THE BRAIN
The brainconsists of many parts that function as an
integrated whole. The major parts are the medulla,
pons, and midbrain (collectively called the brain
stem), the cerebellum, the hypothalamus, the thala-
mus, and the cerebrum. These parts are shown in Fig.
8–6. We will discuss each part separately, but keep in
mind that they are all interconnected and work
together.
VENTRICLES
The ventriclesare four cavities within the brain: two
lateral ventricles, the third ventricle, and the fourth
ventricle (Fig. 8–7). Each ventricle contains a capillary
network called a choroid plexus, which forms cere-
brospinal fluid (CSF) from blood plasma. Cere-
brospinal fluid is the tissue fluid of the central nervous
system; its circulation and functions will be discussed
in the section on meninges.
MEDULLA
The medullaextends from the spinal cord to the pons
and is anterior to the cerebellum. Its functions are
those we think of as vital (as in “vital signs”). The
medulla contains cardiac centers that regulate heart
rate, vasomotor centers that regulate the diameter of
blood vessels and, thereby, blood pressure, and respi-
ratory centers that regulate breathing. You can see
why a crushing injury to the occipital bone may be
rapidly fatal—we cannot survive without the medulla.
Also in the medulla are reflex centers for coughing,
sneezing, swallowing, and vomiting.PONS
The ponsbulges anteriorly from the upper part of the
medulla. Within the pons are two respiratory centers
that work with those in the medulla to produce a nor-
mal breathing rhythm. (The function of all the respi-
ratory centers is discussed in Chapter 15.) The many
other neurons in the pons (ponsis from the Latin for
“bridge”) connect the medulla with other parts of the
brain.MIDBRAIN
The midbrainextends from the pons to the hypothal-
amus and encloses the cerebral aqueduct, a tunnel
that connects the third and fourth ventricles. Several176 The Nervous System
BOX8–3 SPINAL CORD INJURIES
need to urinate or defecate. Nor will voluntary con-
trol of these reflexes be possible, because inhibiting
impulses from the brain can no longer reach the
lower segments of the spinal cord.
Potentially less serious injuries are those in which
the spinal cord is crushed rather than severed, and
treatment is aimed at preserving whatever function
remains. Minimizing inflammation and stimulating
the production of nerve growth factors are aspects
of such treatment.
Perhaps the most challenging research is the
attempt to stimulate severed spinal cords to regen-
erate. Partial success has been achieved in rats
and mice, with Schwann cells transplanted from
their peripheral nerves and nerve growth factors
produced by genetically engineered cells. The use
of stem cells has also been successful in rats. The
researchers caution, however, that it will take some
time before their procedures will be tested on
people.Injuries to the spinal cord are most often caused by
auto accidents, falls, and gunshot wounds. The
most serious injury is transection, or severing, of the
spinal cord. If, for example, the spinal cord is sev-
ered at the level of the 8th thoracic segment, there
will be paralysis and loss of sensation below that
level. Another consequence is spinal shock, the at-
least-temporary loss of spinal cord reflexes. In this
example, the spinal cord reflexes of the lower trunk
and legs will not occur. The stretch reflexes and
flexor reflexes of the legs will be at least temporar-
ily abolished, as will the urination and defecation
reflexes. Although these reflexes do not depend
directly on the brain, spinal cord neurons depend
on impulses from the brain to enhance their own
ability to generate impulses.
As spinal cord neurons below the injury recover
their ability to generate impulses, these reflexes,
such as the patellar reflex, often return. Urination
and defecation reflexes may also be reestablished,
but the person will not have an awareness of the