In neurophysiology, an action potential (also known as a nerve impulse or spike) is a pulse-like wave of voltage that travels along several types of cell membranes. Neurophysiology (from Greek grc νεῦρον neuron, "nerve" grc φύσις physis, "nature origin" and grc -λογία Electrical tension (or voltage after its SI unit, the Volt) is the difference of electrical potential between two points of an electrical The cell membrane (also called the plasma membrane, plasmalemma, or "phospholipid bilayer" is a Selectively permeable Lipid bilayer The best-understood example is generated on the membrane of the axon of a neuron, but also appears in other types of excitable cells, such as cardiac muscle cells, and even plant cells. An axon or nerve fiber is a long slender projectionof a nerve cell or Neuron, that conducts electrical impulses away from the neuron's Cell Neurons (ˈnjuːɹɒn also known as neurones and nerve cells) are responsive cells in the Nervous system that process and transmit information The cardiac muscle is a type of involuntary striated Muscle found in the walls of the Heart. Plants are living Organisms belonging to the kingdom Plantae. The resting voltage across the axonal membrane is typically −70 millivolts (mV), with the inside being more negative than the outside. Membrane potential (or transmembrane potential) is the Voltage difference (or Electrical potential difference between the interior and exterior of a The volt (symbol V) is the SI derived unit of electric Potential difference or Electromotive force. As an action potential passes through a point, this voltage rises to roughly +40 mV in one millisecond, then returns to −70 mV. The action potential moves rapidly down the axon, with a conduction velocity as high as 100 meters/second (225 miles per hour). A nerve conduction study (NCS is a test commonly used to evaluate the function especially the ability of Electrical conduction, of the motor and Sensory nerves Because they are able to transmit information so fast, the flow of action potentials is a very efficient form of data transmission, considering that each neuron the signal passes through can be up to a meter in length. [note 1]
An action potential is provoked on a patch of membrane when the membrane is depolarized sufficiently, i. e. , when the voltage of the cell's interior relative to the cell's exterior is raised above a threshold. Such a depolarization opens voltage-sensitive channels, which allows current to flow into the axon, further depolarizing the membrane. This will cause the membrane to "fire", initiating a positive feedback loop that suddenly and rapidly causes the voltage inside the axon to become more positive. Positive feedback, sometimes referred to as "cumulative causation" is a Feedback loop system in which the system responds to perturbation in the same direction After this rapid rise, the membrane voltage is restored to its resting value by a combination of effects: the channels responsible for the initial inward current are inactivated, while the raised voltage opens other voltage-sensitive channels that allow a compensating outward current. Because of the positive feedback, an action potential is all-or-none; there are no partial action potentials. In neurons, a typical action potential lasts for just a few thousandths of a second at any given point along their length. The passage of an action potential can leave the ion channels in a non-equilibrium state, making them more difficult to open, and thus inhibiting another action potential at the same spot: such an axon is said to be refractory. Ion channels are pore-forming Proteins that help establish and control the small Voltage Gradient across the Plasma membrane of all living
The principal ions involved in an action potential are sodium and potassium cations; sodium ions enter the cell, and potassium ions leave, restoring equilibrium. Sodium (ˈsoʊdiəm is an element which has the symbol Na( Latin natrium, from Arabic natrun) atomic number 11 atomic mass 22 Potassium (pəˈtæsiəm is a Chemical element. It has the symbol K (kalium from qalīy Atomic number 19 and Atomic mass 39 An ion is an Atom or Molecule which has lost or gained one or more Valence electrons giving it a positive or negative electrical charge 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. The few ions that do cross are pumped out again by the continual action of the sodium–potassium pump, which, with other ion transporters, maintains the normal ratio of ion concentrations across the membrane. "Ion pump" redirects here For pumps that reduce pressure see Ion pump (physics. 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-celled alga Acetabularia, respectively. Calcium (ˈkælsiəm is the Chemical element with the symbol Ca and Atomic number 20 An ion is an Atom or Molecule which has lost or gained one or more Valence electrons giving it a positive or negative electrical charge The chloride Ion is formed when the element Chlorine picks up one Electron to form an Anion (negatively-charged ion Cl&minus An ion is an Atom or Molecule which has lost or gained one or more Valence electrons giving it a positive or negative electrical charge The cardiac action potential is a specialized Action potential in the Heart, with unique properties necessary for function of the Electrical conduction system Algae ( sing. alga are a large and diverse group of simple typically Autotrophic organisms ranging from Unicellular to Multicellular forms In taxonomy, Acetabularia is a genus of Green algae, specifically of Polyphysaceae family Typically found in subtropical waters Acetabularia
The action potential "travels" along the axon without fading out because the signal is regenerated at each patch of membrane. This happens because an action potential at one patch raises the voltage at nearby patches, depolarizing them and provoking a new action potential there. In unmyelinated neurons, the patches are adjacent, but in myelinated neurons, the action potential "hops" between distant patches, making the process both faster and more efficient. Myelin is an electrically-insulating Dielectric Phospholipid layer that surrounds only the Axons of many Neurons It is an outgrowth For Saltation definition and other use disambiguation see Saltation Saltatory conduction (from the Latin saltare, to hop or leap is The axons of neurons generally branch, and an action potential often travels along both forks from a branch point. In the mathematical field of Complex analysis, a branch point may be informally thought of as a point z 0 at which a " multi-valued The action potential stops at the end of these branches, but usually causes the secretion of neurotransmitters at the synapses that are found there. See Chemical synapse for an introduction to concepts and terminology used in this article Chemical synapses are specialized junctions through which Neurons signal to each other and to non-neuronal cells such as those in Muscles or Glands These neurotransmitters bind to receptors on adjacent cells. These receptors are themselves ion channels, although—in contrast to the axonal channels—they are generally opened by the presence of a neurotransmitter, rather than by changes in voltage. The opening of these receptor channels can help to depolarize the membrane of the new cell (an excitatory channel) or work against its depolarization (an inhibitory channel). In Neuroscience, an excitatory postsynaptic potential ( EPSP) is a temporary depolarization of postsynaptic Membrane potential caused by the flow of positively An Inhibitory Postsynaptic Potential (commonly abbreviated as IPSP) is the change in membrane voltage of a postsynaptic Neuron which results from synaptic If these depolarizations are sufficiently strong, they can provoke another action potential in the new cell.
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Biophysical and cellular context
Ions and the forces driving their motion
Electrical signals within biological organisms are generally by ions, which may be either positively charged cations or negatively charged anions. An ion is an Atom or Molecule which has lost or gained one or more Valence electrons giving it a positive or negative electrical charge An ion is an Atom or Molecule which has lost or gained one or more Valence electrons giving it a positive or negative electrical charge An ion is an Atom or Molecule which has lost or gained one or more Valence electrons giving it a positive or negative electrical charge [1] The most important cations for the action potential are sodium (Na+) and potassium (K+),[2] which are both monovalent cations that carry a single positive charge. Sodium (ˈsoʊdiəm is an element which has the symbol Na( Latin natrium, from Arabic natrun) atomic number 11 atomic mass 22 Potassium (pəˈtæsiəm is a Chemical element. It has the symbol K (kalium from qalīy Atomic number 19 and Atomic mass 39 Action potentials can also involve calcium (Ca2+),[3] which is a divalent cation that carries a double positive charge. Calcium (ˈkælsiəm is the Chemical element with the symbol Ca and Atomic number 20 The chloride anion (Cl−) plays a major role in the action potentials of some algae,[4] but plays a negligible role in the action potentials of most animals. The chloride Ion is formed when the element Chlorine picks up one Electron to form an Anion (negatively-charged ion Cl&minus Algae ( sing. alga are a large and diverse group of simple typically Autotrophic organisms ranging from Unicellular to Multicellular forms [5]
Ions cross the cell membrane under two influences: diffusion and electric fields. Diffusion is the net movement of particles (typically molecules from an area of high concentration to an area of low concentration by uncoordinated random movement In Physics, the space surrounding an Electric charge or in the presence of a time-varying Magnetic field has a property called an electric field (that can [6] Diffusion allows net flow of ions from regions where the ions are highly concentrated into regions of low concentration. In Chemistry, concentration is the measure of how much of a given substance there is mixed with another substance Ions also move in response to an electric field. In Physics, the space surrounding an Electric charge or in the presence of a time-varying Magnetic field has a property called an electric field (that can By definition, the integral of the electric field across a patch of membrane equals the voltage Vm across that patch. The European Space Agency 's INTErnational Gamma-Ray Astrophysics Laboratory ( INTEGRAL) is detecting some of the most energetic radiation that comes from space Membrane potential (or transmembrane potential) is the Voltage difference (or Electrical potential difference between the interior and exterior of a [note 2] Likewise by definition, the flows of different ions through that patch are the ionic currents at that patch; the total current is the sum of all the individual ionic currents. Electric current is the flow (movement of Electric charge. The SI unit of electric current is the Ampere. Using these definitions of voltage and current, such a membrane patch can be modeled by an equivalent electronic circuit. An equivalent circuit refers to the simplest form of a circuit that retains all of the electrical characteristics of the original (and more complex circuit [7] In particular, for each type of ion the patch will have a capacitance C and a conductance g; according to Ohm's law, the current I of each ion type is related to the transmembrane voltage Vm by the equation I = g Vm. Capacitance is a measure of the amount of Electric charge stored (or separated for a given Electric potential. Electrical conductance is a measure of how easily Electricity flows along a certain path through an Electrical element. Ohm's law applies to Electrical circuits it states that the current through a conductor between two points is directly proportional to the For a given set of ionic conductances, there is an equilibrium voltage E at which the total current across the membrane is zero; the natural flow of ions generally causes the membrane voltage Vm to approach E. [8]
Cell membrane
