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Encyclopedia > Action potential
A. A schematic view of an idealized action potential illustrates its various phases as the action potential passes a point on a cell membrane. B. Actual recordings of action potentials are often distorted compared to the schematic view because of variations in electrophysiological techniques used to make the recording
A. A schematic view of an idealized action potential illustrates its various phases as the action potential passes a point on a cell membrane. B. Actual recordings of action potentials are often distorted compared to the schematic view because of variations in electrophysiological techniques used to make the recording

An action potential is an pulse-like wave of membrane voltage that can travel on certain types of cell membranes, most commonly the membrane of the axon of a neuron, but also in other types of excitable cells, such as cardiac muscle cells and even plant cells. The resting voltage across the axonal membrane is typically -60 mV to -70 mV, with the inside being more negative than the outside; this voltage results mainly from a strong difference in concentrations of potassium inside and outside the cell, as described by the Goldman equation. As an action potential passes through a point, the voltage rises to roughly +40 mV in roughly a millisecond, and then returns to -60 mV somewhat more slowly. The action potential moves along the axon at various speeds, depending on the axon's diameter; the conduction velocity can range from roughly 1-100 meters/second. Because they move so quickly, action potentials are useful in conveying information along neurons, which are extraordinarily long cells, sometimes over a meter in length. (For comparison, ordinary eukaryotic cells are typically 100,000 times smaller, being roughly of radius 10 μm.) The transport of a chemical signal over such distances would be much slower. Image File history File links Download high resolution version (795x1130, 290 KB)Modified version of older [[Image:Action potential reloaded. ... Image File history File links Download high resolution version (795x1130, 290 KB)Modified version of older [[Image:Action potential reloaded. ... Look up cell membrane in Wiktionary, the free dictionary. ... Current Clamp is a common technique in electrophysiology. ... Look up cell membrane in Wiktionary, the free dictionary. ... An axon or nerve fiber, is a long, slender projection of a nerve cell, or neuron, that conducts electrical impulses away from the neurons cell body or soma. ... This article is about cells in the nervous system. ... Cardiac muscle is a type of involuntary striated muscle found within the heart. ... Look up cell in Wiktionary, the free dictionary. ... For other uses, see Plant (disambiguation). ... Membrane potential (or transmembrane potential or transmembrane potential difference or transmembrane potential gradient), is the electrical potential difference (voltage) across a cells plasma membrane. ... The Goldman-Hodgkin-Katz voltage equation, more commonly known as the Goldman equation is used in cell membrane physiology to determine the potential across a cells membrane taking into account all of the ions that are permeant through that membrane. ... The metre, or meter (symbol: m) is the SI base unit of length. ... This article is about the unit of time. ...


An action potential is stimulated by depolarizing the membrane, i.e., by making the voltage of the cell's interior less negative relative to the cell's exterior. Such a depolarization opens voltage-sensitive channels, which in turn allow positive current to flow inwards, further depolarizing the membrane. Weak depolarizations are damped out, restoring the resting potential; however, a sufficiently strong stimulus causes the membrane to "fire", initiating a positive feedback loop that raises the membrane voltage drastically in a short time. The membrane voltage is restored to its resting value by a combination of several effects; the channels responsible for the initial inwards current become inactivated, while the raised voltage opens other voltage-sensitive channels that allow a compensating outwards current. In neurons, the rise and fall of membrane voltage is usually very rapid (typically, a few thousandth of a second); hence, action potentials are sometimes called "spikes". The passage of an action potential can leave the ionic channels in a non-equilibrium state, making it transiently harder to open them and produce another action potential at the same spot; the axon is said to be refractory. The principal ions involved in an action potential are sodium and potassium cations; the sodium ions enter the cell initially, and the potassium ions leave it, restoring equilibrium. Relatively few ions are required to cross the membrane to change the membrane voltage drastically; hence, the ions exchanged during an action potential make a negligible change in the interior and exterior ionic concentrations. Positive feedback is a feedback system in which the system responds to the perturbation in the same direction as the perturbation (It is sometimes referred to as cumulative causation). ... For sodium in the diet, see Salt. ... General Name, symbol, number potassium, K, 19 Chemical series alkali metals Group, period, block 1, 4, s Appearance silvery white Standard atomic weight 39. ... A cation is an ion with positive charge. ...


The action potential "travels" because it raises the voltage at one patch of membrane dramatically, causing a similar rise at adjacent patches of membrane, as described by the cable equation. The axon generally forms many branches, and the action potential usually travels both forks of a branch point. The action potential stops at the termini of these branches, but make provoke the extracellular release of neurotransmitters at these synapses. These neurotransmitters diffuse and may bind to receptors on an adjacent excitable cell (usually a muscle cell or the dendrites of another neuron). These receptors are usually ionic channels although, in contrast to the axonal channels, they are opened by chemical binding and generally not by changes in voltage. The binding of neurotransmitters can help to depolarize the membrane (an excitatory channel) or make shunt any excitatory currents back outside (an inhibitory channel). If these polarizations are sufficiently strong, they can provoke another action potential (usually at the axon hillock), beginning the process anew. // [edit] Overview Figure. ... Chemical structure of D-aspartic acid, a common amino acid neurotransmitter. ... Illustration of the major elements in a prototypical synapse. ... Dendrites (from Greek dendron, “tree”) are the branched projections of a neuron that act to conduct the electrical stimulation received from other neural cells to the cell body, or soma, of the neuron from which the dendrites project. ... In neuroscience, an excitatory postsynaptic potential (EPSP) is a temporary increase in postsynaptic membrane potential caused by the flow of positively charged ions into the postsynaptic cell. ... Inhibitory Postsynaptic Potential is commonly abbreviated to Impulses are transmitted from neuron to neuron by the release of a chemical transmitter across synaptic clefts from the synaptic vesicles along the axon to the postsynaptic receptors of another neuron. ... The arrow labeled axon is pointing directly at the axon hillock. ...

