5. Action Potential

At the threshold potential, permeability suddenly changes at the trigger zone of the neuron being stimulated. Channels highly selective for sodium ions open and allow sodium to diffuse freely inward. This movement is aided by the negative electrical condition on the inside of the membrane, which attracts the positively charged sodium ions.

As sodium ions diffuse inward, the membrane loses its negative electrical charge and becomes depolarized. At almost the same time, however, membrane channels open that allow potassium ions to pass through, and as these positive ions diffuse outward, the inside of the membrane becomes negatively charged once more. The membrane potential may briefly become overly negative (hyperpolarization), but the membrane quickly returns to the resting potential (repolarization), and it remains in this state until stimulated again.

This rapid sequence of depolarization and repolarization, which takes about one-thousandth of a second, is the action potential. Because only a small fraction of the sodium and potassium ions move through the membrane during an action potential, many action potentials can occur, and resting potentials be reestablished, before the original concentrations of these ions change significantly. Also, active transport within the membrane maintains the original concentrations of sodium and potassium ions on either side.

Figure 7- Action potential. (a) At rest, the membrane potential is negative. (b) When the membrane reaches threshold, sodium channels open, some sodium (Na+ ) diffuses in, and the membrane is depolarized. (c) Soon afterward, potassium channels open, potassium (K+ ) diffuses out, and the membrane is repolarized. (For simplicity, negative ions are not shown.)

5. Action Potential

At the threshold potential, permeability suddenly changes at the trigger zone of the neuron being stimulated. Channels highly selective for sodium ions open and allow sodium to diffuse freely inward. This movement is aided by the negative electrical condition on the inside of the membrane, which attracts the positively charged sodium ions.

As sodium ions diffuse inward, the membrane loses its negative electrical charge and becomes depolarized. At almost the same time, however, membrane channels open that allow potassium ions to pass through, and as these positive ions diffuse outward, the inside of the membrane becomes negatively charged once more. The membrane potential may briefly become overly negative (hyperpolarization), but the membrane quickly returns to the resting potential (repolarization), and it remains in this state until stimulated again.

This rapid sequence of depolarization and repolarization, which takes about one-thousandth of a second, is the action potential. Because only a small fraction of the sodium and potassium ions move through the membrane during an action potential, many action potentials can occur, and resting potentials be reestablished, before the original concentrations of these ions change significantly. Also, active transport within the membrane maintains the original concentrations of sodium and potassium ions on either side.

Figure 7- Action potential. (a) At rest, the membrane potential is negative. (b) When the membrane reaches threshold, sodium channels open, some sodium (Na+ ) diffuses in, and the membrane is depolarized. (c) Soon afterward, potassium channels open, potassium (K+ ) diffuses out, and the membrane is repolarized. (For simplicity, negative ions are not shown.)