HO
CholesterolSeveral types of protein are also found in myelin. One function of
the proteins is to link layers together, so that the myelin doesn’t un-
ravel from the axon.
Myelin was first noted by neuroanatomists in the mid-nineteenth
century. It was in the 1870s that Louis-Antoine Ranvier (1835-1922)
observed that in axons covered with myelin (myelinated axons), the
myelin was not continuous along the axon but occurred in sections
separated by very small gaps. These gaps were later named the nodes
of Ranvier, in honor of their discoverer (see Fig. 5.9).
So what’s up with myelin? The generation and propagation of ac-
tion potentials along an axon depend on the controlled flow of sodium
and potassium ions across the membrane via the opening and closing
of voltage-gated channels. If the axon is covered with layers and layers
of fatty material (myelin), how can ions flow through channels across
the membrane? Well, they couldn’t—so the action potential couldn’t
propagate if the axon were completely covered with myelin. The key
lies in the nodes of Ranvier. Along a myelinated axon there is a gap
or node every few hundred micrometers, and all the voltage-gated