Glossary

activation The time-dependent growth of a membrane conductance in response to membrane depolarization.

all-or-nothing A term that refers to the property that action potentials, if they occur, have a stereotyped shape that is largely independent of the size and form of the suprathreshold stimulus.

current clamp An experimental protocol in which transmembrane current is controlled, usually at a series of constant values, and resulting transmembrane potentials are measured.

deactivation The time-dependent reversal of activation in response to membrane hyperpolarization; leads to a decrease in membrane conductance.

deinactivation The time-dependent reversal of inactivation, triggered by hyperpolarization; leads to an increase in membrane conductance.

depolarization Making membrane potential less negative.

hyperpolarization Making membrane potential more negative.

inactivation The time-dependent decline of a conductance (e.g., the Na+ conductance), which follows after its activation; triggered by depolarization.

membrane potential The voltage difference across the neural membrane, determined by the balance of ionic fluxes across the plasma membrane.

refractory period The period immediately after an action potential, in which it is difficult or impossible to induce a second action potential.

space clamp The condition in which membrane potential is the same throughout the spatial extent of the cell.

threshold The value of membrane current or membrane potential necessary to induce an action potential.

voltage clamp An experimental protocol in which membrane potential is controlled, usually in a stepwise fashion, and resulting transmembrane currents are measured.

voltage-gated ion channels Transmembrane proteins that open in response to changes in membrane potential, allowing a particular ionic species to cross the membrane.

The action potential is the all-or-nothing electrical impulse used to communicate information between neurons and from neurons to muscle fibers. The energy used to generate action potentials is in the form of electrochemical gradients of ions (in particular, sodium and potassium) that are established by ion pumps. The rising phase of action potentials is caused by the autocatalytic activation of many Na+-selective ion channels in response to sufficiently large increases in membrane potential. The falling phase of the action potential is caused by two factors that develop more slowly but dominate the electrical response after a few milliseconds: the inactivation of sodium channels and the activation of potassium channels, both of which occur in response to depolarization. Understanding the diverse mechanisms underlying electrical excitability in neurons remains a rich field of experimental and theoretical study, with wide-ranging implications for human health.

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