Because the membrane surrounding cells is nearly impermeable to ions,[9] cells have evolved systems for transporting ions across the membrane. The cell membrane (also called the plasma membrane, plasmalemma, or "phospholipid bilayer" is a Selectively permeable Lipid bilayer The cell is the structural and functional unit of all known living Organisms It is the smallest unit of an organism that is classified as living and is often called An ion is an Atom or Molecule which has lost or gained one or more Valence electrons giving it a positive or negative electrical charge These systems can be divided into two classes: pores ("channels") that allow passive transport of ions, and ion pumps that use adenosine triphosphate for active transport of ions. Passive transport means moving biochemicals and atomic or molecular substances across the Cell membrane. "Ion pump" redirects here For pumps that reduce pressure see Ion pump (physics. Adenosine-5'-triphosphate ( ATP) is a multifunctional Nucleotide that is most important as a " molecular currency" of intracellular Energy Active transport is the mediated process of moving particles across Biological membrane against the concentration gradient The ion pumps tend to work continuously, as long as there are ions to be pumped. By contrast, the ion channels open and close in response to signals from their environment. The two classes play complementary roles; the ion pumps generate the differences in ion concentrations across the membrane, which the ion channels exploit to carry out electrical signaling. In Chemistry, concentration is the measure of how much of a given substance there is mixed with another substance As an analogy, ion pumps play the role of the battery that allows a radio circuit (the ion channels) to transmit a signal. [10]
Ion channels
Ion channels are integral membrane proteins through which ions can cross the membrane. Ion channels are pore-forming Proteins that help establish and control the small Voltage Gradient across the Plasma membrane of all living Passive transport means moving biochemicals and atomic or molecular substances across the Cell membrane. Ion channels are pore-forming Proteins that help establish and control the small Voltage Gradient across the Plasma membrane of all living An Integral Membrane Protein ( IMP) is a Protein Molecule (or assembly of proteins that is permanently attached to the Biological membrane. Most channels are specific for one ion; whereas that ion passes through relatively quickly, other similar ions pass through very infrequently. [12] For example, although potassium and sodium ions have the same charge and differ only slightly in their radius, potassium channels allow few sodium ions through, and vice versa. The pore through which the ion passes is typically so small that ions must pass through it alone and single-file. [13] Channels are either fully open or fully closed. When the channel is open, ions flow through it by passive transport, i. Passive transport means moving biochemicals and atomic or molecular substances across the Cell membrane. e. , at a rate determined by the membrane voltage Vm and concentration difference across the membrane. [8] The action potential is a manifestation of different ion channels opening and closing at different times. [14]
A channel may have several different states (corresponding to different conformations of the protein), but each such state is either open or closed. Proteins are an important class of biological Macromolecules present in all biological organisms made up of such elements as Carbon, Hydrogen In general, closed states correspond either to a contraction of the pore—making it impassable to the ion—or to a separate part of the protein stoppering the pore. For example, the voltage-dependent sodium channel undergoes inactivation, in which a portion of the protein swings into the pore, sealing it. [15] This inactivation shuts off the sodium current and plays a critical role in the action potential.
Ion channels can be classified by how they respond to their environment. [16] For example, the ion channels involved in the action potential are voltage-sensitive channels; they open and close in response to the voltage across the membrane. Ligand-gated channels form another important class; these ion channels open and close in response to the binding of a ligand molecule, such as a neurotransmitter. In Biochemistry, a ligand ( latin ligare = to bind is a substance that is able to bind to and form a complex with a Biomolecule See Chemical synapse for an introduction to concepts and terminology used in this article Still other ion channels—such as those of sensory neurons—open and close in response to other stimuli, such as light, temperature or pressure. Sensory neurons are Neurons that are activated by sensory input (vision touch hearing etc
Ion pumps
The ionic currents of the action potential flow in response to concentration differences of the ions across the cell membrane. "Ion pump" redirects here For pumps that reduce pressure see Ion pump (physics. Active transport is the mediated process of moving particles across Biological membrane against the concentration gradient In Chemistry, concentration is the measure of how much of a given substance there is mixed with another substance The cell membrane (also called the plasma membrane, plasmalemma, or "phospholipid bilayer" is a Selectively permeable Lipid bilayer These concentration differences are established by ion transporters, which are integral membrane proteins that carry out active transport, i. "Ion pump" redirects here For pumps that reduce pressure see Ion pump (physics. An Integral Membrane Protein ( IMP) is a Protein Molecule (or assembly of proteins that is permanently attached to the Biological membrane. Active transport is the mediated process of moving particles across Biological membrane against the concentration gradient e. , use cellular energy (ATP) to "pump" the ions against their concentration gradient. [17] Such ion pumps take in ions from one side of the membrane (decreasing its concentration there) and release them on the other side (increasing its concentration there). The ion pump most relevant to the action potential is the sodium–potassium pump, which transports three sodium ions out of the cell and two potassium ions in. [18] Consequently, the concentration of potassium ions K+ inside the neuron is roughly 20-fold larger than the outside concentration, whereas the sodium concentration outside is roughly ninefold larger than inside. Potassium (pəˈtæsiəm is a Chemical element. It has the symbol K (kalium from qalīy Atomic number 19 and Atomic mass 39 [19][20] Similarly, other ions have different concentrations inside and outside the neuron, such as calcium, chloride and magnesium. Calcium (ˈkælsiəm is the Chemical element with the symbol Ca and Atomic number 20 The chloride Ion is formed when the element Chlorine picks up one Electron to form an Anion (negatively-charged ion Cl&minus Magnesium (mægˈniːziəm is a Chemical element with the symbol Mg, Atomic number 12 Atomic weight 24 [20]
Ion pumps influence the action potential only by establishing the relative ratio of intracellular and extracellular ion concentrations. The action potential mainly involves the opening and closing of ion channels, not ion pumps. If the ion pumps are turned off by removing their energy source, or by adding an inhibitor such as ouabain, the axon can still fire hundreds of thousands of action potentials before their amplitudes begin to decay significantly. Ouabain ('waben wa'bein (through French from Somali 'waabaayo' an arrow poison is the familiar name of g-strophanthin a poisonous Cardiac glycoside. [17] In particular, ion pumps play no significant role in the repolarization of the membrane after an action potential. [2]
Resting potential
Each type of ion has a reversal potential E—also called its equilibrium voltage or equilibrium potential—at which the net current of that ion across the membrane is zero; the ionic current due to the electric field exactly cancels the current due to the differences in concentration across the membrane. The Membrane potential, or better Membrane Voltage, is the difference of Electric potentials between two Aqueous solutions separated by a ( Membrane potential (or transmembrane potential) is the Voltage difference (or Electrical potential difference between the interior and exterior of a In a Biological membrane, the reversal potential (also known as the Nernst potential) of an Ion is the Membrane potential at which there In a Biological membrane, the reversal potential (also known as the Nernst potential) of an Ion is the Membrane potential at which there That equilibrium voltage is given by the Nernst equation[21][22]
The constants in this equation are the charge valence n of the ion (e. In Electrochemistry, the Nernst equation is an equation which can be used (in conjunction with other information to determine the equilibrium Reduction potential Electric charge is a fundamental conserved property of some Subatomic particles which determines their Electromagnetic interaction. In Chemistry, valence, also known as valency or valency number, is a measure of the number of Chemical bonds formed by the Atoms g. , +1 for K+, +2 for Ca2+ and −1 for Cl−), the temperature T (in Kelvins), the molar gas constant R, and the Faraday F, which is the total charge of a mole of electrons. Temperature is a physical property of a system that underlies the common notions of hot and cold something that is hotter generally has the greater temperature The kelvin (symbol K) is a unit increment of Temperature and is one of the seven SI base units The Kelvin scale is a thermodynamic Relationship with the Boltzmann constant The Boltzmann constant kB (often abbreviated k) may be used in place of the gas constant by working Michael Faraday, FRS ( September 22 1791 – August 25 1867) was an English The mole (symbol mol) is a unit of Amount of substance: it is an SI base unit, and almost the only unit to be used to measure this The electron is a fundamental Subatomic particle that was identified and assigned the negative charge in 1897 by J For illustration, at a typical physiological ratio of concentrations, the potassium equilibrium voltage EK is −75 mV, whereas the sodium equilibrium voltage ENa is +55mV. Since these disagree, there is no voltage at which the currents of potassium and sodium ions are both zero. [note 3]
However, there is a voltage Em at which the net current of all ions across the membrane is zero; this voltage is given by the Goldman equation[23][24]
![E_{m} = \frac{RT}{F} \ln{ \left( \frac{ P_{\mathrm{K}}[\mathrm{K}^{+}]_\mathrm{out} + P_{\mathrm{Na}}[\mathrm{Na}^{+}]_\mathrm{out} + P_{\mathrm{Cl}^{-}}[\mathrm{Cl}^{-}]_\mathrm{in}}{ P_{\mathrm{K}}[\mathrm{K}^{+}]_\mathrm{in} + P_{\mathrm{Na}}[\mathrm{Na}^{+}]_\mathrm{in} + P_{\mathrm{Cl}}[\mathrm{Cl}^{-}]_\mathrm{out}} \right) }](../../../../math/4/f/f/4fffa2b5f50b4266b2497afc362b8574.png)
for the three monovalent ions most important to action potentials: potassium (K+), sodium (Na+), and chloride (Cl−). 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 cell's membrane Being an anion, the chloride terms are treated differently than the cation terms; the inside concentration is in the numerator, and the outside concentration is in the denominator, which is reversed from the cation terms. Pi stands for the permeability of the ion type i. If calcium ions are also considered, the formula for the equilibrium voltage becomes more complicated. [25]
The membrane voltage Vm need not equal its equilibrium value Em. However, since Vm can change drastically when only a few ions cross the membrane,[note 4] Vm tracks Em closely, so that the two are effectively equivalent. In a typical action potential, where Vm changes by roughly 100 mV, the ionic concentrations inside the axon change only by roughly 1 part in 10 million;[26] hence, hundreds of thousands of action potentials can be fired before the ion pumps are needed to restore the standard ratio of ionic concentrations. [17]
The equilibrium voltage Em under normal, unstimulated conditions is called the resting potential Vrest, typically −70 mV. The Membrane potential, or better Membrane Voltage, is the difference of Electric potentials between two Aqueous solutions separated by a ( [27] (The word "potential" or "potential difference" is sometimes a synonym for voltage. Electrical tension (or voltage after its SI unit, the Volt) is the difference of electrical potential between two points of an electrical ) Under those conditions, the membrane is much more permeable to potassium than to any other ion; thus, consistent with the Goldman equation, the resting potential is close to the potassium equilibrium potential EK. [28][29] In the middle of the action potential, however, the sodium permeability dominates, so that Em is +45 mV, close to the sodium equilibrium voltage ENa. [30][29]
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). The cardiac action potential is a specialized Action potential in the Heart, with unique properties necessary for function of the Electrical conduction system However, the main excitable cell is the neuron, which also has the simplest mechanism for the action potential. Neurons (ˈnjuːɹɒn also known as neurones and nerve cells) are responsive cells in the Nervous system that process and transmit information
