358
UNIT THREE
Neuron
cell body
Dendrite
Neuron
nucleus
Schwann cell
nucleus
Neurilemma
Myelin sheath
Neurofibrils
Axon
Node of Ranvier
Myelinated region of axon
Unmyelinated
region of axon
Unmyelinated
axon
Axon
Longitudinal
groove
Enveloping
Schwann cell
Schwann
cell nucleus
Axon
Node of Ranvier
Myelin
(a)
(b)
(c)
FIGURE 10.4
A myelinated axon. (
a
) The part of a Schwann
cell that winds tightly around an axon forms the myelin
sheath. The cytoplasm and nucleus of the Schwann cell,
remaining on the outside, form the neurilemma. (
b
) Light
micrograph of a myelinated axon (longitudinal section)
(650×). (
c
) An axon lying in a longitudinal groove of a
Schwann cell lacks a myelin sheath.
Schwann cells also enclose, but do not wind around, the
smallest axons of peripheral neurons. Consequently, these
axons lack myelin sheaths. Instead, the axon or a group of
axons may lie partially or completely in a longitudinal groove
of Schwann cells.
Axons that have myelin sheaths are called
myelinated
(medullated) axons, and those that lack these sheaths are
unmyelinated
axons
(f
g. 10.5)
. Groups of myelinated axons
appear white. Masses of such axons impart color to the
white
matter
in the brain and spinal cord, but in the CNS another
type of neuroglial cell called an
oligodendrocyte
produces
myelin. In the brain and spinal cord, myelinated axons lack
neurilemmae.
Unmyelinated nerve tissue appears gray. Thus, the
gray
matter
in the CNS contains many unmyelinated axons and
neuron cell bodies. Clinical Application 10.2 discusses mul-
tiple sclerosis, in which neurons in the brain and spinal cord
lose their myelin.
previous page 388 David Shier Hole's Human Anatomy and Physiology 2010 read online next page 390 David Shier Hole's Human Anatomy and Physiology 2010 read online Home Toggle text on/off