3.2 Myelinated fiber<->Non-myelinated Axon

This example demonstrates the problem an impulse, traveling in a myelinated nerve fiber, faces when it encounters a demyelinated region - as is the case in multiple sclerosis. There is not enough reserve energy in the nodes to depolarize the bare (demylinated) axon sufficiently to cause the generation of an impulse there. This figure shows that the bare axon presents, to the upstream node, a far larger capacitance than that of a node which it normally has to discharge. This means that depolarization of the bare axon will be so much slower than in a node that it sill fail to provoke a regenerative spike in it even in the presence of an abundance of Na channels. You can check out the excitability of the axon by moving the current injecting electrode to the distal end of the axon. A spike generated in the axon will progagate through the transition into the myelinated region! There are several ways to increase the depolarization in the bare axon sufficiently to cause the generation of an impulse:

• Cooling. This increases the duration of the nodal action potentials and can restore transmission in the bare axon region. One may be surprised at the small change in temperature required for this. Cooling also improves the condition of some patients with multiple sclerosis.
• Increasing the duration of the nodal action potentials in other ways such as partial blocking of the potassium channels.
• Decreasing the internodal length of the myelinated regions proximal myelin [0] and myelin [1] to the bare axon.

Run this example to experiment with these and other variables. Because of the large number of parameters to vary in this example, a "Panel Manager" panel has been added to allow convenient control of the panels on the screen at any time.

A careful study of the problems of demyelination and remyelination, with coupled experimentation and simulation, has been published by Hines & Shrager.