Most neurons have numerous branched tendrils called neurites,[31] which are divided into two main types, dendrites and axons. Any projection from the Cell body of a Neuron can be referred to as a neurite. Dendrites (from Greek δένδρον déndron, “tree” are the branched projections of a Neuron that act to conduct the electrochemical An axon or nerve fiber is a long slender projectionof a nerve cell or Neuron, that conducts electrical impulses away from the neuron's Cell Most neurons have only one axon but numerous dendrites;[32] the beginning of the axon is called the axon hillock. "Hillock" redirects here A hillock is also a small Hill. [33] Action potentials almost always begin at the axon hillock, and travel down the axon; it is very rare for an action potential to occur in the dendrites. [31] A typical axon has a few branch points, forming a tree-like shape; the tips of this tree (the axonal termini) are generally called the synaptic knobs. These knobs are usually adjacent to the dendrites of another neuron or, more generally, to another excitable cell; the contact between them is called the synapse. Chemical synapses are specialized junctions through which Neurons signal to each other and to non-neuronal cells such as those in Muscles or Glands [34]
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In some animals (mostly vertebrates), segments of the axon are sheathed in myelin,[35] which generally increases the conduction velocity at which action potentials travel down the axon. Dendrites (from Greek δένδρον déndron, “tree” are the branched projections of a Neuron that act to conduct the electrochemical The soma, or cyton or perikaryon, is the bulbous end of a Neuron, containing the Cell nucleus. An axon or nerve fiber is a long slender projectionof a nerve cell or Neuron, that conducts electrical impulses away from the neuron's Cell In Cell biology, the nucleus (pl nuclei; from Latin la ''nucleus'' or la ''nuculeus'' "little nut" or kernel is a membrane-enclosed Nodes of Ranvier are known as the gaps (about 1 micrometer in diameter formed between myelin sheath cells along axons or nerve fibers Chemical synapses are specialized junctions through which Neurons signal to each other and to non-neuronal cells such as those in Muscles or Glands Named after the German physiologist Theodor Schwann, Schwann cells (also referred to as neurolemmocytes) are a variety of Glial cell that mainly Myelin is an electrically-insulating Dielectric Phospholipid layer that surrounds only the Axons of many Neurons It is an outgrowth Myelin is an electrically-insulating Dielectric Phospholipid layer that surrounds only the Axons of many Neurons It is an outgrowth A nerve conduction study (NCS is a test commonly used to evaluate the function especially the ability of Electrical conduction, of the motor and Sensory nerves [36] Myelin is composed of Schwann cells that wrap themselves multiple times around the axonal segment, forming a thick fatty layer that prevents ions from entering or escaping the axon. Named after the German physiologist Theodor Schwann, Schwann cells (also referred to as neurolemmocytes) are a variety of Glial cell that mainly Ions can flow into and out of the axon only at the nodes of Ranvier, which are the gaps between the Schwann cells, the "chinks" in the myelin armor. Nodes of Ranvier are known as the gaps (about 1 micrometer in diameter formed between myelin sheath cells along axons or nerve fibers [35] Therefore, the action potential "hops" from one node of Ranvier to the next (the process of saltatory conduction); it does not move continuously down the axon, as it does in unmyelinated axons. For Saltation definition and other use disambiguation see Saltation Saltatory conduction (from the Latin saltare, to hop or leap is [37]
Phases
The course of the action potential is determined by two coupled effects. [38] First, voltage-sensitive ion channels open and close in response to changes in the membrane voltage Vm, thus changing the membrane's permeability to those ions. Membrane potential (or transmembrane potential) is the Voltage difference (or Electrical potential difference between the interior and exterior of a [39] However, by the Goldman equation, changes in the ionic permeabilities causes changes in the equilibrium potential Em, and, thus, the membrane voltage Vm. 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 cell's membrane [24] This two-way interaction between Vm and the ion channels sets up the possibility for positive feedback, which is a key part of the rising phase of the action potential. Positive feedback, sometimes referred to as "cumulative causation" is a Feedback loop system in which the system responds to perturbation in the same direction [40] 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. [41][42] For example, although raising Vm opens most gates in the voltage-sensitive sodium channel, it also closes the channel's "inactivation gate", albeit more slowly. [43] Hence, when Vm is raised suddenly, the sodium channels open initially, but then close due to the slower inactivation.
The course of the action potential can be divided into four parts: the rising phase, the falling phase, the undershoot phase, and the refractory period. The initial membrane permeability to potassium is low, but much higher than that of other ions, making the resting potential close to EK. [28] 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. 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. [44] This is the rising phase. [45] At this point, the sodium channels begin to inactivate, lowering the membrane's permeability to sodium and driving Vm back down toward the original resting potential. [43] Meanwhile, the potassium channels open more fully; the increased permeability to potassium likewise helps to drive the membrane voltage back down towards EK, the potassium equilibrium voltage. [46] This is the falling phase. [45] The potassium conductance remains unusually high, causing the membrane voltage to dip below even the resting potential; this is the undershoot phase. [45] 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. [45]
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,[42] for which they were awarded the Nobel Prize in Physiology or Medicine in 1963. Sir Alan Lloyd Hodgkin, OM, KBE, FRS (5 February 1914 Banbury, Oxfordshire, England – 20 December 1998 Cambridge Sir Andrew Fielding Huxley, OM, FRS (born 22 November 1917, Hampstead, London) is an English physiologist The Nobel Prize in Physiology or Medicine (Nobelpriset i fysiologi eller medicin is awarded once a year by the Swedish Karolinska Institute. [47] 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,[16] and they do not always open and close independently. [48]
Stimulation and rising phase
A typical action potential begins at the axon hillock[49] with a sufficiently strong depolarization, e. "Hillock" redirects here A hillock is also a small Hill. g. , a stimulus that increases Vm. This depolarization is often caused by the injection of extra sodium cations into the cell; these cations can come from a wide variety of sources, such as chemical synapses, sensory neurons or pacemaker potentials. An ion is an Atom or Molecule which has lost or gained one or more Valence electrons giving it a positive or negative electrical charge Chemical synapses are specialized junctions through which Neurons signal to each other and to non-neuronal cells such as those in Muscles or Glands Sensory neurons are Neurons that are activated by sensory input (vision touch hearing etc In the Heart, the pacemaker potential is the voltage created by impulses from an artificial electronic pacemaker or the SA node which drives the rhythmic firing of
The depolarization opens both the sodium and potassium channels in the membrane, allowing the ions to flow into and out of the axon, respectively. If the depolarization is small (say, increasing Vm from −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. [50] The "failed initiations" shown in Figure 1 illustrate this response. 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: the more inward current flows, the more Vm increases, which in turn further increases the inward current. Positive feedback, sometimes referred to as "cumulative causation" is a Feedback loop system in which the system responds to perturbation in the same direction [40] The sharp rise in Vm and sodium permeability correspond to the rising phase of the action potential. [44]
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. The period during which no new action potential can be fired is called the absolute refractory period. In Physiology, a refractory period is a period of time during which an organ or cell is incapable of repeating a particular action or (more precisely the amount of time [51] 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. The period during which action potentials are unusually difficult to provoke is called the relative refractory period. [51]
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. 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 stoppering their pores; the sodium channels become inactivated. [43] This lowers the membrane's permeability to sodium, driving the membrane voltage back down. At the same time, the raised voltage opens voltage-sensitive potassium channels; the increase in the membrane's potassium permeability drives Vm towards EK. 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. [46]
Hyperpolarization ("undershoot")
The raised voltage opened many more potassium channels than usual, and these do not close right away when the membrane returns to its normal resting voltage. The potassium permeability of the membrane is transiently unusually high, driving the membrane voltage Vm even closer to the potassium equilibrium voltage EK. Hence, there is an undershoot, a hyperpolarization in technical language, that persists until the membrane potassium permeability returns to its usual value. [52]
Refractory period
The opening and closing of the sodium and potassium channels during an action potential may leave some of them in a "refractory" state, in which they are unable to open again until they have recovered. [51] In the absolute refractory period, so many ion channels are refractory that no new action potential can be fired. Significant recovery (de-inactivation) requires that the membrane potential remain hyperpolarized for a certain length of time. In the relative refractory period, enough channels have recovered that an action potential can be provoked, but only with a stimulus much stronger than usual. These refractory periods ensure that the action potential travels in only one direction along the axon. In Physiology, a refractory period is a period of time during which an organ or cell is incapable of repeating a particular action or (more precisely the amount of time [53]
Initiation, propagation and termination
A typical action potential is initiated at the axon hillock when the membrane is depolarized sufficiently, i. e. , when its voltage is increased sufficiently. As the membrane voltage is increased, both the sodium and potassium ion channels begin to open up, increasing both the inward sodium current and the balancing outward potassium current. For small voltage increases, the potassium current triumphs over the sodium current and the voltage returns to its normal resting value, typically −70 mV. [50] However, if the voltage increases past a critical threshold, typically 15 mV higher than the resting value, the sodium current dominates and a runaway condition results; the cell "fires", producing an action potential. [40] Once started, the action potential propagates down the axon without diminishing;[54] the inwards current of an action potential at one patch of membrane depolarizes nearby membrane patches, sparking another action potential there. In effect, the action potential is created afresh at each patch of membrane; its energy derives from the local differences in ionic concentrations, not from the depolarization that triggered it. The axon may branch along its length, and there the inward current may not quite suffice to trigger a new action potential in one or both of its branches; the action potential may stop. [55] Action potentials that do reach the ends of the axon generally cause the release of a neurotransmitter into the synapse, which may combine with other inputs to provoke a new action potential in the post-synaptic neuron or muscle cell. See Chemical synapse for an introduction to concepts and terminology used in this article Chemical synapses are specialized junctions through which Neurons signal to each other and to non-neuronal cells such as those in Muscles or Glands
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. An axon or nerve fiber is a long slender projectionof a nerve cell or Neuron, that conducts electrical impulses away from the neuron's Cell "Hillock" redirects here A hillock is also a small Hill. The basic requirement is that the membrane voltage at the hillock be raised above the threshold for firing;[44] there are several ways in which this depolarization can occur.
