Sodium channels are integral membrane proteins that exist in a cell's plasma membrane and regulate the flow of sodium (Na⁺) ions into it. A number of receptors function as Na⁺-permeable ion channels, including Acetylcholine receptors, NMDA receptors, AMPA receptors, and kainate receptors. In neuronal signalling, voltage gated sodium channels are especially important because they are responsible for a large part of the depolarization of the cell. Integral membrane protein of the transmembrane type An Integral Membrane Protein (IMP) is a protein molecule (or assembly of proteins) that in most cases spans the biological membrane with which it is associated (especially the plasma membrane) or which, in any case, is sufficiently embedded in the membrane to remain... A representation of the 3D structure of myoglobin, showing coloured alpha helices. ... Jump to: navigation, search A cell is a single unit or compartment, enclosed by a border or wall. ... Drawing of a cell membrane A component of every biological cell, the cell membrane (or plasma membrane) is a thin and structured bilayer of phospholipid and protein molecules that envelopes the cell. ... Jump to: navigation, search General Name, Symbol, Number sodium, Na, 11 Chemical series alkali metals Group, Period, Block 1, 3, s Appearance silvery white Atomic mass 22. ... Jump to: navigation, search An ion is an atom or group of atoms with a net electric charge. ... Another, unrelated ion channeling process is part of ion implantation. ... An acetylcholine receptor (abbreviated AChR) is an integral membrane protein that responds to the binding of the neurotransmitter acetylcholine. ... The NMDA receptor (NMDAR) is an ionotropic receptor for glutamate (NMDA (N-methyl d-aspartate) is a name of its selective specific agonist). ... The AMPA receptor (AMPAR) is a non-NMDA-type ionotropic transmembrane receptor for glutamate that mediates fast synaptic transmission in the central nervous system. ... Kainate receptors are ionotropic receptors which respond to both glutamate, which is their physiological ligand, or kainate which is a drug first isolated from red alga Digenea simplex. ... Neurons (also spelled neurones or called nerve cells) are the primary cells of the nervous system. ... Voltage-gated ion channel is a ion channel that is specifically activated, or gated, by the surrounding potential difference near the channel (or near the cell, neuron or synapse). ... In biology, depolarization is the event a cell undergoes when its membrane potential grows more positive with respect to the extracellular solution. ...

Voltage-gated Sodium channels

When closed, sodium channels help to maintain a neuron's resting potential, and when open, they allow sodium ions to flow rapidly down their electrochemical gradient, thus depolarizing the neuron. Voltage-gated Na⁺ channels are probably genetically related to potassium and calcium channels; in fact, a change of two amino acids will cause the channel to behave as a calcium channel (Kandel, 2000, p. 164). Neurons (also spelled neurones or called nerve cells) are the primary cells of the nervous system. ... The resting potential of a cell is the membrane potential that would be maintained if there were no action potentials, synaptic potentials, or other active changes in the membrane potential. ... Jump to: navigation, search Electrochemical gradient In celllular biology refers to the diffusion gradient of an ion, representing a type of potential energy that accounts for both the concentration difference of the ions across a cellular membrane and its tendency to move relative to the membrane potential. ... In biology, depolarization is the event a cell undergoes when its membrane potential grows more positive with respect to the extracellular solution. ... In cell biology, potassium channels are the most common type of ion channel. ... Ion channels are present in the membranes that surround all biological cells. ... Jump to: navigation, search In chemistry, an amino acid is any molecule that contains both amino and carboxylic acid functional groups. ...

Structure

The voltage-gated Na⁺ channel has three known subunits: a large glycoprotein called the alpha subunit, which probably forms the channel's pore, and two smaller polypeptides called beta1 and beta2 which regulate the function of the alpha (Kandel, 2000, p. 164). Gamma and delta subunits may also exist to regulate the alpha subunit. The alpha subunit has four repeats of the same 150 amino acid sequence named I through IV, each of which contains six membrane spanning regions labeled S1 through S6 (Kandel, 2000, p. 164). The highly conserved S4 region, thought to be the part of the channel that acts as its voltage sensor, has a positive amino acid at every third spot, with hydrophobic residues between these (Kandel, 2000, p. 164). It is thought that when stimulated, the channel moves this subunit from within the pore toward the extracellular side of the cell, opening the channel. A glycoprotein is a macromolecule composed of a protein and a carbohydrate (a sugar). ... A pore, in general, is some form of opening, usually very small. ... Conservation is a high degree of similarity in the primary or higher structure of homologous proteins amongst various phyla. ...

