Biological Membranes - A. N. Ogurtsov 2012

Electrogenesis of Biomembranes
Mechanisms of Action Potential Generation
Propagation of the Action Potential

If an Action Potential is generated in any region of an excitable membrane, the membrane becomes depolarized, and the excitation begins to spread to adjacent Regions of the membrane.

Local currents arise in both the axoplasm and the surrounding solution between patches of the membrane surface with a higher potential (positively charged) and patches with a lower potential (negatively charged). These local currents are formed both inside the axon and on its outer surface (Figure 129).

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Figure 129 - Propagation of the action potential

In areas close to the excited region, the magnitude of the Membrane Potential rises above the threshold value. Driven by this shift in the membrane potential, sodium channels open, and further depolarization is driven by the influx of sodium ions across the membrane. This triggers an avalanche-like, spontaneous depolarization of the membrane, generating an action potential. Subsequently, the excitation is similarly transmitted further to adjacent resting regions of the membrane.

Local currents flow in both directions from the excited region, but excitation can only propagate into the region of the membrane that is at rest, that is, in only one direction along the axon. The Nerve Impulse cannot travel in the opposite direction because the areas through which the excitation has just passed remain unexcitable—refractory—for a certain period of time.