Neurotransmission
Action potentials are most commonly initiated by excitatory postsynaptic potentials from a presynaptic neuron. Neurotransmission (latin transmissio = passage crossing from transmitto = send let through also called synaptic transmission, is an electrical movement In Neuroscience, an excitatory postsynaptic potential ( EPSP) is a temporary depolarization of postsynaptic Membrane potential caused by the flow of positively [56] Typically, neurotransmitter molecules are released by the presynaptic neuron bound to receptors on the postsynaptic cell. See Chemical synapse for an introduction to concepts and terminology used in this article Chemical synapses are specialized junctions through which Neurons signal to each other and to non-neuronal cells such as those in Muscles or Glands Neurons (ˈnjuːɹɒn also known as neurones and nerve cells) are responsive cells in the Nervous system that process and transmit information This binding opens various types of ion channels, changing the local permeability of the cell membrane and thereby altering the membrane potential. Ion channels are pore-forming Proteins that help establish and control the small Voltage Gradient across the Plasma membrane of all living The cell membrane (also called the plasma membrane, plasmalemma, or "phospholipid bilayer" is a Selectively permeable Lipid bilayer If the binding increases the voltage (depolarizes the membrane), the synapse is excitatory; if 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; typically, the voltage stimulus decays exponentially with the distance from the synapse and with time from the binding of the neurotransmitter. Classical cable theory describes the development of mathematical models that can calculate the flow of electric current (and accompanying Voltage) along passive neuronal 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. "Hillock" redirects here A hillock is also a small Hill. More typically, the excitatory potentials from several synapses must work together at nearly the same time to provoke a new action potential. Spatial summation is a way of achieving Action potential in a Neuron which involves input from multiple cells Temporal summation is an effect generated by a single Neuron as a way of achieving Action potential. Their joint efforts can be thwarted, however, by the counter-acting inhibitory postsynaptic potentials. An Inhibitory Postsynaptic Potential (commonly abbreviated as IPSP) is the change in membrane voltage of a postsynaptic Neuron which results from synaptic
Neurotransmission can also occur through electrical synapses. An electrical synapse is a mechanical and electrically conductive link between two abutting Neuron cells that is formed at a narrow gap between the pre- [57] Due to the direct connection between the excitable cells in such cases, an action potential can well be transmitted directly from one cell to the next. Rectifying channels ensure that action potentials only move in one direction through an electrical synapse.
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. Sensory neurons are Neurons that are activated by sensory input (vision touch hearing etc Sensory neurons are Neurons that are activated by sensory input (vision touch hearing etc Ion channels are pore-forming Proteins that help establish and control the small Voltage Gradient across the Plasma membrane of all living [58] 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. An olfactory receptor neuron also called an olfactory sensory neuron is the primary transduction cell in the Olfactory system. Meissner's corpuscles (or tactile corpuscles) are a type of Mechanoreceptor. Olfaction (also known as olfactics or smell) refers to the Sense of smell. However, not all sensory neurons convert their external signals into action potentials; some do not even have an axon![59] 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. See Chemical synapse for an introduction to concepts and terminology used in this article Receptor potential, a type of graded potential is the Transmembrane potential difference of a Sensory receptor. 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. The ear is the sense organ that detects Sounds The Vertebrate ear shows a common biology from Fish to Humans with variations Hair cells are the Sensory receptors of both the Auditory system and the Vestibular system in all Vertebrates. Stretch-activated or stretch-gated ion channels are Ion channels which open their Pores in response to mechanical deformation of a Neuron 's See Chemical synapse for an introduction to concepts and terminology used in this article Similarly, in the human retina, the initial photoreceptor cells and the next two layers of cells (bipolar cells and amacrine cells) do not produce action potentials; only the third layer, the ganglion cells, produce action potentials, which then travel up the optic nerve. The vertebrate retina is a light sensitive part inside the inner layer of the Eye. A photoreceptor, or photoreceptor cell, is a specialized type of Neuron (nerve cell found in the Eye 's Retina that is capable of A bipolar cell is a type of Neuron which has two extensions Bipolar cells are specialized sensory neurons for the transmission of special senses Amacrine cells are Interneurons in the Retina. Amacrine cells are responsible for 70% of input to retinal ganglion cells A ganglion cell (more correctly a retinal ganglion cell or RGC) is a type of Neuron typically located near the inner surface of the Retina The optic nerve, also called cranial nerve II, is the Nerve that transmits visual information from the Retina to the Brain.
Pacemaker potentials
In the cases of neurotransmission and sensory neurons, action potentials result from an external stimulus. However, some excitable cells require no such stimulus to fire: they spontaneously depolarize their axon hillock and fire action potentials at a regular rate, like an internal clock. [60] The voltage traces of such cells are known as pacemaker potentials. In the Heart, the pacemaker potential is the voltage created by impulses from an artificial electronic pacemaker or the SA node which drives the rhythmic firing of [61] The cardiac pacemaker cells of the sinoatrial node in the heart provide a good example. The contractions of the Heart are controlled by chemical impulses which fire at a rate which controls the beat of the heart The Sinoatrial node (abbreviated SA node or SAN, also called the sinus node) is the impulse generating (pacemaker tissue located in the Right atrium The heart is a muscular organ in all Vertebrates responsible for pumping Blood through the Blood vessels by repeated rhythmic [62] 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 parasympathetic nerves. Measuring heart rate The Pulse rate (which in most people is identical to the heart rate can be measured at any point on the body where an Artery 's pulsation The Sympathetic Nervous System ( SNS) is a branch of the Autonomic nervous system along with the Enteric nervous system and Parasympathetic nervous The parasympathetic Nervous system ( PSNS) is a division of the Autonomic nervous system (ANS along with the Sympathetic nervous system [63] The external stimuli do not cause the cell's repetitive firing, but merely alter its timing. [61] In some cases, the regulation of frequency can be more complex, leading to patterns of action potentials, such as bursting. Bursting is a rapid signaling mode in Neurons whereby clusters of two or more Action potentials (spikes are emitted as a single signaling event
Propagation
The action potential propagates as a wave along the axon. A nerve conduction study (NCS is a test commonly used to evaluate the function especially the ability of Electrical conduction, of the motor and Sensory nerves [64] 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. 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. Sir Alan Lloyd Hodgkin, OM, KBE, FRS (5 February 1914 Banbury, Oxfordshire, England – 20 December 1998 Cambridge 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. [65]
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, this absolute refractory period corresponds to the time required for its ion channels to return to their normal open or closed states. [66] Although it limits the frequency of firing,[67] the absolute refractory period ensures that the action potential moves in only one direction along an axon. [53] The currents flowing in due to an action potential spread out in both directions along the axon. [68] However, only the unfired part of the axon can respond with an action potential; the part that has just fired is unresponsive until the action potential is safely out of range and cannot restimulate that part. In the usual orthodromic 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. An orthodromic impulse runs along an Axon in its normal direction away from the soma. For most neurons their Dendrites, soma, or Axons are depolarized forming an Action potential that moves from the starting point of the depolarization [69] 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
The axons of some neurons are ensheathed in myelin, a fatty (ie, lipid-rich) insulating material that increases the speed and energy efficiency of action potential conduction. Myelin is an electrically-insulating Dielectric Phospholipid layer that surrounds only the Axons of many Neurons It is an outgrowth For Saltation definition and other use disambiguation see Saltation Saltatory conduction (from the Latin saltare, to hop or leap is Myelin is an electrically-insulating Dielectric Phospholipid layer that surrounds only the Axons of many Neurons It is an outgrowth Lipids are broadly defined as any fat- Soluble ( lipophilic) naturally-occurring Molecule, such as fats oils waxes cholesterol sterols fat-soluble [70] Axons are myelinated by specialized cells, Schwann cells and oligodendrocytes, that wrap themselves multiple times around segments of axon. Named after the German physiologist Theodor Schwann, Schwann cells (also referred to as neurolemmocytes) are a variety of Glial cell that mainly Origin Oligodendroglia arise during development from an Oligodendrocyte precursor cell, which can be identified by its expression of a number of Antigens, including [71] The gaps between these segments are known as the nodes of Ranvier. Nodes of Ranvier are known as the gaps (about 1 micrometer in diameter formed between myelin sheath cells along axons or nerve fibers
Myelin prevents ions from entering or leaving the axon along myelinated segments. Myelination is found mainly in vertebrates, but an analogous system has been discovered in a few invertebrates, such as some species of shrimp. Vertebrates are members of the Subphylum Vertebrata, Chordates with backbones or spinal columns The grouping sometimes includes True shrimp are swimming decapod Crustaceans classified in the Infraorder Caridea, found widely around the world in both fresh [72] As a general rule, myelination increases the conduction velocity of action potentials and makes them more energy-efficient. A nerve conduction study (NCS is a test commonly used to evaluate the function especially the ability of Electrical conduction, of the motor and Sensory nerves However, not all neurons in vertebrates are myelinated. Whether saltatory or not, the mean conduction velocity of an action potential ranges from 1 m/s to over 100 m/s, and generally increases with axonal diameter. A nerve conduction study (NCS is a test commonly used to evaluate the function especially the ability of Electrical conduction, of the motor and Sensory nerves [73]
Action potentials cannot propagate through the myelinated segments of the axon, since no ions can flow across the membrane there. Instead, the ionic current from an action potential at one node of Ranvier provokes another action potential at the next node; this "hopping" of the action potential from node to node is known as saltatory conduction. Nodes of Ranvier are known as the gaps (about 1 micrometer in diameter formed between myelin sheath cells along axons or nerve fibers For Saltation definition and other use disambiguation see Saltation Saltatory conduction (from the Latin saltare, to hop or leap is Although the mechanism of saltatory conduction was suggested in 1925 by Ralph Lillie,[74] the first experimental evidence for saltatory conduction came from Ichiji Tasaki[75] and Taiji Takeuchi[76] and from Alan Hodgkin and Robert Stämpfli. Dr Ichiji Tasaki (田崎一二 is a Japanese Biophysicist and physician involved in research relating to the electical impulses in the nervous system Sir Alan Lloyd Hodgkin, OM, KBE, FRS (5 February 1914 Banbury, Oxfordshire, England – 20 December 1998 Cambridge [77] By contrast, in unmyelinated axons, the action potential provokes another in the membrane immediately adjacent, and moves continuously down the axon like a wave.
