Axonal Conduction: How an action potential moves along the axon

1. Axonal Conduction: How an action potential moves along the axon

2. What determines the conduction velocity, i.e., how fast action potentials move along the axon membrane? 1. Large diameter (like the squid�s giant axon) favors rapid conduction. Why? The resistance of current flow in the center of the axon becomes smaller as the diameter becomes larger. 2. Insulation speeds conduction by allowing the signal to jump from uninsulated node to node down the axon (saltatory conduction). Myelination, the wrapping of axons by glial cells (Schwann cells or oligodendrocytes) eliminates the passive leakage so that the depolarization is relayed �instantly� as an electrical field effect to the next node, where exposed membrane allows the opening of Na+ channels that can �recharge� the action potential, to send it to the next node. (Myelination increases the rate of action potential spread most effectively, but the fastest conducting axons in our nervous system combine relatively large diameter and myelination. The smallest axons are not myelinated and their messages of dull pain and temperature arrive at the central nervous system very slowly.)

3. Axonal conduction in the absence of myelination: every part of the membrane must go through the changes in potential