entry, it will trigger sodium channels to open only in the downstream
direction. The upstream sodium channels will be in their refractory
period and unable to open again for another millisecond or two. This
prevents the neural signal from bouncing around in both directions
along the axon, which could make for quite a confusing mess.
Now consider this: many nerve cell axons in the human brain and
body (and, indeed, in the brains and bodies of all vertebrate animals)
are covered along their length by something called myelin (Fig. 5.9).
Myelin is formed when particular types of glial cells—oligodendro-
cytes in the brain and Schwann cells in the peripheral nervous system
—develop large flattened bodies and wrap around and around the
axon (Fig. 5.10). Myelin is largely composed of layers of lipid bilayer
membrane. About 70 percent of the dry weight of myelin is lipid.
And more than 25 percent of this lipid is cholesterol. Cholesterol is
an important component in all lipid bilayer membranes in animals,
contributing to the structural integrity and the fluidity of the mem-
branes. The cholesterol molecule orients so that its -OH group is
within a hydrophilic edge portion of the bilayer, and the main body of
the molecule is within the hydrophobic core of the bilayer.