Myelin has two important advantages: fast conduction speed and energy efficiency. For axons larger than a minimum diameter (roughly 1 micron), myelination increases the conduction velocity of an action potential, typically tenfold. A nerve conduction study (NCS is a test commonly used to evaluate the function especially the ability of Electrical conduction, of the motor and Sensory nerves [80] Conversely, for a given conduction velocity, myelinated fibers are smaller than their unmyelinated counterparts. 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 100-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. This saving is a significant selective advantage, since the human nervous system uses approximately 20% of the body's metabolic energy. Natural selection is the process by which favorable Heritable traits become more common in successive Generations of a Population of [80]
The length of axons' myelinated segments is important to the success of saltatory conduction. 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. Thus, the safety factor of saltatory conduction is high, allowing transmission to bypass nodes in case of injury. Factor of safety ( FoS) can mean either the fraction of structural capability over that required or a Multiplier applied to the maximum expected load ( Force 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. [81]
Some diseases degrade myelin and impair saltatory conduction, reducing the conduction velocity of action potentials. [82] The most well-known of these is multiple sclerosis, in which the breakdown of myelin impairs coordinated movement. Multiple sclerosis (abbreviated MS also known as disseminated sclerosis or encephalomyelitis disseminata) is an autoimmune condition in which the [83]
Cable theory
The flow of currents within an axon can be described quantitatively by cable theory[84] and its elaborations, such as the compartmental model. Classical cable theory describes the development of mathematical models that can calculate the flow of electric current (and accompanying Voltage) along passive neuronal [85] Cable theory was developed in 1855 by Lord Kelvin to model the transatlantic telegraph cable[86] and was shown to be relevant to neurons by Hodgkin and Rushton in 1946. William Thomson 1st Baron Kelvin (or Lord Kelvin) OM, GCVO, PC, PRS, FRSE, (26 June 1824 &ndash 17 December 1907 Sir Alan Lloyd Hodgkin, OM, KBE, FRS (5 February 1914 Banbury, Oxfordshire, England – 20 December 1998 Cambridge William Albert Hugh Rushton FRS (8 December 1901 - 21 June 1980 was professor of Physiology at Trinity College Cambridge. [87] In simple cable theory, the neuron is treated as an electrically passive, perfectly cylindrical transmission cable, which can be described by a partial differential equation[84]
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 λ and τ are the characteristic length and time scales on which those voltages decay in response to a stimulus. In Mathematics, partial differential equations ( PDE) are a type of Differential equation, i Referring to the circuit diagram above, these scales can be determined from the resistances and capacitances per unit length[88]
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. For example, the time-scale τ 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. Capacitance is a measure of the amount of Electric charge stored (or separated for a given Electric potential. Similarly, 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 λ becomes longer and the conduction velocity of an action potential should increase. A nerve conduction study (NCS is a test commonly used to evaluate the function especially the ability of Electrical conduction, of the motor and Sensory nerves If the transmembrane resistance rm is increased, that lowers the average "leakage" current across the membrane, likewise causing λ to become longer, increasing the conduction velocity.
Termination
Chemical synapses
Action potentials that reach the synaptic knobs generally cause a neurotransmitter to be released into the synaptic cleft. Chemical synapses are specialized junctions through which Neurons signal to each other and to non-neuronal cells such as those in Muscles or Glands See Chemical synapse for an introduction to concepts and terminology used in this article In Neuroscience, an excitatory postsynaptic potential ( EPSP) is a temporary depolarization of postsynaptic Membrane potential caused by the flow of positively An Inhibitory Postsynaptic Potential (commonly abbreviated as IPSP) is the change in membrane voltage of a postsynaptic Neuron which results from synaptic See Chemical synapse for an introduction to concepts and terminology used in this article [89] 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 pre-synaptic cell; the influx of calcium causes vesicles filled with neurotransmitter to migrate to the cell's surface and release their contents into the synaptic cleft. In a Neuron, synaptic vesicles, also called neurotransmitter vesicles, store the various Neurotransmitters that are released during Calcium -regulated Exocytosis (ek-soh-sy-TOH-sis Greek: Έξω - external and κύτος - cell is the durable process by which a cell directs secretory vesicles out of the Cell Chemical synapses are specialized junctions through which Neurons signal to each other and to non-neuronal cells such as those in Muscles or Glands [90] This complex process is inhibited by the neurotoxins tetanospasmin and botulinum toxin, which are responsible for tetanus and botulism, respectively. A neurotoxin is a Toxin that acts specifically on nerve cells ( Neurons, usually by interacting with Membrane proteins such as Ion channels Tetanospasmin is the Neurotoxin produced by the vegetative Spore of Clostridium tetani in anaerobic conditions causing Botulinum toxin is a Neurotoxin Protein produced by the Bacterium Clostridium botulinum. Tetanus is a medical condition that is characterized by a prolonged contraction of Skeletal muscle fibres Botulism ( Latin, botulus, "sausage" is a rare but serious Paralytic illness caused by Botulin Toxin. [91]
Electrical synapses
Some synapses dispense with the "middleman" of the neurotransmitter, and connect the presynaptic and postsynaptic cells together. An electrical synapse is a mechanical and electrically conductive link between two abutting Neuron cells that is formed at a narrow gap between the pre- A gap junction or nexus is a specialized Intercellular connection between certain animal cell -types Connexins, or Gap junction Proteins, are a family of structurally-related transmembrane proteins that assemble to form vertebrate gap junctions (an entirely different [92] 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. Connexins, or Gap junction Proteins, are a family of structurally-related transmembrane proteins that assemble to form vertebrate gap junctions (an entirely different [93] 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. See Chemical synapse for an introduction to concepts and terminology used in this article 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. Escape reflex, a kind of Escape response, is a simple Reflectory reaction in response to stimuli indicative of danger that initiates an escape motion of The vertebrate retina is a light sensitive part inside the inner layer of the Eye. Vertebrates are members of the Subphylum Vertebrata, Chordates with backbones or spinal columns The grouping sometimes includes The heart is a muscular organ in all Vertebrates responsible for pumping Blood through the Blood vessels by repeated rhythmic
Neuromuscular junctions
A special case of a chemical synapse is the neuromuscular junction, in which the axon of a motor neuron terminates on a muscle fiber. A neuromuscular junction ( NMJ) is the Synapse or junction of the Axon terminal of a Motoneuron with the motor end plate, the An acetylcholine receptor (abbreviated AChR) is an Integral membrane protein that responds to the binding of the Neurotransmitter Acetylcholine In Biochemistry, cholinesterase is an enzyme that catalyzes the Hydrolysis of the Neurotransmitter Acetylcholine into A neuromuscular junction ( NMJ) is the Synapse or junction of the Axon terminal of a Motoneuron with the motor end plate, the An axon or nerve fiber is a long slender projectionof a nerve cell or Neuron, that conducts electrical impulses away from the neuron's Cell In Vertebrates the term motor neuron (or motoneuron) classically applies to Neurons located in the Central nervous system (or CNS that project Skeletal muscle is a type of Striated muscle, which usually attaches to tendons [94] In such cases, the released neurotransmitter is acetylcholine, which binds to the acetylcholine receptor, an integral membrane protein in the membrane (the sarcolemma) of the muscle fiber. The Chemical compound acetylcholine (often abbreviated ACh) is a Neurotransmitter in both the Peripheral nervous system (PNS and Central The sarcolemma is the Cell membrane of a muscle cell It consists of a true cell membrane called the plasma membrane and an outer coat made up of a thin layer of polysaccharide [95] However, the acetylcholine does not remain bound; rather, it dissociates and is hydrolyzed by the enzyme, acetylcholinesterase, located in the synapse. Hydrolysis is a Chemical reaction during which one or more water molecules are split into hydrogen and hydroxide ions which may go on to participate in further reactions This enzyme quickly reduces the stimulus to the muscle, which allows the degree and timing of muscular contraction to be regulated delicately. Some poisons inactivate acetylcholinesterase to prevent this control, such as the nerve agents sarin and tabun,[96] and the insecticides diazinon and malathion. Nerve agents (also being referred to as nerve gases, though these chemicals are liquid at room temperature are a class of Phosphorus -containing organic chemicals Sarin, also known by its NATO designation of GB, is an extremely toxic substance whose sole application is as a Nerve agent. Diazinon (OO-diethyl-O-(2-isopropyl-6-methyl-pyrimidine-4-ylphosphorothioate a colorless to dark brown liquid is a thiophosphoric acid ester developed in 1952 by Malathion is an Organophosphate Parasympathomimetic which binds irreversibly to Cholinesterase. [97]
Other cell types
Cardiac action potentials
The cardiac action potential differs from the neuronal action potential by having an extended plateau, in which the membrane is held at a high voltage for a few hundred milliseconds prior to being repolarized by the potassium current as usual. [98] This plateau is due to the action of slower calcium channels opening and holding the membrane voltage near their equilibrium potential even after the sodium channels have inactivated. Calcium (ˈkælsiəm is the Chemical element with the symbol Ca and Atomic number 20
