462
UNIT 3
Regulation and Integration of the Body
12
move passively by facilitated diffusion through the endothelial
cell membranes. Bloodborne metabolic wastes, proteins, cer-
tain toxins, and most drugs are denied entry. Small nonessential
amino acids and potassium ions not only are prevented from
entering the brain, but also are actively pumped from the brain
across the capillary endothelium.
Te barrier is ineffective against fats, fatty acids, oxygen, car-
bon dioxide, and other fat-soluble molecules that diffuse easily
through all plasma membranes. Tis explains why bloodborne
alcohol, nicotine, and anesthetics can affect the brain.
Te structure of the blood brain barrier is not completely uni-
form. In some brain areas surrounding the third and fourth ventri-
cles, the barrier is entirely absent and the capillary endothelium is
quite permeable, allowing bloodborne molecules easy access to the
neural tissue. One such region is the vomiting center of the brain
stem, which monitors the blood for poisonous substances. An-
other is in the hypothalamus, which regulates water balance, body
temperature, and many other metabolic activities. Lack of a blood
brain barrier here is essential to allow the hypothalamus to sample
the chemical composition of the blood. Te barrier is incomplete
in newborn and premature infants, and potentially toxic substances
can enter the CNS and cause problems not seen in adults.
Injury to the brain, whatever the cause, may result in a lo-
calized breakdown of the blood brain barrier. Most likely, this
breakdown reflects some change in the capillary endothelial
cells or their tight junctions.
Homeostatic Imbalances
of the Brain
Describe the cause (if known) and major signs and
symptoms of cerebrovascular accidents, Alzheimer’s disease,
Parkinson’s disease, and Huntington’s disease.
Brain dysfunctions are unbelievably varied and extensive. We
have mentioned some of them already, but here we will focus on
traumatic brain injuries, cerebrovascular accidents, and degen-
erative brain disorders.
Traumatic Brain Injuries
Head injuries are a leading cause of accidental death in North
America. Consider, for example, what happens if you forget to
fasten your seat belt and then rear-end another car. Your head is
moving and then stops suddenly as it hits the windshield. Brain
damage is caused not only by localized injury at the site of the
blow (the
coup
injury), but also by the ricocheting effect as the
brain hits the opposite end of the skull (the
contrecoup
injury).
A
concussion
is an alteration in brain function, usually tem-
porary, following a blow to the head. Te victim may be dizzy
or lose consciousness. Although typically mild and short-lived,
even a seemingly mild concussion can be damaging, and multi-
ple concussions over time produce cumulative damage.
More serious concussions can bruise the brain and cause per-
manent neurological damage, a condition called a
contusion
. In
cortical contusions, the individual may remain conscious. Severe
Hydrocephalus enlarges the head of a newborn baby because
the skull bones have not yet fused
(Figure 12.25)
. In adults,
however, the skull is rigid and hard, and hydrocephalus is likely
to damage the brain because accumulating fluid compresses
blood vessels and crushes the soF nervous tissue. Hydrocepha-
lus is treated by inserting a shunt into the ventricles to drain
excess fluid into the abdominal cavity.
Blood Brain Barrier
Te
blood brain barrier
is a protective mechanism that helps
maintain a stable environment for the brain. No other body tissue
is so absolutely dependent on a constant internal environment as is
the brain. In other body regions, the extracellular concentrations of
hormones, amino acids, and ions are in constant flux, particularly
aFer eating or exercise. If the brain were exposed to such chemi-
cal variations, its neurons would fire uncontrollably, because some
hormones and amino acids serve as neurotransmitters and certain
ions (particularly K
1
) modify the threshold for neuronal firing.
Bloodborne substances in the brain’s capillaries must pass
through three layers before they reach the neurons: (1) the en-
dothelium of the capillary wall, (2) a relatively thick basal lam-
ina surrounding the external face of each capillary, and (3) the
bulbous “feet” of the astrocytes clinging to the capillaries.
Which of these layers constitutes the blood brain barrier? As
you might expect, the astrocyte “feet” play a role but they are
not themselves the barrier. Instead, they supply required signals
to the endothelial cells, causing them to make
tight junctions
.
Tese tight junctions seamlessly join together the endothelial
cells, forming the blood brain barrier and making these the least
permeable capillaries in the body.
Te blood brain barrier is selective, not absolute. Nutrients
such as glucose, essential amino acids, and some electrolytes
Figure 12.25
Hydrocephalus in a newborn.
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