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It is almost impossible for charged anions and cations to cross cell membranes without the help of an ion transport protein. Ion channels are ion transport proteins that allow for the most rapid movement of ions across cell membranes. Ion channels are important for action potentials, when the membrane potential must change rapidly.Voltage-gated ion channels can shift between open and closed forms in response to changes in the membrane potential. Examples: voltage-gated calcium and potassium channels produce action potentials in cardiac pacemaker cells.
Ligand-gated ion channels can be opened in response to the binding of a chemical ligand. Example: the acetylcholine receptors of skeletal muscle are opened by the neurotransmitter acetylcholine. Example: "funny" channels are gated open by the intracellular second messenger molecule cyclic-AMP. This this happens when signals from the sympathetic division of the autonomic nervous system speed up the heart rate during exercise.
Animation. Ion channel proteins can change conformation (shape) and shift between open and closed conformations. When open, ion channels provide a trans-membrane pore through which ions and small molecules can pass. Click on the image to view the animation.
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