Home > Conscious > Chapter 2 > 2.4. The Propagation of Nerve Impulses

 

In a physiological system, the nerve impulse is initiated at the "axon initial segment" (AIS), although in experiments using a single axon the nerve impulse can be initiated at other positions. AIS contains a high density of voltage-gated sodium channels so that a large number of sodium ions can easily cause depolarization. Entry of sodium ions into the cell will raise membrane potential at AIS, thereby driving the movement of positive ions within the cell toward both sides, resulting in depolarization and subsequent generation of nerve impulses at adjacent areas. By this mechanism, a nerve impulse can travel from the AIS to the axon terminal. It may also propagate from AIS toward the dendrites. However, the nerve impulse cannot travel back toward AIS because of the refractory period.

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Figure 2-11. Changes in sodium ion concentration during the propagation of nerve impulses. Local increase in sodium ion concentration will raise the membrane potential there, driving the movement of positive ions inside the cell toward both sides, resulting in depolarization in adjacent areas. Due to the refractory period, the side that has just generated an impulse cannot produce another one. As a result, the nerve impulse will propagate along a definite direction, rather than back and forth. [Source: Wikipedia]

The propagation mechanisms for myelinated and unmyelinated axons are similar. The major difference is that the myelinated portions do not contain sodium and potassium channels. Therefore, nerve impulses will jump directly to the Node of Ranvier that contain ion channels. This type of propagation is called "saltatory conduction" which is much faster than the continuous conduction in unmyelinated axons.

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Figure 2-12. The saltatory conduction in the myelinated axon. [Source: Duflocq et al, 2011 ]