FoundationalConceptsNeuroscience

channel proteins and Na/K pump proteins that would be distributed
along the entire length of an unmyelinated axon are jammed together
at the nodes. The density of voltage-gated sodium channels in an
unmyelinated axon is around a hundred channel proteins per square
micrometer of axonal membrane, whereas in a myelinated axon the
density of voltage-gated sodium channels at the nodes of Ranvier
is more than one hundred times more concentrated—ten thousand
channel proteins per square micrometer.

Oligodendrocyte ~

soma

Axon

interior

Figure 5.10. Electron micrograph of a cross section of a myelinated axon from
human cerebral cortex. Note the multiple-layered structure of the myelin.
The diameter of the axon interior is approximately 1.3 micrometers, and the
thickness of the myelin surrounding the axon is around 200 nanometers. The
spots visible in the interior of the axon are microtubules and microfilaments,
components of the neuronal cytoskeleton (see Chapter 10).

As described earlier, what happens in an unmyelinated axon is this:
when voltage-gated sodium channels open and sodium ions come
pouring into the axon, the ions drift around inside the axon and de-