The cardiac action potential plays an important role in coordinating the contraction of the heart. [98] The cardiac cells of the sinoatrial node provide the pacemaker potential that synchronizes the heart. The Sinoatrial node (abbreviated SA node or SAN, also called the sinus node) is the impulse generating (pacemaker tissue located in the Right atrium In the Heart, the pacemaker potential is the voltage created by impulses from an artificial electronic pacemaker or the SA node which drives the rhythmic firing of The action potentials of those cells propagate to and through the atrioventricular node (AV node), which is normally the only conduction pathway between the atria and the ventricles. The atrioventricular node (abbreviated AV node) is an area of specialized tissue between the atria and the ventricles of the Heart, specifically In Anatomy, the atrium (plural atria) refers to a chamber or space In the Heart, a ventricle is a heart chamber which collects Blood from an atrium (another heart chamber that is smaller than a ventricle and Action potentials from the AV node travel through the bundle of His and thence to the Purkinje fibers. The bundle of His, also known as the AV bundle or atrioventricular bundle is a collection of heart muscle cells specialized for electrical conduction that transmits the electrical For the nervous cells see Purkinje cell Purkinje fibers (or Purkyne tissue are located in the inner ventricular walls of the Heart, just [note 5] Conversely, anomalies in the cardiac action potential—whether due to a congenital mutation or injury—can lead to human pathologies, especially arrhythmias. Dysrhythmia redirects here For the American band see Dysrhythmia (band. [98] Several anti-arrhythmia drugs act on the cardiac action potential, such as quinidine, lidocaine, beta blockers, and verapamil. Quinidine is a Pharmaceutical agent that acts as a Class I antiarrhythmic agent in the Heart. Lidocaine ( INN) (ˈlaɪdoʊkeɪn or lignocaine (former BAN) (/ˈlɪgnoʊkeɪn/ is a common Local anesthetic and antiarrhythmic drug Beta blockers (sometimes written as β-blocker) are a class of drugs used for various indications but particularly for the management of Cardiac arrhythmias Verapamil (brand names Isoptin, Verelan, Calan, Bosoptin, Covera-HS) is an L-type Calcium channel blocker. [99]
Muscular action potentials
The action potential in a normal skeletal muscle cell is similar to the action potential in neurons. A neuromuscular junction ( NMJ) is the Synapse or junction of the Axon terminal of a Motoneuron with the motor end plate, the A muscles contraction (also known as a muscle twitch or simply twitch) occurs when a Muscle fibre generates tension through the action of Actin [100] Action potentials result from the depolarization of the cell membrane (the sarcolemma), which opens voltage-sensitive sodium channels; these becomes inactivated and the membrane is repolarized through the outward current of potassium ions. The sarcolemma is the Cell membrane of a muscle cell It consists of a true cell membrane called the plasma membrane and an outer coat made up of a thin layer of polysaccharide The resting potential prior to the action potential is typically −90mV, somewhat more negative than typical neurons. The muscle action potential lasts roughly 2–4 ms, the absolute refractory period is roughly 1–3 ms, and the conduction velocity along the muscle is roughly 5 m/s. The action potential releases calcium ions that free up the tropomyosin and allow the muscle to contract. Calcium (ˈkælsiəm is the Chemical element with the symbol Ca and Atomic number 20 Tropomyosin is an actin-binding protein that regulates actin mechanics Muscle action potentials are provoked by the arrival of a pre-synaptic neuronal action potential at the neuromuscular junction, which is a common target for neurotoxins. A neuromuscular junction ( NMJ) is the Synapse or junction of the Axon terminal of a Motoneuron with the motor end plate, the A neurotoxin is a Toxin that acts specifically on nerve cells ( Neurons, usually by interacting with Membrane proteins such as Ion channels [96]
Plant action potentials
Many plants also exhibit action potentials that travel via their phloem to coordinate activity. In Vascular plants phloem is the living tissue that carries organic Nutrients (known as photosynthate particularly Sucrose, a sugar to The physiology of these ion movements has been studied most in algae such as charophytes. Algae ( sing. alga are a large and diverse group of simple typically Autotrophic organisms ranging from Unicellular to Multicellular forms The Charophyta are a division of Green algae, including the closest relatives of the Embryophyte plants [101] 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. Potassium (pəˈtæsiəm is a Chemical element. It has the symbol K (kalium from qalīy Atomic number 19 and Atomic mass 39 Calcium (ˈkælsiəm is the Chemical element with the symbol Ca and Atomic number 20 Potassium (pəˈtæsiəm is a Chemical element. It has the symbol K (kalium from qalīy Atomic number 19 and Atomic mass 39 Sodium (ˈsoʊdiəm is an element which has the symbol Na( Latin natrium, from Arabic natrun) atomic number 11 atomic mass 22 These signals are used by plants to rapidly transmit information from environmental signals such as temperature, light, touch or wounding. [102]
Taxonomic distribution and evolutionary advantages
Action potentials are found throughout multicellular organisms, including plants, invertebrates such as insects, and vertebrates such as reptiles and mammals. Multicellular organisms are Organisms consisting of more than one cell, and having Differentiated cells that perform specialized functions Plants are living Organisms belonging to the kingdom Plantae. An invertebrate is an Animal lacking a Vertebral column. The group includes 98% of all animal Species — all animals except those in the Chordate Insects ( Class Insecta) are a major group of Arthropods and the most diverse group of Animals on the Earth with over a million described Vertebrates are members of the Subphylum Vertebrata, Chordates with backbones or spinal columns The grouping sometimes includes Reptiles, or members of the class Reptilia are air-breathing Cold-blooded Vertebrates that have skin covered in scales as opposed to hair or feathers Mammals ( class Mammalia) are a class of Vertebrate Animals characterized by the presence of Sweat glands, including sweat glands [102] Sponges seem to be the main phylum of multicellular eukaryotes that does not transmit action potentials, although some studies have suggested that these organisms have a form of electrical signaling, too. The sponges or poriferans (from Latin porus "pore" and ferre "to bear" are Animals A phylum ( Plural: phyla) is a Taxonomic rank between Kingdom and above Class. Animals Plants fungi, and Protists are eukaryotes (juːˈkærɪɒt or -oʊt Organisms whose cells are organized into complex [103] The resting potential, as well as the size and duration of the action potential, have not varied much with evolution, although the conduction velocity does vary dramatically with axonal diameter and myelination. A nerve conduction study (NCS is a test commonly used to evaluate the function especially the ability of Electrical conduction, of the motor and Sensory nerves
| Animal | Cell type | Resting potential (mV) | AP increase (mV) | AP duration (ms) | Conduction speed (m/s) |
|---|---|---|---|---|---|
| Squid (Loligo) | Giant axon | −60 | 120 | 0. 75 | 35 |
| Earthworm (Lumbricus) | Median giant fiber | −70 | 100 | 1. 0 | 30 |
| Cockroach (Periplaneta) | Giant fiber | −70 | 80–104 | 0. 4 | 10 |
| Frog (Rana) | sciatic nerve axon | −60 to −80 | 110–130 | 1. 0 | 7–30 |
| Cat (Felis) | Spinal motor neuron | −55 to −80 | 80–110 | 1–1. 5 | 30–120 |
Given its conservation throughout evolution, the action potential seems to confer evolutionary advantages. One function of action potentials is rapid, long-range signaling within the organism; the conduction velocity can exceed 110 m/s, which is one-third the speed of sound. Sound is a vibration that travels through an elastic medium as a Wave. No material object could convey a signal that rapidly throughout the body; for comparison, a hormone molecule carried in the bloodstream moves at roughly 8 m/s in large arteries. Part of this function is the tight coordination of mechanical events, such as the contraction of the heart. A second function is the computation associated with its generation. Being an all-or-none signal that does not decay with transmission distance, the action potential has similar advantages to digital electronics. Digital electronics are Electronics systems that use Digital signals Digital electronics are representations of Boolean algebra also see The integration of various dendritic signals at the axon hillock and its thresholding to form a complex train of action potentials is another form of computation, one that has been exploited biologically to form central pattern generators and mimicked in artificial neural networks. " Central pattern generators (CPGs can be defined as neural networks that can endogenously (i An artificial neural network (ANN, often just called a "neural network" (NN is a Mathematical model or Computational model based on Biological neural
Experimental methods
- See also: Electrophysiology
The study of action potentials has required the development of new experimental methods. The initial work, prior to 1955, focused on three goals: isolating signals from single neurons or axons, developing fast, sensitive electronics, and shrinking electrodes enough that the voltage inside a single cell could be recorded. An electrode is an Electrical conductor used to make contact with a nonmetallic part of a circuit (e
The first problem was solved by studying the giant axons found in the neurons of the squid genus Loligo. Squid are marine Cephalopods of the order Teuthida, which comprises around 300 species Loligo is a genus of Squids and one of the most representative and widely distributed group of myopsid squids The genus was first described by [105] These axons are so large in diameter (roughly 1 mm, or 100-fold larger than a typical neuron) that they can be seen with the naked eye, making them easy to extract and manipulate. [106] However, the Loligo axons are not representative of all excitable cells, and numerous other systems with action potentials have been studied.
