Action Potentials Jump from Node to Node in Myelinated Axons

The myelin sheath surrounding an axon is formed from many glial cells. Each region of myelin formed by an individual glial cell is separated from the next region by an un-myelinated area of axonal membrane about 1 ^m in length called the node of Ranvier (or simply, node). The axonal membrane is in direct contact with the extracellular fluid only at the nodes. Moreover, all the voltage-gated Na+ channels and all the Na+/K+ pumps, which maintain the ionic gradients in the axon, are located in the nodes.

As a consequence of this localization, the inward movement of Na+ ions that generates the action potential can occur only at the myelin-free nodes (Figure 7-40). The excess cytosolic positive ions generated at a node during the membrane depolarization associated with an action potential spread passively through the axonal cytosol to the next node with very little loss or attenuation, since they cannot cross the myelinated axonal membrane. This causes a depolarization at one node to spread rapidly to the next node, permitting, in effect, the action potential to "jump" from node to node. This phenomenon explains why the conduction velocity of myelinated neurons is about the same as that of much larger diameter unmyelinated neurons. For instance, a 12-^m-diameter myelinated vertebrate axon and a 600-^m-diameter unmyelinated squid axon both conduct impulses at 12 m/s.

Several factors contribute to the clustering of voltage-gated Na+ channels and Na+/K+ pumps at the nodes of

Action potential (depolarized region of membrane) Na+

Node of Ranvier

Myelin sheath

Axon f

Excess positive charge f

Excess positive charge

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