Human Physiology, 14th edition (2016)

168 Chapter 7

cell wraps only about a millimeter of axon, leaving gaps of
exposed axon between the adjacent Schwann cells. These gaps
in the myelin sheath are known as the nodes of Ranvier. The
successive wrappings of Schwann cell membrane provide
insulation around the axon, leaving only the nodes of Ranvier
exposed to produce nerve impulses.
The Schwann cells remain alive as their cytoplasm is forced
to the outside of the myelin sheath. As a result, myelinated axons
of the PNS are surrounded by a living sheath of Schwann cells,
or neurilemma ( figs. 7.6 and 7.7 ). Unmyelinated axons are also
surrounded by a neurilemma, but they differ from myelinated
axons in that they lack the multiple wrappings of Schwann cell
plasma membrane that compose the myelin sheath.

Myelin Sheath in CNS

As mentioned earlier, the myelin sheaths of the CNS are
formed by oligodendrocytes. This process occurs mostly post-
natally (after birth) and continues into late adolescence. It may
continue later into adulthood in some brain regions, and there
is evidence that learning certain activities (such as playing
piano), particularly in childhood, may increase myelination.

CLINICAL APPLICATION

Demyelinating diseases are those in which the myelin sheaths

are specifically attacked. Because Schwann cells form the

myelin in the PNS, whereas oligodendrocytes form the myelin

of the CNS, the immune system attacks one or the other type of

myelin. In Guillain-Barre syndrome, the T cells of the immune

system (chapter 15) attack myelin sheaths of the PNS. This pro-

duces rapid onset of symptoms that include muscle weakness

(which can dangerously affect the muscles of breathing) due

to dysfunction of somatic motor axons, and cardiac and blood

pressure problems due to dysfunction of autonomic axons.

Multiple sclerosis ( MS ) is produced by autoimmune

attack (chapter 15, section 15.6) mediated by T cells on the

myelin sheaths in the CNS. This leads to areas of hardening, or

scleroses, followed by axonal degeneration. MS is usually diag-

nosed in people between the ages of 20 and 40, and is twice

as common among woman than men. It is a chronic remitting

and relapsing disease with highly variable symptoms, including

sensory impairments, motor dysfunction and spasticity, blad-

der and intestinal problems, fatigue, and others. Treatment for

MS includes drugs that reduce autoimmune activity and inflam-

mation, and drugs that interfere with the entry of autoreactive

T cells into the CNS. These drugs help in treating the symptoms

of MS, but do not restore myelin sheaths or cure the disease.

Myelin sheath

Schwann cellSchwann cell

cytoplasmcytoplasm

Schwann cell

cytoplasm

MyelinatedMyelinated

axonaxon

Myelinated

axon

UnmyelinatedUnmyelinated

axonaxon

Unmyelinated

axon Schwann cellSchwann cell

cytoplasmcytoplasm

Schwann cell

cytoplasm

Figure 7.7 An electron micrograph of unmyelinated
and myelinated axons. Notice that myelinated axons have
Schwann cell cytoplasm to the outside of their myelin sheath,
and that Schwann cell cytoplasm also surrounds unmyelinated
axons.

Figure 7.8 The formation of myelin sheaths in the

CNS by an oligodendrocyte. One oligodendrocyte forms

myelin sheaths around several axons.

Oligodendrocyte

Node of Ranvier

Axon

Myelin

sheath

Unlike a Schwann cell, which forms a myelin sheath around

only one axon, each oligodendrocyte has extensions, like the

tentacles of an octopus, that form myelin sheaths around several

axons ( fig. 7.8 ). The myelin sheaths around axons of the CNS