VPB 121: Veterinary Physiology - II (2+1)

THE STAGES OF ACTION POTENTIAL

Depolarization stage

• The resting membrane suddenly becomes very permeable to sodium ions (stimulus applied causes this permeability) allowing tremendous number of Na⁺ to flow to the interior of the fiber. The normal resting state of -75mV is lost with the potential rising or changing rapidly in the positive direction. This is called depolarization.

• In large fibers the potential over shoots beyond the zero level and becomes slightly positive (reversal potential). In some smaller fibers as well as in many CNS neurons the potential approaches the zero but does not overshoot or reverse.

Repolarization stage

• Within a fraction of a second (few 10,000 ^ths of a second) after the membrane becomes highly permeable for Na⁺ the channels for the Na⁺ close almost as rapidly as they had opened. Then rapid diffusion of K⁺ to the exterior re-establishes the normal negative resting potential. This is called repolarization of the membrane.