460
UNIT 3
Regulation and Integration of the Body
12
Pia Mater
Te
pia mater
(pi
9
ah), meaning “gentle mother,” is composed
of delicate connective tissue and richly invested with tiny blood
vessels. It is the only meninx that clings tightly to the brain like
cellophane wrap, following its every convolution. Small arter-
ies entering the brain tissue carry ragged sheaths of pia mater
inward with them for short distances.
Homeostatic Imbalance
12.9
Meningitis
, inflammation of the meninges, is a serious threat to
the brain because a bacterial or viral meningitis may spread to
the CNS. Brain inflammation is called
encephalitis
(en-sef
0
ah-
li
9
tis). Meningitis is usually diagnosed by obtaining a sample of
cerebrospinal fluid via a lumbar tap (see Figure 12.27, p. 466)
and examining it for microbes.
Cerebrospinal Fluid (CSF)
Cerebrospinal fluid
, found in and around the brain and spinal
cord, forms a liquid cushion that gives buoyancy to CNS struc-
tures. By floating the jellylike brain, the CSF effectively reduces
brain weight by 97% and prevents the delicate brain from crush-
ing under its own weight. CSF also protects the brain and spinal
cord from blows and other trauma. Additionally, although the
brain has a rich blood supply, CSF helps nourish the brain, and
there is some evidence that it carries chemical signals (such as
hormones and sleep- and appetite-inducing molecules) from
one part of the brain to another.
CSF is a watery “broth” similar in composition to blood
plasma, from which it is formed. However, it contains less pro-
tein than plasma and its ion concentrations are different. For
example, CSF contains more Na
1
, Cl
2
, and H
1
than does blood
plasma, and less Ca
2
1
and K
1
.
Falx cerebri
Superior
sagittal sinus
Straight
sinus
Crista galli of the
ethmoid bone
Pituitary gland
Falx cerebelli
Occipital lobe
Dura mater
Transverse
sinus
Temporal
bone
(b)
Posterior dissection
(a)
Midsagittal view
Scalp
Parietal
bone
Tentorium
cerebelli
Cerebellum
Arachnoid mater
over medulla
oblongata
Figure 12.23
Dural septa and dural venous sinuses.
(a)
Dural septa are partitioning folds
of dura mater in the cranial cavity.
(b)
Dural venous sinuses (injected with blue latex) are spaces
between the periosteal and meningeal dura containing venous blood.
Te
choroid plexuses
that hang from the roof of each ventri-
cle form CSF. Tese plexuses are frond-shaped clusters of broad,
thin-walled capillaries (
plex
5
interwoven) enclosed first by pia
mater and then by a layer of ependymal cells lining the ventri-
cles
(Figure 12.24b)
. Tese capillaries are fairly permeable, and
tissue fluid filters continuously from the bloodstream. However,
the choroid plexus ependymal cells are joined by tight junc-
tions, and they have ion pumps that allow them to modify this
filtrate by actively transporting only certain ions across their
membranes into the CSF pool. Tis careful regulation of CSF
composition is important because CSF mixes with the extracel-
lular fluid bathing neurons and influences the composition of
this fluid. Ion pumping also sets up ionic gradients that cause
water to diffuse into the ventricles.
In adults, the total CSF volume of about 150 ml (about half
a cup) is replaced every 8 hours or so. About 500 ml of CSF is
formed daily. Te choroid plexuses also help cleanse the CSF by
removing waste products and unnecessary solutes.
Once produced, CSF moves freely through the ventricles.
CSF enters the subarachnoid space via the lateral and median
apertures in the walls of the fourth ventricle (Figure 12.24a).
Te long cilia of the ependymal cells lining the ventricles help
keep the CSF in constant motion. In the subarachnoid space,
CSF bathes the outer surfaces of the brain and spinal cord
and then returns to the blood in the dural sinuses via the arach-
noid villi.
Homeostatic Imbalance
12.10
Ordinarily, CSF is produced and drained at a constant rate.
However, if something (such as a tumor) obstructs its circula-
tion or drainage, CSF accumulates and exerts pressure on the
brain. Tis condition is called
hydrocephalus
(“water on the
brain”).
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