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1 Basic features

An ion channel is an integral membrane protein or more typically an assembly of several proteins. Such "multi- subunit" assemblies usually involve a circular arrangement of identical or related proteins closely packed around a water-filled pore through the membrane single file — nearly as fast as the ions move through free fluid. Access to the pore is governed by "gates," which may be opened or closed by chemical or electrical signals, or mechanical force, depending on the variety of channel.

2 Biological role

Because "voltage-gated" channels underlie the nerve impulse and because "transmitter-gated" channels mediate conduction across the synapses, channels are especially prominent components of the nervous system. Indeed, most of the offensive and defensive toxins that organisms have evolved for shutting down the nervous systems of predators and prey (e.g. the venoms produced by spiders, scorpions, snakes, fish, bees, sea snails and others) work by plugging ion channel pores. But ion channels figure in a wide variety of biological processes that involve rapid changes in cells. In the search for any drug, ion channels are a favorite target.

2.1 Diversity and activation

• Voltage-gated channels sense the transmembrane potential and open or close in response to depolarization or hyperpolarization, respectively. Examples include the sodium and potassiumIn cell biology, potassium channels are the most common type of ion channel. They form potassium-selective pores that span cell membranes. Potassium channels are found in most cells, and control the electrical excitability of the cell membrane. They shape voltage-gated channels of nerve and muscle, and the voltage-gated calcium channels that control neurotransmitterA neurotransmitter is a type of molecule that carries signals between neurons (nerve cells) at synapses in the nervous system. Neurotransmitters may be either excitatory ( EPSPs) or inhibitory ( IPSPs). That is, they may foster the initiation of a nerve i release in pre-synaptic endings.
• Ligand-gated channels open in response to a specific ligand molecule on the external face of the membrane in which the channel resides. Examples include the "nicotinic" Acetylcholine receptorAn acetylcholine receptor (abbreviated AChR is an integral membrane protein that responds to the binding of the neurotransmitter acetylcholine by opening a pathway in the membrane for the diffusion of ions across the cell membrane. Classification Like oth, AMPA receptorThe AMPA receptor AMPAR is a non- NMDA-type ionotropic transmembrane receptor for glutamate that mediates fast synaptic transmission in the central nervous system. Its name is derived from its ability to be activated by the artificial glutamate analog, AM and other neurotransmitterA neurotransmitter is a type of molecule that carries signals between neurons (nerve cells) at synapses in the nervous system. Neurotransmitters may be either excitatory ( EPSPs) or inhibitory ( IPSPs). That is, they may foster the initiation of a nerve i-gated channels.
• cyclic nucleotide-gated channels , Calcium-activated channels and others open in response to internal solutes and mediate cellular responses to second messengerIn biology, second messengers are low-weight diffusible molecules that are used in signal transduction to relay a signal within a cell. They are synthesized or released by specific enzymatic reactions, usually as a result of an external signal that was res.
• Stretch-activated channels open or close in response to mechanical forces that arise from local stretching or compression of the membrane around them; for example when their cells swell or shrink. Such channels are believed to underlie touch sensation and the transduction of acoustic vibrations into the sensation of sound.
• G-protein-gated channels open in response to G proteinG-Proteins short for Guanine Nucleotide Binding Proteins are a family of proteins involved in second messenger cascades. They are so-called because of their signaling mechanism, which uses the exchange of guanine di phosphate ( GDP) for guanine triphospha-activation via its receptor. An example is the "muscarinic" Acetylcholine receptor

Certain channels respond to multiple influences. For instance, the NMDA receptor is partially activated by interaction with its ligand, glutamate, but is also voltage-sensitive and only conducts when the membrane is depolarized. Some calcium-sensitive potassium channels respond to both calcium and depolarization, with an excess of one apparently being sufficient to overcome an absence of the other.