dispense with the "middleman" of the neurotransmitter, and connect the presynaptic and postsynaptic cells together.[65] When an action potential reaches such a synapse, the ionic currents flowing into the presynaptic cell can cross the barrier of the two cell membranes and enter the postsynaptic cell through pores known as connexins.Sony VAIO VPCF112FX/B Battery

Thus, the ionic currents of the presynaptic action potential can directly stimulate the postsynaptic cell. Electrical synapses allow for faster transmission because they do not require the slow diffusion of neurotransmitters across the synaptic cleft. Hence, electrical synapses are used whenever fast response and coordination of timing are crucial, as in escape reflexes, the retina of vertebrates, and the heart.Sony VAIO VPCF115FG/B Battery

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