The second problem was addressed with the crucial development of the voltage clamp,[107] which permitted experimenters to study the ionic currents underlying an action potential in isolation, and eliminated a key source of electronic noise, the current IC associated with the capacitance C of the membrane. The voltage clamp is used by electrophysiologists to measure the Ion currents across a neuronal membrane while holding the membrane Electronic noise is an unwanted signal characteristic of all electronic circuits. Capacitance is a measure of the amount of Electric charge stored (or separated for a given Electric potential. [108] Since the current equals C times the rate of change of the transmembrane voltage Vm, the solution was to design a circuit that kept Vm fixed (zero rate of change) regardless of the currents flowing across the membrane. Thus, the current required to keep Vm at a fixed value is a direct reflection of the current flowing through the membrane. Other electronic advances included the use of Faraday cages and electronics with high input impedance, so that the measurement itself did not affect the voltage being measured. A Faraday cage or Faraday shield is an enclosure formed by conducting material, or by a mesh of such material The Input impedance, Load impedance, or external impedance of a circuit or electronic device is the Thévenin [109]
The third problem, that of obtaining electrodes small enough to record voltages within a single axon without perturbing it, was solved in 1949 with the invention of the glass micropipette electrode,[110] which was quickly adopted by other researchers. [111][112] Refinements of this method are able to produce electrode tips that are as fine as 100 Å (10 nm), which also confers high input impedance. An ångström or angstrom (symbol Å) (ˈɔːŋstrəm Swedish: ˈɔ̀ŋstrœm is an internationally recognized non- SI unit of length equal A nanometre ( American spelling: nanometer, symbol nm) ( Greek: νάνος nanos dwarf; μετρώ metrό count) is a [113] Action potentials may also be recorded with small metal electrodes placed just next to a neuron, with neurochips containing EOSFETs, or optically with dyes that are sensitive to Ca2+ or to voltage. 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 Calcium imaging is a scientific technique usually carried out in research which is designed to show the Calcium (Ca2+ status of a tissue or medium [114]
While glass micropipette electrodes measure the sum of the currents passing through many ion channels, studying the electrical properties of a single ion channel became possible in the 1970s with the development of the patch clamp by Erwin Neher and Bert Sakmann. The patch clamp technique is a Laboratory technique in Electrophysiology that allows the study of single or multiple Ion channels in cells Erwin Neher (born March 20, 1944 in Landsberg am Lech, Bavaria) is a German biophysicist. Bert Sakmann (born June 12, 1942) is a German cell Physiologist. For this they were awarded the Nobel Prize in Physiology or Medicine in 1991. The Nobel Prize in Physiology or Medicine (Nobelpriset i fysiologi eller medicin is awarded once a year by the Swedish Karolinska Institute. [115] Patch-clamping verified that ionic channels have discrete states of conductance, such as open, closed and inactivated.
Neurotoxins
Several neurotoxins, both natural and synthetic, are designed to block the action potential. A neurotoxin is a Toxin that acts specifically on nerve cells ( Neurons, usually by interacting with Membrane proteins such as Ion channels Tetrodotoxin from the pufferfish and saxitoxin from the Gonyaulax (the dinoflagellate genus responsible for "red tides") block action potentials by inhibiting the voltage-sensitive sodium channel;[116] similarly, dendrotoxin from the black mamba snake inhibits the voltage-sensitive potassium channel. Tetrodotoxin (anhydrotetrodotoxin 4-epitetrodotoxin tetrodonic acid TTX is a potent Neurotoxin with no known antidote which blocks Action potentials in Nerves Tetraodontidae is a family of primarily marine and estuarine fish Saxitoxin ( STX) is a neurotoxin naturally produced by certain species of marine Dinoflagellates ( Alexandrium sp The dinoflagellates are a large group of Flagellate Protists Most are marine Plankton, but Paralytic shellfish poisoning (PSP is one of the four recognized syndromes of Shellfish poisoning (the others being Neurotoxic shellfish poisoning, diarrhetic Dendrotoxins are a class of Neurotoxins produced by Mamba Snakes ( Dendroapsis) that block particular subtypes of Voltage-gated MAMBA stands for Mobile Artillery Monitoring Battlefield Asset, a Counter-battery radar operated by the Royal Artillery. Such inhibitors of ion channels serve an important research purpose, by allowing scientists to "turn off" specific channels at will, thus isolating the other channels' contributions; they can also be useful in purifying ion channels by affinity chromatography or in assaying their concentration. Affinity chromatography is a chromatographic method of separating biochemical mixtures based on a highly specific biologic interaction such as that between Antigen However, such inhibitors also make effective neurotoxins, and have been considered for use as chemical weapons. Chemical warfare involves using the toxic properties of Chemical substances to kill injure or incapacitate an enemy. Neurotoxins aimed at the ion channels of insects have been effective insecticides; one example is the synthetic permethrin, which prolongs the activation of the sodium channels involved in action potentials. An insecticide is a Pesticide used against Insects in all developmental forms Permethrin is a common synthetic chemical widely used as an Insecticide and Acaricide and as an Insect repellent. The ion channels of insects are sufficiently different from their human counterparts that there are few side effects in humans. Many other neurotoxins interfere with the transmission of the action potential's effects at the synapses, especially at the neuromuscular junction. Chemical synapses are specialized junctions through which Neurons signal to each other and to non-neuronal cells such as those in Muscles or Glands A neuromuscular junction ( NMJ) is the Synapse or junction of the Axon terminal of a Motoneuron with the motor end plate, the
History
The role of electricity in the nervous systems of animals was first observed in dissected frogs by Luigi Galvani, who studied it from 1791 to 1797. This article is about the block cipher algorithm For the ultrafast laser pulse measurement technique see Frequency-resolved optical gating. Luigi Galvani was an Italian Physician and Physicist who lived and died in Bologna. [117] Galvani's results stimulated Alessandro Volta to develop the Voltaic pile—the earliest known electric battery—with which he studied animal electricity (such as electric eels) and the physiological responses to applied direct-current voltages. Count Alessandro Giuseppe Antonio Anastasio Volta was a Lombard physicist known especially for the development of the first electric cell in A voltaic pile is a set of individual Voltaic cells placed in series In electronics a battery is a combination of two or more Electrochemical cells which store chemical Energy which can be converted into electrical energy The electric eel, Electrophorus electricus, is a species of Fish. Direct current ( DC) is the unidirectional flow of Electric charge. Electrical tension (or voltage after its SI unit, the Volt) is the difference of electrical potential between two points of an electrical [118]
Scientists of the 19th century studied the propagation of electrical signals in whole nerves (i. A nerve is an enclosed cable-like bundle of peripheral Axons (the long slender projections of Neurons. e. , bundles of neurons) and demonstrated that nervous tissue was made up of cells, instead of an interconnected network of tubes (a reticulum). Neurons (ˈnjuːɹɒn also known as neurones and nerve cells) are responsive cells in the Nervous system that process and transmit information The cell is the structural and functional unit of all known living Organisms It is the smallest unit of an organism that is classified as living and is often called [119] Carlo Matteucci followed up Galvani's studies and demonstrated that cell membranes had a voltage across them and could produce direct current. Carlo Matteucci ( June 21, 1811 - June 25, 1868) was an Italian Physicist and neurophysiologist who was The cell membrane (also called the plasma membrane, plasmalemma, or "phospholipid bilayer" is a Selectively permeable Lipid bilayer Direct current ( DC) is the unidirectional flow of Electric charge. Matteucci's work inspired the German physiologist, Emil du Bois-Reymond, who discovered the action potential in 1848. Emil du Bois-Reymond ( November 7, 1818 &ndash December 26, 1896) was a German Physician and Physiologist, the The conduction velocity of action potentials was first measured in 1850 by du Bois-Reymond's friend, Hermann von Helmholtz. A nerve conduction study (NCS is a test commonly used to evaluate the function especially the ability of Electrical conduction, of the motor and Sensory nerves To establish that nervous tissue was made up of discrete cells, the Spanish physician Santiago Ramón y Cajal and his students used a stain developed by Camillo Golgi to reveal the myriad shapes of neurons, which they rendered painstakingly. Santiago Ramón y Cajal ( May 1 1852 &ndash October 17 1934) was a Spanish histologist, Physician, and Camillo Golgi ( July 7, 1843 &ndash January 21, 1926) was an Italian Physician and Scientist. For their discoveries, Golgi and Ramón y Cajal were awarded the 1906 Nobel Prize in Physiology. The Nobel Prize in Physiology or Medicine (Nobelpriset i fysiologi eller medicin is awarded once a year by the Swedish Karolinska Institute. [120] Their work resolved a long-standing controversy in the neuroanatomy of the 19th century; Golgi himself had argued for the network model of the nervous system. Neuroanatomy is the science for localizing function in the Human brain.