Contents

Overview

An action potential is a depolarizing all-or-nothing stimulus that propagates along a cell's surface without losing intensity. There is always a difference in electrostatic potential between the inside and outside of a cell, i.e. the cell is polarized. This membrane potential is the result of the distribution of ions across the cell membrane and the selective permeability of the membrane to these ions. The voltage of an inactive cell remains close to a resting potential with excess negative charge inside the cell. When the membrane of an excitable cell becomes depolarized beyond a threshold, the cell undergoes an action potential (it "fires"), often called a "spike" (see Threshold and initiation). Membrane potential (or transmembrane potential or transmembrane potential difference or transmembrane potential gradient), is the electrical potential difference (voltage) across a cells plasma membrane. ... Membrane potential (or transmembrane potential or transmembrane potential difference or transmembrane potential gradient), is the electrical potential difference (voltage) across a cells plasma membrane. ... This article is about the electrically charged particle. ... The resting potential of a cell is the membrane potential that would be maintained if there were no action potentials, synaptic potentials, or other active changes in the membrane potential. ... In biology, depolarization is the event a cell undergoes when its membrane potential grows more positive with respect to the extracellular solution. ...


An action potential is a rapid change of the polarity of the voltage from negative to positive and then vice versa, the entire cycle lasting on the order of milliseconds. Each cycle — and therefore each action potential — has a rising phase, a falling phase, and finally an undershoot (see Phases). In specialized muscle cells of the heart, such as cardiac pacemaker cells, a plateau phase of intermediate voltage may precede the falling phase, extending the action potential duration into hundreds of milliseconds. The heart and lungs, from an older edition of Grays Anatomy. ... The contractions of the heart are controlled by electrical impulses, these fire at a rate which controls the beat of the heart. ...


Action potentials are measured with the recording techniques of electrophysiology and more recently with neurochips containing EOSFETs. An oscilloscope recording the membrane potential from a single point on an axon shows each stage of the action potential as the wave passes. These phases trace an arc that resembles a distorted sine wave; its amplitude depends on whether the action potential wave has reached that point on the membrane or has passed it and if so, how long ago. Current Clamp is a common technique in electrophysiology. ... A neurochip is a chip (integrated circuit/microprocessor) that is designed for the interaction with neuronal cells. ... An EOSFET or electrolyte-oxide-semiconductor field effect transistor is a FET, like a MOSFET, but with the metal replaced by electrolyte solution for the detection of neuronal activity. ... Illustration showing the interior of a cathode-ray tube for use in an oscilloscope. ... In mathematics, the trigonometric functions are functions of an angle, important when studying triangles and modeling periodic phenomena. ...


The action potential does not dwell in one location of the cell's membrane, but travels along the membrane (see Propagation). It can travel along an axon for long distances, for example to carry signals from the spinal cord to the muscles of the foot. After traveling the whole length of the axon, the action potential reaches a synapse, where it stimulates the release of neurotransmitters. These neurotransmitters can immediately induce an action potential in the next neuron to propagate the signal, but the response is usually more complex. An axon or nerve fiber, is a long, slender projection of a nerve cell, or neuron, that conducts electrical impulses away from the neurons cell body or soma. ... The Spinal cord nested in the vertebral column. ... For other uses, see Foot (disambiguation). ... Illustration of the major elements in a prototypical synapse. ... Neurotransmitters are chemicals that are used to relay, amplify and modulate electrical signals between a presynaptic and a postsynaptic neuron. ...


Both the speed and complexity of action potentials vary between different types of cells, but their amplitudes tend to be roughly the same. Within any one cell, consecutive action potentials are typically indistinguishable. Neurons are thought to transmit information by generating sequences of action potentials called "spike trains". By varying both the rate as well as the precise timing of the action potentials they generate, neurons can change the information that they transmit.


Underlying mechanism

The hydrophobic cell membrane prevents charged molecules from easily diffusing through it, permitting a potential difference to exist across the membrane.
The hydrophobic cell membrane prevents charged molecules from easily diffusing through it, permitting a potential difference to exist across the membrane.

Image File history File links CellMembraneDrawing. ... Image File history File links CellMembraneDrawing. ... Look up cell membrane in Wiktionary, the free dictionary. ... In membrane biophysics sometimes used interchangeably with cell potential, but applicable to any lipid bilayer or membrane. ...

Resting potential

Main article: Resting potential

The resting potential is what would be maintained were there no action potentials, synaptic potentials, or other changes to the membrane potential. In neurons the resting potential is approximately -70 mV (the negative sign signifies excess negative charge inside the cell relative to the outside). The resting potential is mostly determined by the ion concentrations in the fluids on both sides of the cell membrane and the ion transport proteins in the cell membrane. The term resting is somewhat misleading, for the cell must constantly do work to maintain the resting potential. The establishment of this potential difference involves several factors, the most important of which are the transport of ions across the cell membrane and the selective permeability of the membrane to these ions. The resting potential of a cell is the membrane potential that would be maintained if there were no action potentials, synaptic potentials, or other active changes in the membrane potential. ... In neurobiology, Inhibitory Post-Synaptic Current (IPSC) and the complementary process of Excitatory Post-Synaptic Current (EPSC) are thought to be the foundational concepts that underlie the basic features of neurotransmission in mammalian cells. ... Josephson junction array chip developed by NIST as a standard volt. ... Look up cell membrane in Wiktionary, the free dictionary. ... Drawing of a cell membrane A component of every biological cell, the cell membrane (or plasma membrane) is a thin and structured bilayer of phospholipid and protein molecules that envelopes the cell. ... A representation of the 3D structure of myoglobin showing coloured alpha helices. ...


The active transport of potassium and sodium ions into and out of the cell, respectively, is accomplished by a number of sodium-potassium pumps scattered across the cell membrane. Each pump transports two ions of potassium into the cell for every three ions of sodium pumped out. This establishes a particular distribution of positively charged ions across the cell membrane, with more sodium present outside the cell than inside. In some situations, the electrogenic sodium-potassium pumps make a significant contribution to the resting membrane potential, but in most cells there are potassium leak channels that dominate the value of the resting potential. Sodium-Potassium pump, an example of Primary active transport secondary active transport Active transport (sometimes called active uptake) is the mediated transport of biochemicals, and other atomic/molecular substances, across membranes. ... General Name, symbol, number potassium, K, 19 Chemical series alkali metals Group, period, block 1, 4, s Appearance silvery white Standard atomic weight 39. ... For sodium in the diet, see Salt. ... ... Two-pore-domain potassium channels: This family of 15 members form what is known as leak channels, and they follow Goldman-Hodgkin-Katz (open) rectification. ...


