What is the Action Potential of Neuron?

Depolarization of Nerve Cell Membranes Causes Impulse Transmission

Apr 13, 2009

Neurons allow the brain to communicate with the rest of the body through the propagation of waves of depolarization, known as action potentials, along the axon membrane.

Before being stimulated to transmit an electrical impulse, the cell membrane of a neuron is slightly polarized. The fluid inside has a negative charge relative to that outside of the cell because of the balance of positively-charged ions of potassium and sodium and negatively-charged proteins and chloride ions. This polarized state is known as the resting potential. It prepares a neuron for the propagation of an action potential or nerve impulse.

Stages in the Passage of the Action Potential

The membrane potential of an unmyelinated neuron changes during the transmission of an idealized neural impulse. The changes in the membrane potential are brought about by the opening and closing of certain ion channels, allowing the diffusion of ions into and out of the cell, and by protein transport molecules which pump sodium ions out of the cell and potassium ions into the cell.

Stage 1: The Resting Potential

The neuron is at rest. The outside of the cell has an excess of sodium ions (Na+) and the inside of the cell has an excess of potassium ions (K+). The voltage inside the cell is a constant -70 mV (relative to the extracellular fluid). This potential difference is maintained by closed ion channels and the action of the sodium-potassium pump.

Stage 2: Reaching the Threshold

An external stimulus (either from sensory receptor cells or neurotransmitters from adjacent nerve cells) causes some sodium channels to begin to open. Sodium ions start to move into that part of the cell, causing a local decrease in the polarity of the membrane. If the potential difference across the membrane does not reach the threshold level of -55 mV, no action potential is generated.

Stage 3: Depolarization

If the membrane potential crosses the threshold value, an action potential is generated. The sodium channels open completely, sodium ions flood into the neural axon and the membrane is locally depolarized. This rapid diffusion causes the membrane to become oppositely polarized, that is, it overshoots neutral polarity and reaches a potential of +30 mV. At this point, the outside of the cell is negative relative to the inside.

The reversal in polarity of the membrane causes the sodium channels close. However, the change away from the resting potential activates sodium channels in adjacent parts of the membrane. This causes the wave of depolarization caused by opening and closing ion channels (the action potential) to be transmitted along the length of the axon at speeds ranging from 1 to 100 m/s.

Stage 4: Repolarization

In the declining phase of the action potential, the potassium channels open. This allows potassium ions to diffuse out of the cell and into the extracellular fluid, re-establishing the negative polarity of the membrane. The rapid diffusion results in undershooting of the resting potential by a small amount, at which point the potassium channels close.

Stage 5: Restoration of Original Ion Concentrations

After stage 4, the original sense of polarity of the membrane has been restored. In this state, however, the neuron will be unable to transmit another impulse. Therefore, the sodium-potassium pump uses the energy of ATP (adenosine triphosphate) to pump sodium ions back out of the cell and potassium ions back into the cell to re-establish the original distribution and concentration of ions.

The copyright of the article What is the Action Potential of Neuron? in Biology is owned by Adrienne Larocque. Permission to republish What is the Action Potential of Neuron? in print or online must be granted by the author in writing.