The 20th century was a golden era for electrophysiology. In 1902 and again in 1912, Julius Bernstein advanced the hypothesis that the action potential resulted from a change in the permeability of the axonal membrane to ions. Julius Bernstein ( December 18, 1839 - February 6, 1917) was a German Physiologist who was born in Berlin. [22] Bernstein's hypothesis was confirmed by Ken Cole and Howard Curtis, who showed that membrane conductance increases during an action potential. Kenneth Stewart Cole ( July 10, 1900 &ndash April 18, 1984) was an American Biophysicist described by his peers as "a pioneer [121] In 1949, Alan Hodgkin and Bernard Katz refined Bernstein's hypothesis by considering that the axonal membrane might have different permeabilities to different ions; in particular, they demonstrated the crucial role of the sodium permeability for the action potential. Sir Alan Lloyd Hodgkin, OM, KBE, FRS (5 February 1914 Banbury, Oxfordshire, England – 20 December 1998 Cambridge Sir Bernard Katz, FRS ( 26 March 1911 &ndash 20 April 2003) was a German -born biophysicist, noted for his work [29] This line of research culminated in the five 1952 papers of Hodgkin, Katz and Andrew Huxley, in which they applied the voltage clamp technique to determine the dependence of the axonal membrane's permeabilities to sodium and potassium ions on voltage and time, from which they were able to reconstruct the action potential quantitatively. Sir Andrew Fielding Huxley, OM, FRS (born 22 November 1917, Hampstead, London) is an English physiologist The voltage clamp is used by electrophysiologists to measure the Ion currents across a neuronal membrane while holding the membrane [42] Hodgkin and Huxley correlated the properties of their mathematical model with discrete ion channels that could exist in several different states, including "open", "closed", and "inactivated". Ion channels are pore-forming Proteins that help establish and control the small Voltage Gradient across the Plasma membrane of all living Their hypotheses were confirmed in the mid-1970s and 1980s by Erwin Neher and Bert Sakmann, who developed the technique of patch clamping to examine the conductance states of individual ion channels. Erwin Neher (born March 20, 1944 in Landsberg am Lech, Bavaria) is a German biophysicist. Bert Sakmann (born June 12, 1942) is a German cell Physiologist. The patch clamp technique is a Laboratory technique in Electrophysiology that allows the study of single or multiple Ion channels in cells [122] In the 21st century, researchers are beginning to understand the structural basis for these conductance states and for the selectivity of channels for their species of ion,[123] through the atomic-resolution crystal structures,[13] fluorescence distance measurements[124] and cryo-electron microscopy studies. X-ray crystallography is a method of determining the arrangement of Atoms within a Crystal, in which a beam of X-rays strikes a crystal and scatters Electron cryomicroscopy ( cryo-EM or sometimes cryo-electron microscopy) is a form of Electron microscopy (EM where the sample is studied at Cryogenic [125]
Julius Bernstein was also the first to introduce the Nernst equation for resting potential across the membrane; this was generalized by David E. In Electrochemistry, the Nernst equation is an equation which can be used (in conjunction with other information to determine the equilibrium Reduction potential The Membrane potential, or better Membrane Voltage, is the difference of Electric potentials between two Aqueous solutions separated by a ( Goldman to the eponymous Goldman equation in 1943. 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 cell's membrane [24] The sodium–potassium pump was identified in 1957[126] and its properties gradually elucidated,[17][18][127] culminating in the determination of its atomic-resolution structure by X-ray crystallography. X-ray crystallography is a method of determining the arrangement of Atoms within a Crystal, in which a beam of X-rays strikes a crystal and scatters [128] The crystal structures of related ionic pumps have also been solved, giving a broader view of how these molecular machines work. [129]
Quantitative models
Mathematical and computational models are essential for understanding the action potential, and offer predictions that may be tested against experimental data, providing a stringent test of a theory. The most important and accurate of these models is the Hodgkin–Huxley model, which describes the action potential by a coupled set of four ordinary differential equations (ODEs). The Hodgkin–Huxley model is a Scientific model that describes how Action potentials in Neurons are initiated and propagated In Mathematics, an ordinary differential equation (or ODE) is a relation that contains functions of only one Independent variable, and one or more of its [42] Although the Hodgkin–Huxley model may be a simplification of a realistic nervous membrane, its complexity has inspired several even-more-simplified models,[130] such as the Morris–Lecar model[131] and the FitzHugh–Nagumo model,[132] both of which have only two coupled ODEs. The FitzHugh-Nagumo model (named after Richard FitzHugh, 1922&ndash2007 describes a prototype of an excitable system i The properties of the Hodgkin–Huxley and FitzHugh–Nagumo models and their relatives, such as the Bonhoeffer–van der Pol model,[133] have been well-studied within mathematics,[134] computation[135] and electronics. [136] More modern research has focused on larger and more integrated systems; by joining action-potential models with models of other parts of the nervous system (such as dendrites and synapses), researches can study neural computation[137] and simple reflexes, such as escape reflexes and others controlled by central pattern generators. Traditionally the term neural network had been used to refer to a network or circuit of biological neurons. A reflex action, also known as a reflex, is an involuntary and almost instant movement in response to stimulus. Escape reflex, a kind of Escape response, is a simple Reflectory reaction in response to stimuli indicative of danger that initiates an escape motion of " Central pattern generators (CPGs can be defined as neural networks that can endogenously (i [138]
See also
Notes
- ^ For comparison, ordinary eukaryotic cells are typically 100,000 times smaller than the longest neurons, having a size of roughly 10 μm. Bursting is a rapid signaling mode in Neurons whereby clusters of two or more Action potentials (spikes are emitted as a single signaling event See also Electrophysiology In Biology, a signal or biopotential is an Electric quantity (voltage or current or field strength caused " Central pattern generators (CPGs can be defined as neural networks that can endogenously (i The extraordinary length of neurons may be responsible for some diseases specific to them. For example, defects in the long-distance transport system used to shuttle proteins and organelles from the nucleus to the peripheral synapses and back again may lead to their accumulation and aggregation and, eventually, to cell death by apoptosis.
- ^ Here, a patch of membrane is defined as a segment of the membrane small enough that the transmembrane voltage does not vary significantly over its surface.
- ^ Membrane voltages are defined relative to the exterior of the cell; thus, a potential of −70 mV implies that the interior of the cell is negative relative to the exterior.
- ^ This follows from the capacitance equation, Δq = C ΔV, where ΔV is the change in voltage that results from the transfer of a charge Δq to a capacitor. To obtain the given ΔV ≈ 100 mV of an action potential, the charge Δq transferred per area of membrane is small (≈0. 1 μC per cm2) because the capacitance C per area of the membrane is likewise small (≈1 μF per cm2; see Bullock, Orkand, and Grinnell, p. The coulomb (symbol C) is the SI unit of Electric charge. It is named after Charles-Augustin de Coulomb. Capacitance is a measure of the amount of Electric charge stored (or separated for a given Electric potential. This is about the capacitance unit of measure For the charge unit see Faraday (unit. Theodore Holmes “Ted” Bullock (16 May 1915 – 20 December 2005 is one of the founding fathers of Neuroethology. 135).
- ^ Note that these Purkinje fibers are muscle fibers and not related to the Purkinje cells, which are neurons found in the cerebellum. For the nervous cells see Purkinje cell Purkinje fibers (or Purkyne tissue are located in the inner ventricular walls of the Heart, just For the cells of the Electrical conduction system of the heart, see Purkinje fibers Purkinje cells (or Purkinje neurons) are a class of Neurons (ˈnjuːɹɒn also known as neurones and nerve cells) are responsive cells in the Nervous system that process and transmit information The cerebellum ( Latin: "little brain" is a region of the Brain that plays an important role in the integration of sensory perception
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External links
- Animations
- Ionic flow in action potentials at Blackwell Publishing
- Action potential propagation in myelinated and unmyelinated axons at Blackwell Publishing
- Resting membrane potential from Life: The Science of Biology, by WK Purves, D Sadava, GH Orians, and HC Heller, 8th edition, New York: WH Freeman, ISBN 978-0716776710. Blackwell Publishing Ltd was a Learned society publishing company based in Oxford, England. Blackwell Publishing Ltd was a Learned society publishing company based in Oxford, England.
- Ionic motion and the Goldman voltage for arbitrary ionic concentrations at University of Arizona
- [2] An excellent cartoon illustrating the action potential. The University of Arizona (also referred to as UA, U of A, or Arizona) is a Land-grant and space-grant public institution
- Lecture notes and other materials
- The Action Potential John Kinnamon, University of Denver
- Resting and Action Membrane Potentials Teaching Resources Center, UC Davis. The University of Denver ( DU) founded in 1864 is the oldest private University in the Rocky Mountain Region of the United States. The University of California Davis, commonly known as UC Davis, or just UCD, is a public coeducational university located in the city of Davis, Animated tutorials
- Open-source software to simulate neuronal and cardiac action potentials at SourceForge.net
SourceForgenet is a Source code repository. It acts as a centralized location for software developers to control and manage open source software development
Dictionary
action potential
-noun
- A short term change in the electrical potential that travels along a cell such as a nerve or muscle fiber.
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