Sodium and potassium ions diffuse through open ion channels under the influence of their electrochemical gradients. At the resting potential, the net movement of sodium into the cell equals the net movement of potassium out of the cell. However, the resting cell membrane is approximately 75 times more permeable to potassium than to sodium because potassium leak channels are always open. As a result, the cell's resting membrane potential is closer to the equilibrium potential of potassium (=EK=−90 mV) than the equilibrium potential of sodium (=ENa=+60 mV). diffusion (disambiguation). ... In cellular biology, an electrochemical gradient refers to the electrical and chemical properties across a membrane. ... In a biological membrane, the reversal potential of a particular ion is the membrane voltage at which there is no net flow of ions from one side of the membrane to the other. ...


Like the resting potential, action potentials depend upon the permeability of the cell membrane to sodium and potassium ions. Transient changes in conductance for different ions cause the changes in membrane potential necessary to initiate, sustain, and terminate action potentials.


Phases

The sequence of events that underlie the action potential are outlined below:


Resting potential

At rest in many neurons, potassium channels (both Kir and K2P) are open while sodium channels are closed. Though no net current flows, the resting potential is pulled toward the K+ reversal potential as K+ is the primary permeant ion. Other tissue types, such as skeletal muscle, can also have a large resting Cl- conductance, increasing the resting potential to more positive values. In cell biology, potassium channels are the most common type of ion channel. ... Inwardly rectifing potassium channels (Kir or IRK) are potassium selective ion channels. ... Two-pore-domain potassium channels: This family of 15 members form what is known as leak channels, and they follow Goldman-Hodgkin-Katz (open) rectification. ... Sodium channels (also known as voltage-gated sodium channels) are integral membrane proteins that are localized in and conduct sodium ions (Na+) through a cells plasma membrane. ... A top-down view of skeletal muscle Skeletal muscle is a type of striated muscle, usually attached to the skeleton. ... The resting potential of a cell is the membrane potential that would be maintained if there were no action potentials, synaptic potentials, or other active changes in the membrane potential. ...


Stimulation

A local membrane depolarization caused by an excitatory stimulus causes some voltage-gated sodium channels in the axon hillock membrane to open, causing net inward movement of sodium ions through the channels along their electrochemical gradient. This movement of sodium ions across the membrane is an example of facilitated diffusion[1]. Because they are positively charged, the inward moving sodium ions make the potential difference across the membrane less negative inside. This initial inward movement of sodium ions is favored by both the negative-inside membrane potential and the concentration gradient of sodium ions across the membrane (less sodium inside). The movement of individual sodium ions involves many random molecular collisions and at any particular moment a sodium ion might be moving outward, but the net movement of sodium is inward, as determined by the electrochemical gradient. Sodium channels are integral membrane proteins that exist in a cells plasma membrane and regulate the flow of sodium (Na+) ions into it. ... In cellular biology, an electrochemical gradient refers to the electrical and chemical properties across a membrane. ... facilitated diffusion in cell membrane, showing ion channels and carrier proteins Facilitated diffusion (or facilitated transport) is a process of diffusion, a form of passive transport, where molecules diffuse across membranes, with the assistance of transport proteins. ...


Depolarization ("Rising phase")

As sodium ions enter and the membrane potential becomes less negative, more sodium channels open, causing an even greater influx of sodium ions. This is an example of positive feedback. As more sodium channels open, the sodium current dominates over the potassium leak current and the membrane potential becomes positive inside. Recent experiments on cortical neurons suggest that sodium channels open cooperatively,[2] allowing for a much faster uptake than is possible for Hodgkin-Huxley–type dynamics. Positive feedback is a feedback system in which the system responds to the perturbation in the same direction as the perturbation (It is sometimes referred to as cumulative causation). ... Membrane potential (or transmembrane potential or transmembrane potential difference or transmembrane potential gradient), is the electrical potential difference (voltage) across a cells plasma membrane. ...


Peak

See also: Goldman-Hodgkin-Katz voltage equation

By the time the membrane potential has reached a peak value of around +50 mV, time-dependent inactivation gates on the sodium channels have already started to close, reducing and finally preventing further influx of sodium ions. While this occurs, the voltage-sensitive activation gates on the voltage-gated potassium channels begin to open. In cell biology, potassium channels are the most common type of ion channel. ...


It is important to appreciate that very few ions actually cross the membrane at any stage in the action potential. There is no 'flood' of sodium into the cell; the gross intracellular and extracellular concentrations of sodium and potassium change so little during the action potential as to be negligible. Instead, the change in membrane polarity occurs due to the permeability for sodium, PNa, increasing greatly via the positive feedback system described (depolarization causes voltage-gated sodium channels to open, so membrane becomes more depolarized etc). Increasing PNa relative to potassium permeability (PK) affects voltage because it lifts the membrane potential towards that of the equilibrium potential for sodium (ENa), which is approximately +55mV.


This can be measured quantitatively using the Goldman equation, The Goldman-Hodgkin-Katz voltage equation, more commonly known as the Goldman equation is used in cell membrane physiology to determine the potential across a cells membrane taking into account all of the ions that are permeant through that membrane. ...


 V = frac{RT}{F} ln frac{P_{hbox{K}}[hbox{K}^+]_{hbox{extracellular}} + P_{hbox{Na}}[hbox{Na}^+]_{hbox{extracellular}}}{P_{hbox{K}}[hbox{K}^+]_{hbox{intracellular}} + P_{hbox{Na}}[hbox{Na}^+]_{hbox{intracellular}}}


Concentrations of Na and K in and out of the cell do not change much, but PNa and PK values do change markedly, and it is this that changes the value for V.


Repolarization ("Falling phase")

As voltage-gated potassium channels open, there is a large outward movement of potassium ions driven by the potassium concentration gradient and initially favored by the positive-inside electrical gradient. As potassium ions diffuse out, this movement of positive charge causes a reversal of the membrane potential to negative-inside and repolarization of the neuron back towards the large negative-inside resting potential.


Again, it is not the movement of potassium ions that changes membrane voltage. It is the value for PK rising above that for PNa, dragging membrane voltage back towards the equilibrium constant for potassium (around -70mV) (see Goldman Constant Field Equation).


