363
CHAPTER TEN
Nervous System I
pp. 385–386). Here, gap junctions join ependymal
cells, forming a porous layer through which substances
diffuse freely between the interstitial fl
uid of the
brain tissues and the fl
uid (cerebrospinal fl
uid) in the
ventricles.
Ependymal cells also cover the specialized
capillaries called
choroid plexuses
associated with the
ventricles of the brain. Here they help regulate the
composition of the cerebrospinal fl
uid.
Neuroglia, which comprise more than half of the volume
of the brain
and outnumber neurons 10 to 1, are critical to
neuron function.
Abnormal neuroglia are associated with certain disorders.
Most brain tumors, for example, consist of neuroglia that
divide too often. Neuroglia that produce toxins may lie behind
some neurodegenerative disorders. In one familial form of
amyotrophic lateral sclerosis (ALS, or Lou Gehrig’s disease),
astrocytes release a toxin that destroys motor neurons, caus-
ing progressive weakness. In Huntington disease (HD), which
causes uncontrollable movements and cognitive impairment,
microglia in the brain release a toxin that damages neurons.
In both ALS and HD, only speciF c sets of neurons are affected.
Identifying the unexpected roles of neuroglia in nervous sys-
tem disorders suggests new targets for treatments.
Neuroglia of the PNS
The two types of neuroglia in the peripheral nervous system
are Schwann cells and
satellite cells:
1. Schwann cells produce the myelin on peripheral
myelinated neurons, as described earlier.
2. Satellite cells support clusters of neuron cell bodies
called
ganglia
, in the PNS.
Table 10.2
summarizes the characteristics and functions of
neuroglia.
Neuron
cell body
Neuroglia
FIGURE 10.9
A scanning electron micrograph of a neuron cell body
and some of the neuroglia associated with it (10,000×). (Tissues and
Organs:
A Text-Atlas of Scanning Electron Microscopy,
by R. G. Kessel and
R. H. Kardon, (c) 1979 W. H. Freeman and Company.)
a single oligodendrocyte may provide myelin for many
axons. However, these cells do not form neurilemmae.
3.
Microglia.
Microglial cells are small and have fewer
processes than other types of neuroglia. These cells are
scattered throughout the CNS, where they help support
neurons and phagocytize bacterial cells and cellular
debris. They usually proliferate whenever the brain or
spinal cord is infl
amed because of injury or disease.
4.
Ependyma.
Ependymal cells are cuboidal or columnar
in shape and may have cilia. They form the inner lining
of the
central canal
that extends downward through
the spinal cord. Ependymal cells also form a one-cell-
thick epithelial-like membrane that covers the inside
of spaces in the brain called
ventricles
(see chapter 11,
TABLE
10.2
|
Types of Neuroglia
Type
Characteristics
Functions
CNS
Astrocytes
Star-shaped cells between neurons and blood vessels
Structural support, formation of scar tissue, transport of substances between
blood vessels and neurons, communicate with one another and with neurons,
mop up excess ions and neurotransmitters, induce synapse formation
Oligodendrocytes
Shaped like astrocytes, but with fewer cellular processes,
occur in rows along axons
Form myelin sheaths in the brain and spinal cord, produce nerve growth
factors
Microglia
Small cells with few cellular processes and found throughout
the CNS
Structural support and phagocytosis (immune protection)
Ependyma
Cuboidal and columnar cells in the inner lining of the
ventricles of the brain and the central canal of the spinal
cord
Form a porous layer through which substances di±
use between the
interstitial ²
uid of the brain and spinal cord and the cerebrospinal ²
uid
PNS
Schwann cells
Cells with abundant, lipid-rich membranes that wrap tightly
around the axons of peripheral neurons
Speed neurotransmission
Satellite cells
Small, cuboidal cells that surround cell bodies of neurons in
ganglia
Support ganglia in the PNS
previous page 393 David Shier Hole's Human Anatomy and Physiology 2010 read online next page 395 David Shier Hole's Human Anatomy and Physiology 2010 read online Home Toggle text on/off