When ion channels are in a 'closed' (non-conducting) state, they are impermeable to ions and do not conduct electrical current. When ion channels are in an open state, they conduct electrical current by allowing some ions to pass through them, and thus across the plasma membrane of the cell. These flows of ions across the membrane result in an electrical current across the membrane. Gating is the process of an ion channel transforming between any of its conducting and non-conducting states.
The name 'gating' derives from the idea that an ion channel protein includes a pore that is guarded by a gate or several gates, and the gate(s) must be in the open position for any ions to pass through the pore. A variety of cellular changes can trigger gating, depending on the ion channel, including changes in voltage across the cell membrane (voltage-gated ion channels), drugs or hormones interacting with the ion channel (ligand-gated ion channels), changes in temperature, stretching or deformation of the cell membrane, addition of a phosphate group to the ion channel (phosphorylation), and interaction with other molecules in the cell (e.g., G proteins). The rate at which any of these gating processes occurs in response to these triggers are known as the 'kinetics of gating.' Some drugs and many ion channel toxins act as 'gating modifiers' of voltage-gated ion channels by changing the kinetics of gating.
The voltage-gated ion channels of the action potential are often described as having four gating processes: activation, deactivation, inactivation, and reactivation (also called 'recovery from inactivation'). In a model of an ion channel that has two gates (an activation gate and an inactivation gate) that must both be open for ions to be conducted through the channel, 'activation' is the process of opening the activation gate, which occurs in response to the voltage inside the cell membrane (the membrane potential) becoming more positive with respect to the outside of the cell (depolarization); 'deactivation' is the opposite process of the activation gate closing in response to the inside of the membrane becoming more negative (repolarization). 'Inactivation' is the closing of the inactivation gate; as with activation, inactivation occurs in response to the voltage inside the membrane becoming more positive, but often inactivation is found to be delayed in time compared to activation. 'Recovery from inactivation' is the opposite of inactivation. Thus, both inactivation and deactivation are processes that lead to the channel becoming non-conducting, but they are different processes in that inactivation is triggered by the membrane potential becoming more positive, whereas deactivation is triggered by the membrane potential becoming more negative.
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- Hille, Bertil (2001). Ion channels of excitable membranes. Sunderland, Mass: Sinauer. ISBN 0-87893-321-2.