Hyperpolarization ("Undershoot")

Closing of voltage-gated potassium channels is both voltage- and time-dependent. As potassium exits the cell, the resulting membrane repolarization initiates the closing of voltage-gated potassium channels. These channels do not close immediately in response to a change in membrane potential; rather, voltage-gated potassium channels (also called delayed rectifier potassium channels) have a delayed response, such that potassium continues to flow out of the cell even after the membrane has fully repolarized. Thus the membrane potential dips below the normal resting membrane potential of the cell for a brief moment; this dip of hyperpolarization is known as the undershoot.


Refractory Period

During the next, ~ 1–3 ms, action potential initiation becomes difficult. This is the 'Refractory Period', consisting of an absolute and relative phase. In the absolute refractory period, the Na+ Channels cannot be opened by a stimulus; they have entered an inactivated state. This is time-dependent, and during this phase no action potential, irrespective of applied voltage, will be fired. In the relative refractory period (immediately after the absolute phase), action potentials can be initiated, but the threshold is greater. There are two reasons for this: the cell may still be slightly hyperpolarized due to still higher than resting value for PK, so more voltage is required to reach threshold, and also the threshold itself is higher than usual because some of the sodium channels will still be inactivated. (Note that the sodium channel therefore has at least three states: closed, open and inactivated — closed and not able to open). The refractory period is important because it ensures unidirectional (one way) propagation of the action potential. Schematic of an electrophysiological recording of an action potential showing the various phases which occur as the wave passes a point on a cell membrane. ...


There is a common misconception that the Na+/K+ pump restores the resting potential during the action potential falling phase by actively pumping Na+ out of, and K+ into the neuron. This (along with the misconception that sodium 'floods' the cell to cause the action potential), is not correct. The Na+/K+/ATPase (another name for the pump) does ultimately maintain the resting potential by maintaining the concentration gradients for Na and K, but does so on a much slower time scale; days as opposed to milliseconds. During the falling phase of the action potential, the resting potential is restored exclusively by PK rising to once again be far larger than PNa (i.e. membrane permeability to potassium far exceeds its permeability to sodium, thus bringing the membrane potential back down towards EK (the potassium equilibrium potential). The time-course of the role of the Na+/K+/ATPase in maintaining resting potentials can be demonstrated by the fact that the poison Ouabain inactivates the Na+/K+/ATPase, yet many thousands of action potentials can still be fired without significantly running down the concentration gradients.


Threshold and initiation

A plot of current (ion flux) against voltage (transmembrane potential) illustrates the action potential threshold (red arrow) of an idealized cell.
A plot of current (ion flux) against voltage (transmembrane potential) illustrates the action potential threshold (red arrow) of an idealized cell.

Action potentials are triggered when an initial depolarization reaches the threshold. This threshold potential varies, but generally is about 15 millivolts more positive than the cell's resting membrane potential, occurring when the inward sodium current exceeds the outward potassium current. The net influx of positive charges carried by sodium ions depolarizes the membrane potential, leading to the further opening of voltage-gated sodium channels. These channels support greater inward current causing further depolarization, creating a positive-feedback cycle that drives the membrane potential to a very depolarized level. Image File history File links An illustration (drawn by me) approximating what an I/V (current-voltage) relationship would look like in a hypothetical biological cell that had only two transmembrane ion channels: a non-voltage-dependent potassium channel and a voltate-dependent sodium channel File history Legend: (cur) = this... Image File history File links An illustration (drawn by me) approximating what an I/V (current-voltage) relationship would look like in a hypothetical biological cell that had only two transmembrane ion channels: a non-voltage-dependent potassium channel and a voltate-dependent sodium channel File history Legend: (cur) = this... An I/V curve (current voltage curve) is simply a [[Cartesian coordinate system|Cartesian plot of the voltage across a resistor plotted against the current flowing through that resistor. ... In electricity, current is the rate of flow of charges, usually through a metal wire or some other electrical conductor. ... International safety symbol Caution, risk of electric shock (ISO 3864), colloquially known as high voltage symbol. ... Sodium channels (also known as voltage-gated sodium channels) are integral membrane proteins that are localized in and conduct sodium ions (Na+) through a cells plasma membrane. ...


The action potential threshold can be shifted by changing the balance between sodium and potassium currents. For example, if some of the sodium channels are in an inactivated state, then a given level of depolarization will open fewer sodium channels and a greater depolarization will be needed to trigger an action potential. This is the basis for the refractory period (see Refractory period).


Action potentials are largely dictated by the interplay between sodium and potassium ions (although there are minor contributions from other ions such as calcium and chloride), and are often modeled using hypothetical cells containing only two transmembrane ion channels (a voltage-gated sodium channel and a non-voltage-gated potassium channel). The origin of the action potential threshold may be studied using I/V curves (right) that plot currents through ion channels against the cell's membrane potential. (Note that the illustrated I/V is an "instantaneous" current voltage relationship. It represents the peak current through channels at a given voltage before any inactivation has taken place (i.e. ~ 1 ms after stepping to that voltage) for the Na current. The most positive voltages in this plot are only attainable by the cell through artificial means: voltages imposed by the voltage-clamp apparatus). An I/V curve (current voltage curve) is simply a [[Cartesian coordinate system|Cartesian plot of the voltage across a resistor plotted against the current flowing through that resistor. ...


Four significant points in the I/V curve are indicated by arrows in the figure:

  1. The green arrow indicates the resting potential of the cell and also the value of the equilibrium potential for potassium (Ek). As the K+ channel is the only one open at these negative voltages, the cell will rest at Ek.
  2. The yellow arrow indicates the equilibrium potential for Na+ (ENa). In this two-ion system, ENa is the natural limit of membrane potential beyond which a cell cannot pass. Current values illustrated in this graph that exceed ENa are measured by artificially pushing the cell's voltage past its natural limit. Note however, that ENa could only be reached if the potassium current were absent.
  3. The blue arrow indicates the maximum voltage that the peak of the action potential can approach. This is the actual natural maximum membrane potential that this cell can reach. It cannot reach ENa because of the counteracting influence of the potassium current.
  4. The red arrow indicates the action potential threshold. This is where Isum becomes net-inward. Note that this is a zero-current crossing, but with a negative slope. Any such "negative slope crossing" of the zero current level in an I/V plot is an unstable point. At any voltage negative to this crossing, the current is outward and so a cell will tend to return to its resting potential. At any voltage positive of this crossing, the current is inward and will tend to depolarize the cell. This depolarization leads to more inward current, thus the sodium current become regenerative. The point at which the green line reaches its most negative value is the point where all sodium channels are open. Depolarizations beyond that point thus decrease the sodium current as the driving force decreases as the membrane potential approaches ENa.

