Events in the transmission of a nerve impulse in a neuron.

1) Resting membrane is polarized.

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

(Na+), whereas the chief intracellular ion is potassium (K+).

The membrane is relatively impermeable to both ions.

2) Stimulus initiates local depolarization.

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.

3) Depolarization and generation of an action potential.

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.

4) Propagation of the action potential.

Propagation of the action potential. Depolarization of

the first membrane patch causes permeability changes in the

adjacent membrane, and the events described in step are repeated. Thus, the action potential propagates rapidly along

the entire length of the membrane.

5) Repolarization

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. Repolarization occurs

in the same direction as depolarization.

6) Initial ionic conditions restored

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.