Medical Definition of antioxidant. : any of various substances (as beta-carotene, vitamin C, and alpha-tocopherol) that inhibit oxidation or reactions promoted by oxygen and peroxides and that include many held to protect the living body from the deleterious effects of free radicals.
An action potential is a very rapid change in membrane potential that occurs when a nerve cell membrane is stimulated. Specifically, the membrane potential goes from the resting potential (typically -70 mV) to some positive value (typically about +30 mV) in a very short period of time (just a few milliseconds).
The action potential includes a depolarization (activation) followed by a repolarization (recovery). The action potential occurs in all cardiac cells but its appearance varies depending on cell type. During de- and repolarization ions (Na+ [sodium], K+ [potassium] and Ca2+ [calcium]) flows back and forth across the cell membrane.