The action potential threshold is often confused with the "threshold" of sodium channel opening. This is incorrect, because sodium channels have no threshold. Instead, they open in response to depolarization in a stochastic manner. Depolarization does not so much open the channel as increases the probability of it being open. Even at hyperpolarized potentials, a sodium channel will open very occasionally. In addition, the threshold of an action potential is not the voltage at which sodium current becomes significant; it is the point where it exceeds the potassium current. This article discusses the term in the context of biological membranes. ... This article discusses the term in the context of biological membranes. ... Stochastic, from the Greek stochos or goal, means of, relating to, or characterized by conjecture; conjectural; random. ...


Biologically in neurons, depolarization typically originates in the dendrites at synapses. In principle, however, an action potential may be initiated anywhere along a nerve fiber. In his discovery of "animal electricity," Luigi Galvani made a leg of a dead frog kick as in life by touching a sciatic nerve with his scalpel, to which he had inadvertently transferred a negative, static-electric charge, thus initiating an action potential. Illustration of the major elements in a prototypical synapse. ... An axon, or nerve fiber, is a long slender projection of a nerve cell, or neuron, which conducts electrical impulses away from the neurons cell body or soma. ... Luigi Galvani (September 9, 1737 – December 4, 1798) Italian physician and physicist who lived and died in Bologna. ... The sciatic nerve (also known as the ischiatic nerve) is a large nerve that runs down the lower limb. ...


Circuit model

A. A basic RC circuit superimposed on an image of a membrane bilayer shows the relationship between the two. B. More elaborate circuits can be used to model membranes containing ion channels, such as this one containing at channels for sodium (blue) and potassium (green).

Cell membranes that contain ion channels can be modeled as RC circuits to better understand the propagation of action potentials in biological membranes. In such a circuit, the resistor represents the membrane's ion channels, while the capacitor models the insulating lipid membrane. Variable resistors are used for voltage-gated ion channels, as their resistance changes with voltage. A fixed resistor represents the potassium leak channels that maintain the membrane's resting potential. The sodium and potassium gradients across the membrane are modeled as voltage sources (batteries). Image File history File links Download high resolution version (1423x604, 100 KB)A drawing I made of a basic RC circuit superimposed on a membrane bilayer. ... Image File history File links Download high resolution version (1423x604, 100 KB)A drawing I made of a basic RC circuit superimposed on a membrane bilayer. ... resistor-capacitor circuit (RC circuit), or RC filter or RC network, is one of the simplest analogue electronic filters. ... Ion channels are pore-forming proteins that help to establish and control the small voltage gradient that exists across the plasma membrane of all living cells (see cell potential) by allowing the flow of ions down their electrochemical gradient. ... resistor-capacitor circuit (RC circuit), or RC filter or RC network, is one of the simplest analogue electronic filters. ... Resistor symbols (American) Resistor symbols (Europe, IEC) Axial-lead resistors on tape. ... See Capacitor (component) for a discussion of specific types. ... It has been suggested that Determining emf of primary cells using potentiometer be merged into this article or section. ... A battery is of one or more electrochemical cells, which store chemical energy and make it available in an electrical form. ...


Propagation

Propagating action potentials can be modeled by joining several RC circuits, each one representing a patch of membrane.

In unmyelinated axons, action potentials propagate as an interaction between passively spreading membrane depolarization and voltage-gated sodium channels. When one patch of cell membrane is depolarized enough to open its voltage-gated sodium channels, sodium ions enter the cell by facilitated diffusion. Once inside, positively-charged sodium ions "nudge" adjacent ions down the axon by electrostatic repulsion (analogous to the principle behind Newton's cradle) and attract negative ions away from the adjacent membrane. As a result, a wave of positivity moves down the axon without any individual ion moving very far. Once the adjacent patch of membrane is depolarized, the voltage-gated sodium channels in that patch open, regenerating the cycle. The process repeats itself down the length of the axon, with an action potential regenerated at each segment of membrane. Image File history File links Download high resolution version (1225x961, 273 KB)A diagram in three parts showing how an action potential propagates down a membrane, using the electrical membrane model as a guide. ... Image File history File links Download high resolution version (1225x961, 273 KB)A diagram in three parts showing how an action potential propagates down a membrane, using the electrical membrane model as a guide. ... facilitated diffusion in cell membrane, showing ion channels and carrier proteins Facilitated diffusion (or facilitated transport) is a process of diffusion, a form of passive transport, where molecules diffuse across membranes, with the assistance of transport proteins. ... The valence shell electron pair repulsion theory or VSEPR is a model in chemistry that aims to generally represent the shapes of individual molecules. ... The cradle in motion. ...


Speed of propagation

See also: Time constant and Length constant

Action potentials propagate faster in axons of larger diameter, other things being equal. They typically travel from 10 – 100 m/s. The main reason is that the axial resistance of the axon lumen is lower with larger diameters, because of an increase in the ratio of cross-sectional area to membrane surface area. As the membrane surface area is the chief factor impeding action potential propagation in an unmyelinated axon, increasing this ratio is a particularly effective way of increasing conduction speed. In physics and engineering, the time constant usually denoted by the Greek letter , (tau), characterizes the frequency response of a first-order, linear time-invariant (LTI) system. ... Length constant is a constant used in neurobiology signified by the Greek letter lambda (λ). In an action potential in a neuron, the constant λ is where rm is the resistance across the membrane, ri is the resistance inside the membrane, and ro is the resistance outside the membrane. ...


