Impulse traveling from an unmyelinated axon into a myelinated axon

Myelination of an axon prroduces an enormous increase in the speed of impulse propagation due to the extensive forward spread of the depolarization. This movie shows, in the lower panel, an action potential (AP) traveling from left to right in a length (1 cm) of axon (10um diam) that is unmyelinated (bare) on its left half and myelinated along its right half. In the myelinated region, 5 nodes are spaced 1000um (1 mm) apart and are numbered from left to right starting with 0. The upper panel is a voltage-vs-time plot of recordings at three electrodes: one (red) in the center of the bare axon; a second (blue) at node 0; and a third (black) at node 4. A stimulating electrode at the left end of the axon triggers the AP with a short sharp shock.

Although the AP propagates normally in the bare axon, there is an abrupt change in its shape and speed as it enters the myelinated region. By stopping the movie, one can see the depolarization spreading far in advance of the AP and then the peak of the AP spreading over many nodes.