Thus, the effect of incoming Na’* is very rapidly transmitted along
the interior of the axon as a sort of current flow of positive charge. In
this way, the impact of the incoming positive charge and resulting
membrane depolarization is quickly transmitted from one node of
Ranvier to the next. This jumping from node to node results in a more
rapid movement of action potentials along the axon than in the un-
myelinated case, where each small region of the axon impacts the im-
mediately adjacent small region in a slower, more continuous flow.
It is analogous to traveling by a local bus or train, which stops at
many intersections or towns to pick up and let off passengers, versus
traveling via an express bus or train, which makes many fewer stops—
the express is much faster than the local. And the neural signal prop-
agation in a myelinated axon is much faster than in an unmyelinated
axon. The speed of an action potential moving along an unmyelinated
axon might be several miles per hour (less than 10 meters per second),
while the speed in a myelinated axon can be 100 meters per second—
more than 200 miles per hour!
The dramatic increase in signaling speed within the nervous
system provided by myelin allows for more rapid communication
throughout the body, permitting such things as coordinated control
of muscles in large animals. Insects don’t have myelin, and there are
no giant insects in part because it would be difficult to coordinate the
synchronous movement of muscles in a large body using more slowly
conducting unmyelinated nerves. Faster neuronal signaling also un-
derlies increased computational power and complexity in the brain,
related, we believe, to more sophisticated abilities to perceive, think,
and feel. All from just wrapping axons with lipids—another amazing
innovation of biological evolution!
Axons and ions,