Ionchannel

An ion channel is a pore-forming transmembrane protein that allows ions to pass through a cell membrane. Ion channels enable selective flow of ions such as potassium, sodium, calcium, and chloride, typically down their electrochemical gradient, and are central to electrical signaling, osmoregulation, and secretion.

Most ion channels are gated, meaning they open or close in response to stimuli. The main gate types are voltage-gated channels (opening in response to changes in membrane potential), ligand-gated channels (opening when a chemical messenger binds), and mechanically gated channels (opening in response to membrane stretch or pressure). Some channels respond to temperature or other intracellular signals.

Structurally, ion channels vary. Potassium channels are often tetramers with a selective filter that favors K+ ions, formed by the pore loop. Voltage-gated channels typically have a voltage-sensing domain that moves in response to voltage changes, triggering pore opening. Sodium and calcium channels usually consist of a single polypeptide with four homologous domains forming the pore.

Physiological roles include generation and propagation of action potentials in neurons and muscle cells, synaptic transmission, hormone secretion, and regulation of cell volume and pH. They are distributed broadly in nervous, cardiovascular, and other systems.

Mutations or dysfunction of ion channels cause channelopathies, including epilepsy, cardiac arrhythmias, cystic fibrosis, and certain neuropathies. Because of their central role in excitability, ion channels are prominent targets in pharmacology; drugs such as local anesthetics, antiarrhythmics, and calcium channel blockers modulate channel activity.

Research methods like patch-clamp recording and structural techniques such as cryo-electron microscopy have advanced understanding of channel gating, selectivity, and pharmacology.