An extreme example of an animal using axon diameter to speed action potential conduction is found in the Atlantic squid. The squid giant axon controls the muscle contraction associated with the squid's predator escape response. This axon can be more than 1 mm in diameter, and is presumably an adaptation to allow very fast activation of the escape behavior. The velocity of nerve impulses in these fibers is among the fastest in nature. Squids are notable examples of organisms with unmyelinated axons; the first tests to try to determine the mechanism by which impulses travel along axons, involving the detection of a potential difference between the inside and the surface of a neuron, were undertaken in the 1940s by Alan Hodgkin and Andrew Huxley using squid giant axons because of their relatively large axon diameter. Hodgkin and Huxley won their shares of the 1963 Nobel Prize in Physiology or Medicine for their work on the electrophysiology of nerve action potentials. Binomial name Loligo pealei Lesueur, 1821 The Longfin Inshore Squid (Loligo pealei) is a species of squid of the family Loliginidae. ... The squid giant axon is the very large (up to 1 mm in diameter; typically around 0. ... Escape response, escape reaction, or escape behaviour is a possible reaction in response to stimuli indicative of danger, in particular, it initiates an escape motion of an animal. ... The eye is an adaptation. ... Sir Alan Lloyd Hodgkin (February 5, 1914 _ December 20, 1998) was a British physiologist and biophysicist, who won the 1963 Nobel Prize in Physiology or Medicine for his work with Andrew Fielding Huxley on the basis of nerve action potentials, the electrical impulses that enable the activity of an... Andrew Huxley at Trinity College, Cambridge, July 2005 Family tree Sir Andrew Fielding Huxley, OM, FRS (born 22 November 1917, Hampstead, London) is an English physiologist and biophysicist, who won the 1963 Nobel Prize in Physiology or Medicine for his work with Alan Lloyd Hodgkin on the basis of nerve... Emil Adolf von Behring was the first person to receive the Nobel Prize in Physiology or Medicine, for his work on the treatment of diphtheria. ...


In the autonomic nervous system in mammals, postganglionic neurons are unmyelinated. The small diameter of these axons (about 2 µ) results in a propagatory speed of approximately 1 m/s, as opposed to approximately 18 m/s in myelinated nerve fibers of comparable diameter, thus highlighting the effect of myelination on the speed of transmission of impulses. This article or section is in need of attention from an expert on the subject. ...


Saltatory conduction

In myelinated axons, saltatory conduction is the process by which an action potential appears to jump along the length of an axon, being regenerated only at uninsulated segments (the nodes of Ranvier). Saltatory conduction increases nerve conduction velocity without having to dramatically increase axon diameter. Saltatory conduction (from the Latin saltare, to hop or leap) is a means by which action potentials are transmitted along myelinated nerve fibers. ... Nodes of Ranvier are regularly spaced gaps in the myelin sheath around an axon or nerve fiber. ...


Saltatory conduction has played an important role in the evolution of larger and more complex organisms whose nervous systems must rapidly transmit action potentials across greater distances. Without saltatory conduction, conduction velocity would need large increases in axon diameter, resulting in organisms with nervous systems too large for their bodies.


Detailed mechanism

The main impediment to conduction speed in unmyelinated axons is membrane capacitance. In an electric circuit, the capacitance of a capacitor can be decreased by decreasing the cross-sectional area of its plates, or by increasing the distance between plates. The nervous system uses myelin as its main strategy to decrease membrane capacitance. Myelin is an insulating sheath wrapped around axons by Schwann cells in the peripheral nervous system (PNS) and oligodendrocytes, in the Central Nervous System (CNS) neuroglia that flatten their cytoplasm to form large sheets made up mostly of plasma membrane. These sheets wrap around the axon, moving the conducting plates (the intra- and extracellular fluid) farther apart to decrease membrane capacitance. Capacitance is a measure of the amount of electric charge stored (or separated) for a given electric potential. ... An electrical network or electrical circuit is an interconnection of analog electrical elements such as resistors, inductors, capacitors, diodes, switches and transistors. ... See Capacitor (component) for a discussion of specific types. ... Myelin is an electrically insulating phospholipid layer that surrounds the axons of many neurons. ... Named after the German physiologist Theodor Schwann, Schwann cells are a variety of neuroglia that mainly provide myelin insulation to axons in the peripheral nervous system of jawed vertebrates. ... Oligodendrocytes (from Greek literally meaning few tree cells), or oligodendroglia (Greek, few tree glue),[1] are a variety of neuroglia. ... Neuroglia cells of the brain shown by Golgis method. ...


The resulting insulation allows the rapid (essentially instantaneous) conduction of ions through a myelinated segment of axon, but prevents the regeneration of action potentials through those segments. Action potentials are only regenerated at the unmyelinated nodes of Ranvier which are spaced intermittently between myelinated segments. An abundance of voltage-gated sodium channels on these bare segments (up to three orders of magnitude greater than their density in unmyelinated axons[3]) allows action potentials to be efficiently regenerated at the nodes of Ranvier. Nodes of Ranvier are regularly spaced gaps in the myelin sheath around an axon or nerve fiber. ...


As a result of myelination, the insulated portion of the axon behaves like a passive wire: it conducts action potentials rapidly because its membrane capacitance is low, and minimizes the degradation of action potentials because its membrane resistance is high. When this passively propagated signal reaches a node of Ranvier, it initiates an action potential, which subsequently travels passively to the next node where the cycle repeats.


Resilience to injury

The length of myelinated segments of axon is important to saltatory conduction. They should be as long as possible to maximize the length of fast passive conduction, but not so long that the decay of the passive signal is too great to reach threshold at the next node of Ranvier. In reality, myelinated segments are 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. Thus, the safety factor of saltatory conduction is high, allowing transmission to bypass nodes in case of injury. Factor of safety (FoS), also known as Safety Factor, is a multiplier applied to the calculated maximum load (force, torque, bending moment or a combination) to which a component or assembly will be subjected. ...


Role in disease

Some diseases degrade saltatory conduction and reduce the speed of action potential conductance. The most well-known of these diseases is multiple sclerosis, in which the breakdown of myelin impairs coordinated movement.


Refractory period

Where membrane has undergone an action potential, a refractory period follows. Thus, although the passive transmission of action potentials across myelinated segments would suggest that action potentials propagate in either direction, most action potentials travel unidirectionally because the node behind the propagating action potential is refractory. Schematic of an electrophysiological recording of an action potential showing the various phases which occur as the wave passes a point on a cell membrane. ...


