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).
After the Action Potential. So what happens inside the neuron after the action potential has taken place? After the neuron has fired, there is a refractory period in which another action potential is not possible. During this time, the potassium channels reopen and the sodium channels close, gradually returning the neuron to its resting potential.