Voltage-gated sodium channels can have three states: resting (closed), activated (open), and inactivated (closed). Channels in the resting state are thought to be blocked on their intracellular side by an "activation gate", which is removed in response to stimulation that opens the channel (Kandel, 2000, p. 163). The ability to inactivate is thought to be due to a tethered plug (formed by domains III and IV of the alpha subunit), called an inactivation gate, that blocks the inside of the channel shortly after it has been activated (Kandel, 2000, p. 166). The channel remains inactivated for a few milliseconds after the neuron is finished depolarizing (Kandel, 2000, p. 156). Genetic diseases that cause Na⁺ channels to be unable to inactivate cause muscle stiffness because muscles fire repetitive trains of action potentials (Kandel, 2000, p. 169). A millisecond is an SI-derived unit of time, equal to one thousandth of a second. ...

Impermeability to Other Ions

The inner pore of sodium channels contains a selectivity filter made of negatively charged amino acid residues, which attract the positive Na⁺ ion and keep out negatively charged ions such as chloride (Kandel, 2000, p. 163). The cations flow into a more constricted part of the pore that is .3 X .5 nm wide, which is just large enough to allow a single Na⁺ ion with a water molecule associated to pass through (Kandel, 2000, p. 163-164). The larger K⁺ ion cannot fit through this area. Differently sized ions also cannot interact as well with the negatively charged glutamic acid residues that line the pore (Kandel, 2000, p. 163-164). A pore, in general, is some form of opening, usually very small. ... The chloride ion is formed when the element chlorine picks up one electron to form the anion (negatively charged ion) Cl−. The salts of hydrochloric acid HCl contain chloride ions and are also called chlorides. ... A nanometre (American spelling: nanometer) is 1. ... Jump to: navigation, search Water (from the Old English word wæter; c. ... Jump to: navigation, search A molecule is the smallest particle of a pure chemical substance that still retains its chemical composition and properties. ... Glutamate is the anion of glutamic acid. ...

Role in Action Potential

Voltage-gated sodium channels play a significant role in action potentials. If enough channels open when there is a change in the neuron's membrane potential, a large number of Na⁺ ions will rush into the cell down their electrochemical gradient, further depolarizing it. Thus the more Na⁺ channels exist in a neuron's membrane, the faster the action potential will propagate down the axon, and the more excitable that area of the cell will be (Kandel, 2000, p. 160-161). Na⁺ channels both open and close more quickly than K⁺ channels, producing an influx of positive charge toward the beginning of the action potential and an efflux toward the end (Kandel, 2000, p. 160-161). Voltage-gated ion channel is a ion channel that is specifically activated, or gated, by the surrounding potential difference near the channel (or near the cell, neuron or synapse). ... A. Schematic of an electrophysiological recording of an action potential showing the various phases which occur as the wave passes a point on a cell membrane. ... Neurons (also spelled neurones or called nerve cells) are the primary cells of the nervous system. ... Membrane potential (or transmembrane potential or transmembrane potential difference or transmembrane potential gradient), is the electrical potential difference across a cells plasma membrane. ... Jump to: navigation, search Electrochemical gradient In celllular biology refers to the diffusion gradient of an ion, representing a type of potential energy that accounts for both the concentration difference of the ions across a cellular membrane and its tendency to move relative to the membrane potential. ... In biology, depolarization is the event a cell undergoes when its membrane potential grows more positive with respect to the extracellular solution. ... An axon, or nerve fiber, is a long slender projection of a nerve cell, or neuron, which conducts electrical impulses away from the neurons cell body or soma. ... In cell biology, potassium channels are the most common type of ion channel. ... A. Schematic of an electrophysiological recording of an action potential showing the various phases which occur as the wave passes a point on a cell membrane. ...

See Also

• Ion channels
• Calcium channels
• Potassium channels
• Voltage-gated ion channels
• Resting ion channels
• Sodium (biology)

Another, unrelated ion channeling process is part of ion implantation. ... Ion channels are present in the membranes that surround all biological cells. ... In cell biology, potassium channels are the most common type of ion channel. ... Voltage-gated ion channel is a ion channel that is specifically activated, or gated, by the surrounding potential difference near the channel (or near the cell, neuron or synapse). ...

Reference

• Kandel E.R., Schwartz, J.H., Jessell, T.M. 2000. Principles of Neural Science, 4th ed., pp.154-169. McGraw-Hill, New York.

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Sodium ion channel

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