This period arises primarily because of the time-dependent inactivation of sodium channels, as described by Hodgkin and Huxley in 1952. Immediately after an action potential, during the absolute refractory period, virtually all sodium channels are inactivated and thus it is impossible to fire another action potential in that segment of membrane. Alan Lloyd Hodgkin photo: taken 1963 Nobel prize photo Sir Alan Lloyd Hodgkin, OM, KBE, FRS (February 5, 1914 – December 20, 1998) was a British physiologist and biophysicist, who won the 1963 Nobel Prize in Physiology or Medicine for his work with Andrew Fielding Huxley on the basis of nerve... Andrew Huxley at Trinity College, Cambridge, July 2005 Family tree Sir Andrew Fielding Huxley, OM, FRS (born 22 November 1917, Hampstead, London) is an English physiologist and biophysicist, who won the 1963 Nobel Prize in Physiology or Medicine for his work with Alan Lloyd Hodgkin on the basis of nerve...


With time, sodium channels are reactivated in a stochastic manner. As they become available, it becomes possible to fire an action potential, albeit one with a much higher threshold. This is the relative refractory period and together with the absolute refractory period, lasts approximately five milliseconds.


Termination and consequences

An action potential proceeding along a membrane is prevented from reversing its direction by the refractory period, and will eventually depolarize the entire cell. When the action potential reaches an area where all the cell membrane is already depolarized or still in the refractory period, the action potential can no longer propagate. Because an action potential propagates only along contiguous membrane, another mechanism is necessary to transmit action potentials between cells. Neurons communicate with each other at a chemical synapse. Other cell types, such as cardiac muscle cells, can communicate action potentials via electrical synapses. Illustration of the major elements in a prototypical synapse. ... Cardiac muscle is a type of involuntary mononucleated, or uninucleated, striated muscle found exclusively within the heart. ... An electrical synapse is a mechanical and electrically conductive link between two abutting neurons that is formed at a narrow gap between the pre- and postsynaptic cells known as a gap junction. ...


The synapse is a very small gap between neurons that allows one-way communication. As the presynaptic neuron undergoes an action potential, voltage-sensitive calcium channels open and cause the release of neurotransmitters into the synapse. These chemical transmitters can initiate an action potential in the postsynaptic neuron, allowing communication between neurons. Some neurotransmitters inhibit action potentials, and the interaction of excitatory and inhibitory signals allows complex modulation of signals in the nervous system.


Evolutionary advantage

The action potential, as a method of long-distance communication, fits a particular biological need seen most readily when considering the transmission of information along a nerve axon. To move a signal from one end of an axon to the other, nature must contend with physics similar to those that govern the movement of electrical signals along a wire. Due to the resistance and capacitance of a wire, signals tend to degrade as they travel along that wire over a distance. These properties, known collectively as cable properties set the physical limits over which signals can travel. Thus, nonspiking neurons (which carry signals without action potentials) tend to be small. Proper function of the body requires that signals be delivered from one end of an axon to the other without loss. An action potential does not so much propagate along an axon, as it is newly regenerated by the membrane voltage and current at each stretch of membrane along its path. In other words, the nerve membrane recreates the action potential at its full amplitude as it travels down the axon, thus overcoming the limitations imposed by cable physics. Electrical resistance is a measure of the degree to which an electrical component opposes the passage of current. ... Capacitance is a measure of the amount of electric charge stored (or separated) for a given electric potential. ... // [edit] Overview Figure. ...


Plant action potentials

Many plants also exhibit action potentials that travel via their phloem to coordinate activity. The main difference between plant and animal action potentials is that plants primarily use potassium and calcium currents while animals typically use currents of potassium and sodium. In vascular plants, phloem is the living tissue that carries organic nutrients, particularly sucrose, a sugar, to all parts of the plant where needed. ... General Name, symbol, number potassium, K, 19 Chemical series alkali metals Group, period, block 1, 4, s Appearance silvery white Standard atomic weight 39. ... For other uses, see Calcium (disambiguation). ... General Name, symbol, number potassium, K, 19 Chemical series alkali metals Group, period, block 1, 4, s Appearance silvery white Standard atomic weight 39. ... For sodium in the diet, see Salt. ...


Alternative models

The model of electrical signal propagation in neurons employing voltage-gated ion channels described above is accepted by almost all scientists working in the field. However there are a few observations not easily reconciled with the model:

  • A signal traveling along a neuron is accompanied by a slight local thickening of the membrane and a force acting outwards.[4]
  • An action potential traveling along a neuron results in a slight increase in temperature followed by a decrease in temperature;[5] electrical charges traveling through a resistor however always produce heat.

One recent alternative, the soliton model, attempts to explain signals in neurons as pressure (or sound) solitons traveling along the membrane, accompanied by electrical field changes resulting from piezo-electric effects. The Soliton model in neuroscience is a recently developed model that attempts to explain how signals are conducted within neurons. ... In mathematics and physics, a soliton is a self-reinforcing solitary wave (a wave packet or pulse) that maintains its shape while it travels at constant speed; solitons are caused by a delicate balance between nonlinear and dispersive effects in the medium. ... Piezoelectricity is the ability of some materials (notably crystals and certain ceramics) to generate an electric potential[1] in response to applied mechanical stress. ...


See also

The cardiac action potential is a specialized action potential in the heart, with unique properties necessary for function of the electrical conduction system of the heart. ... At rest, the ventricular myocyte membrane potential is about -80 mV, which is close to the potassium reversal potential. ... Membrane potential (or transmembrane potential or transmembrane potential difference or transmembrane potential gradient), is the electrical potential difference (voltage) across a cells plasma membrane. ... In biology, depolarization is the event a cell undergoes when its membrane potential grows more positive with respect to the extracellular solution. ... The Hodgkin-Huxley Model is a set of non-linear ordinary differential equations, named after Alan Lloyd Hodgkin and Andrew Huxley, that approximates the electrical characteristics of excitable cells such as neurons and cardiac myocytes. ... In biology, hyperpolarization is any change in a cells membrane potential that makes it more polarized. ... Signals in Biology refer to an electric quantity (voltage or current or field strength) whose modulation represents coded information about the biological source from which it comes, and are also known as Biopotentials. ... In physics and engineering, the time constant usually denoted by the Greek letter , (tau), characterizes the frequency response of a first-order, linear time-invariant (LTI) system. ... Length constant is a constant used in neurobiology signified by the Greek letter lambda (λ). In an action potential in a neuron, the constant λ is where rm is the resistance across the membrane, ri is the resistance inside the membrane, and ro is the resistance outside the membrane. ... Bursting is a rapid signaling mode in neurons whereby clusters of two or more action potentials are emitted as a single signaling event. ...

