HUMAN BIOLOGY

the nerVOus system 253

What are the main functions of the

hindbrain, midbrain, and forebrain?

• In the hindbrain and midbrain, various centers control reflexes
  related to body functions (such as coughing and vomiting). In
  the forebrain, the cerebrum handles overall processing and
  integration of sensory information and motor responses.


• Cerebrospinal fluid fills cavities and canals in the brain and
  spinal cord to provide a protective cushion.


• The blood–brain barrier prevents many potentially harmful
  substances in blood from entering the CSF.

taKe-hoMe Message

blood–brain barrier Fea-
tures of brain capillaries that
prevent many substances
from passing from the blood
into cerebrospinal fluid.
brain The master con-
trol center of the nervous
system.
brain stem Brain region
made up of the pons,
medulla, and midbrain.
cerebellum The hindbrain
region that coordinates vol-
untary movements.
cerebral hemispheres The
two halves of the cerebrum.
cerebrospinal fluid Fluid
that fills spaces in the
brain and the spinal cord
and cushions them against
physical shocks; the CSF.
cerebrum The forebrain’s
main center for processing
information.
hypothalamus Part of the
brain that controls homeo-
static adjustments in the
functions of internal organs.
medulla oblongata The
hindbrain center that
controls reflexes such as
swallowing, sneezing, and
vomiting.
meninges The set of
three membranes that cover
and help protect the brain.
pons Hindbrain center that
helps regulate breathing.
thalamus Forebrain
region that relays sensory
is chemically similar to it. It is secreted from specialized information.
capillaries inside the ventricles and canals in the brain.
The ventricles connect with each other and with the central
canal of the spinal cord, and are filled with cerebrospinal
fluid. The CSF also fills the space enclosed by the two inner
meninges (the pia matter and arachnoid). Because this
enclosed cerebrospinal fluid can’t be compressed, it helps
cushion the brain and spinal cord from jarring movements.
A blood–brain barrier helps control which blood-
borne substances enter the CSF. The barrier is set up by
the unusual structure of brain capillaries. Tight junctions
between the cells of the capillary walls (Section 4.6) make
the walls much less permeable than those of capillaries
elsewhere in the body. Specialized transport proteins in
the plasma membrane of wall cells allow glucose and a few
other needed substances to move out of the bloodstream
and into the CSF. Water crosses the barrier freely. So do
lipid-soluble molecules, including oxygen and carbon diox-
ide, which diffuse through the membrane’s lipid bilayer.
This is one reason why lipid-soluble substances such as
alcohol, nicotine, caffeine, and anesthetics can rapidly
affect brain function.

ventricles

pia mater

arachnoid mater three meninges

dura mater

spinal cord

scalp

skull bone

arachnoid

mater

pia mater dura

mater

cerebrospinal

fluid

cerebrospinal fluid in spinal canal

Figure 13.17 The three meninges help protect the brain and spinal cord. Cerebrospinal fluid fills the space
between the arachnoid and the pia mater. (© Cengage Learning)

The blood–brain barrier stops

viruses, bacteria, many toxins, and

hormones in blood from gaining

access to most neurons in the brain and spinal cord. The

barrier doesn’t protect the hypothalamus, which has a

central role in homeostasis. Instead, the hypothalamus is

directly exposed to the bloodstream and can monitor the

chemical makeup and temperature of blood.

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