Steps in the neuron action potential

Stimulus initiates local depolarization

a stimulus changes the permeability of a local "patch" of the membrane, and sodium ions diffuse rapidly into the cell. This changes the polarity of the membrane (the inside becomes more positive; the outside becomes more negative) at that site

Propagation of the action potential

depolarization of the first membrane patch causes permeability changes in the adjacent membrane, and the events described in step 2 are repeated. Thus, the action potential propagates rapidly among the entire length of the membrane.

Depolarization and generation of an action potential;

if the stimulus is strong enough, depolarization causes membrane polarity to be completely reversed and an action potential is initiated.

Resting membrane is polarized

in the resting state, the external face of the membrane is slightly positive; its internal face is slightly negative. The chief extracellular ion is sodium whereas the chief intracellular ion is potassium. the membrane is relatively impermeable to both ions.

Initial ionic conditions restored

the ionic conditions of the resting state are restored later by the activity of the sodium-potassium pump. Three sodium ions are ejected for every two potassium ions carried back into the cell

Repolarization

potassium ions diffuse out of the cell as the membrane permeability changes again, restoring the negative charge on the inside of the membrane and the positive charge on the outside surface. This occurs in the same direction as depolarization