References

  1. ^ Chapter 6. "Electrical Excitability and Ion Channels" by Bertil Hille and William A. Catterall in Basic Neurochemistry sixth edition. (1999). Published by Lippincott Williams and Wilkins. ISBN 0-397-51820-X
  2. ^ Letter to Nature, Unique features of action potential initiation in cortical neurons. 445, E1-E2 (4 January 2007), online version accessed April 20, 2007)
  3. ^ P. Århem, G. Klement and C. Blomberg, "Channel Density Regulation of Firing Patterns in a Cortical Neuron Model", Biophysical Journal 90:4392-4404 (2006)
  4. ^ Iwasa K, Tasaki I, Gibbons RC (1980). Swelling of nerve fibers associated with action potentials. Science Vol. 210. no. 4467, pp. 338 – 339
  5. ^ Ritchie JM, Keynes RD (1985). The production and absorption of heat associated with electrical activity in nerve and electric organ. Q Rev Biophys. 1985 Nov;18(4):451-76.

is the 110th day of the year (111th in leap years) in the Gregorian calendar. ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ...

Bibliography

General sources

  • Bear MF, Connors BW, Paradiso MA (2001). Neuroscience: Exploring the Brain. Baltimore: Lippincott. ISBN 0781739446. 
  • Bullock TH, Orkand R, Grinell A (1977). Introduction to Nervous Systems. New York: W. H. Freeman. ISBN 0-7167-0030-1. 
  • Hoppensteadt FC (1986). An Introduction to the Mathematics of Neurons. Cambridge: Cambridge University Press. ISBN 0-521-31574-3. 
  • Junge D (1981). Nerve and Muscle Excitation, 2nd ed., Sunderland MA: Sinauer Associates. ISBN 0-87893-410-3. 
  • Kandel ER, Schwartz JH, Jessell TM (2000). Principles of Neural Science, 4th ed., New York: McGraw-Hill. ISBN 0-8385-7701-6. 
  • Dale Purves, et al. Neuroscience, 2nd ed. 2001. Sinauer Associates, Inc. Ion Channels Underlying Action Potentials. ISBN 0878937250 Release of Transmitters from Synaptic Vesicles
  • {{cite book | author = Stevens CF | date = 1966 | title = Neurophysiology: A Primer | publisher = John Wiley and Sons | location = New York | id = LCCN 66-15872

Eric Richard Kandel (born November 7, 1929) is a psychiatrist, a neuroscientist and professor of biochemistry and biophysics at Columbia University. ... Principles of Nerual Science cover First published in 1981, Principles of Neural Science is a neuroscience textbook edited by Eric R. Kandel, James Schwartz, and Thomas Jessell. ... The Library of Congress Control Number or LCCN is a serially based system of numbering books in the Library of Congress in the United States. ...

Original sources

  • Hodgkin AL, Huxley AF. Currents carried by sodium and potassium ions through the membrane of the giant axon of Loligo. J Physiol. 1952 Apr;116(4):449-72. PMID 14946713
  • Hodgkin AL, Huxley AF. The components of membrane conductance in the giant axon of Loligo. J Physiol. 1952 Apr;116(4):473-96. PMID 14946714
  • Hodgkin AL, Huxley AF. The dual effect of membrane potential on sodium conductance in the giant axon of Loligo. J Physiol. 1952 Apr;116(4):497 – 506. PMID 14946715
  • Hodgkin AL, Huxley AF. A quantitative description of membrane current and its application to conduction and excitation in nerve. J Physiol. 1952 Aug;117(4):500-44. PMID 12991237
  • Clay JR. Axonal excitability revisited. Prog Biophys Mol Biol. 2005 May;88(1):59 – 90. PMID 15561301

External links

Image File history File links Action_potential. ... Image File history File links Sound-icon. ... Year 2005 (MMV) was a common year starting on Saturday (link displays full calendar) of the Gregorian calendar. ... is the 173rd day of the year (174th in leap years) in the Gregorian calendar. ... Case Western Reserve University is a private research university located in Cleveland, Ohio, United States, with some residence halls on the south end of campus located in Cleveland Heights. ... Blackwell Publishing was formed in 2001 from two Oxford-based academic publishing companies, Blackwell Science and Blackwell Publishers and is the worlds leading society publisher, partnering with 665 academic and professional societies. ... Blackwell Publishing was formed in 2001 from two Oxford-based academic publishing companies, Blackwell Science and Blackwell Publishers and is the worlds leading society publisher, partnering with 665 academic and professional societies. ... SourceForge is a collaborative revision control and software development management system. ... The University of Arizona (UA or U of A) is a land-grant and space-grant public institution of higher education and research located in Tucson, Arizona, United States. ... For other uses, see University of Chicago (disambiguation). ... The University of New Mexico (UNM) is a public university in Albuquerque, New Mexico. ... Le Moyne College is a four-year Jesuit college of approximately 2,300 undergraduate students that uniquely balances a comprehensive liberal arts education with preparation for specific career paths or graduate study. ... Springfield Technical Community College better known as STCC (pronounced stick by locals) is a two-year community college in Springfield, Massachusetts. ... The University of Derby is a university in the city of Derby, England. ... West Chester University surrounded by the rest of West Chester, Pennsylvania. ... The Encyclopædia Britannica is a general English-language encyclopaedia published by Encyclopædia Britannica, Inc. ... The University of Denver (DU) is an independent, coeducational, four-year university in Denver, Colorado. ... Tufts University is a private research university in Medford/Somerville, Massachusetts, suburbs of Boston. ... The University of California, Davis, commonly abbreviated to UC Davis or UCD is one of the ten University of California campuses. ... The University of California, Berkeley (also known as Cal, UC Berkeley, UCB, or simply Berkeley) is a prestigious, public, coeducational university situated in the foothills of Berkeley, California to the east of San Francisco Bay, overlooking the Golden Gate and its bridge. ...


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The mission of Action Potential, Inc. is to advance research and to provide timely, accurate information to doctors so they can administer state of the art patient care.
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The resting membrane potential of a neuron is about -70 mV (mV=millivolt) - this means that the inside of the neuron is 70 mV less than the outside.
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Action potentials are caused by an exchange of ions across the neuron membrane.
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