Anatomy & Physiology

Anatomy & Physiology Study Guide depolarization occurs at the site. In less than a millisecond, the inner membrane surface has changed; it now contains more positive ions. • Step 3 Inactivation of sodium channels and the activation of potassium channels: As the transmembrane potential approaches +30 mV, the inactivation gates of the voltage-gated sodium channels begin closing. This step is known as sodium channel inactivation . While it is under way, voltage-gated potassium channels are opening. The sudden loss of positive charges then shifts the transmembrane potential back toward resting levels, and repolarization begins. • Step 4 The return to normal permeability: The voltage-gated sodium channels remain inactivated until the membrane has repolarized to near threshold levels. At this time, they regain their normal status: closed but capable of opening. Potassium ions continue to move out of the cell at a faster rate than when they are at rest, producing a brief hyperpolarization that brings the transmembrane potential very close to the equilibrium potential for potassium (–90 mV). As the voltage-gated potassium channels close, the transmembrane potential returns to normal resting levels. The membrane is now in a pre-stimulation condition, and the action potential is over.

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