The Central Nervous System
Coordinates All Nervous Activity
LEARNING OBJECTIVES
1.
Name
the coverings of the brain and spinal
cord.
2.
Explain
the circulation of cerebrospinal fluid.
3.
List
the parts of the brain and describe the
function of each.
4.
Describe
the major parts of the spinal cord and
their functions.
5.
Compare
the ascending and descending spinal
tracts.
s you have already learned, the central ner-
vous system (CNS) consists of the brain and
spinal cord. The brain is composed of 100 bil-
lion neurons and 10 to 50 trillion neuroglia.
It has a mass of about 1.3 kg (about 3 lb). The “wiring”
of the brain and spinal cord allows the CNS to coordinate
and process nervous activity. Because different specialized
areas of the brain receive and process different types of
sensory information, knowledge of the structures of the
brain and spinal cord is essential to an understanding of
their functions. Let’s take a closer look.
The Central Nervous System
Requires Protection
As shown in
Figure 7.8
, the brain lies within the cranial
cavity of the skull and is covered by three connective tissue
membranes called
meninges
. The
brain and spinal cord are bathed
in
cerebrospinal
fluid
(CSF),
which is derived from the blood
in spaces found inside the brain
tissue called
ventricles
(
Figure
7.9
). The CSF circulates through
the
ventricles
and
around
the
brain and spinal cord, cushioning
the CNS. The composition of CSF
is very different from that of the
blood. This separation of CSF from the blood (called the
blood-brain barrier) protects the brain cells from harmful
substances and toxins.
meninges
(me-NIN-
jez) Three membranes
covering the brain and
spinal cord, called the
dura mater, arachnoid
mater, and pia mater.
ventricles
(VEN-
tri-kuls) Cavities in
the brain filled with
cerebrospinal fluid.
The cov ering s o f th e brain
• Figure 7.8
The skull and cranial meninges protect the brain.
Frontal plane
Subarachnoid
space
Arachnoid villus
reabsorbs CSF
into blood.
Superior
sagittal sinus
Brain
Skin
Bone of skull
Cranial
Meninges:
Dura mater
—tough connective
tissue covering immediately
beneath the skull.
Arachnoid
mater
—thin, fragile,
web-like tissue through which
cerebrospinal fluid (CSF) flows.
Pia mater
—extremely thin, inner
layer attached to the neurons of
the brain.
200 CHAPTER 7
The Nervous System
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