Figure 14-21.

Axonal Action Potential Conduction and the Effect of Demyelination.

The speed of the conduction process is determined by the diameter of the axon and the degree of myelination. As axons increase in diameter, the resistance to ion flow decreases, allowing the action potential to flow faster. In addition, the degree of myelination is directly proportional to the diameter of the axon. Thus the concept that the more myelin the faster the speed of the impulse is true up to the point in which the myelin is normal in thickness. For an axon whose myelin is reduced, conduction of the action potential is slower. Under normal conditions, locomotion is a well-coordinated event that requires precise timing (speed) of impulse conduction to get coordinated movements. If the speed of the action potential is altered by disease, especially demyelination, then the conduction of the action potential will be delayed, and what are normally coordinated movements become uncoordinated. A, In unmyelinated axons, action potentials are conducted at a relatively “slower” velocity by the process of ion exchange continuous conduction (see E-Fig. 14-4). B, In myelinated axons, action potentials are conducted at a relatively “faster” velocity by a mechanism called saltatory conduction. Optimal function of saltatory conduction is dependent on having the proper degree of myelination of the axon (as determined by axonal diameter) throughout the full length of the axon. C, In axons that have lost some but not all of their myelin lamellae from one or more internodes so that there is a “thinner” covering of myelin, the speed of saltatory conduction is reduced because of leakage of the action potential across this thinner myelin sheath, resulting in clinical dysfunction of the nervous system. D, In axons that have lost all of their myelin from one or more internodes (complete primary demyelination of the internode), the speed of saltatory conduction is reduced because of the conversion from saltatory conduction to ion exchange continuous conduction in the areas where internodes have lost their myelin. Thus the speed and timing of the action potential is substantially reduced, leading to clinical dysfunction of the nervous system.

(Courtesy Dr. J.F. Zachary, College of Veterinary Medicine, University of Illinois.)