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Nervous System
sends a signal via an efferent motor pathway, signaling
an effector response. A signal is also sent to the brain,
alerting it to the sensation and response. This happens
so quickly that people may experience the sensation
of pain almost simultaneously with the development
of the reflex, even though the brain is aware of the
situation.
The
withdrawal reflex
is initiated by a painful (noci-
ceptive) stimulus and quickly moves the body part
away from the offending stimulus, usually by flexing
a limb part. The withdrawal reflex is powerful, taking
precedence over other reflexes associated with locomo-
tion. All the joints of an extremity (e.g., finger, wrist,
elbow, shoulder) typically are involved. This complex,
polysynaptic reflex also shifts postural support to the
opposite side of the body with a crossed extensor reflex
and simultaneously alerts the forebrain to the offending
stimulus event. The withdrawal reflex also can produce
contraction of muscles other than the extremities. For
example, irritation of the abdominal viscera may cause
contraction of the abdominal muscles.
The Brain
The brain is divided into three regions, the hindbrain,
the midbrain, and the forebrain (Fig. 34-14). The hind-
brain includes the medulla oblongata, the pons, and its
dorsal outgrowth, the cerebellum. Midbrain structures
include two pairs of dorsal enlargements, the superior
and inferior colliculi (not shown). The forebrain, which
consists of two hemispheres covered by the cerebral cor-
tex, contains central masses of gray matter, the basal
ganglia (discussed in Chapter 36), and the rostral end of
the neural tube, the diencephalon with its adult deriva-
tives—the thalamus and hypothalamus. The olfactory
and optic nerves are also considered part of the fore-
brain. In contrast, the motor and sensory nuclei of the
3rd to 12th cranial nerves are located in the midbrain
and hindbrain.
An important concept is that the more rostral or
recently developed parts of the neural tube gain domi-
nance or control over regions and functions at lower
levels. They do not replace the earlier developed cir-
cuitry but merely dominate it. After damage to the more
vulnerable parts of the forebrain, as occurs with severe
brain injury, a brain stem–controlled organism remains
that is capable of breathing and may survive if the envi-
ronmental temperature is regulated and nutrition and
other aspects of care are provided. However, all aspects
of intellectual function, experience, perception, and
memory usually are permanently lost.
Hindbrain
During embryonic development, the hindbrain becomes
subdivided into the cerebellum, the medulla oblongata—
also referred to as the
medulla
—and the pons. The
CSF-filled tube becomes the fourth ventricle, which is
continuous with the cerebral aqueduct of the midbrain
(see Fig. 34-14).
The cerebellum, the “little brain,” is an important
movement control center (see Chapter 36). It receives
massive axonal inputs from the spinal cord and the
pons. Neurons in the spinal cord provide information
about the body’s position in space, and those in the pons
relay information from the cerebral cortex specifying
the goals of the intended movements.
Brain Stem.
The brain stem is the region of the brain that
connects the cerebrum with the spinal cord. It consists of
themidbrain, medulla oblongata, and the pons. In the brain
stem, the reticular formation has been greatly expanded.
Certain groups of neurons termed
vital centers
are located
in the reticular formation of the pons and medulla. These
centers include the respiratory, cardiovascular, and vaso-
motor centers. Still other centers in the brain stem are con-
cerned with swallowing, intestinal movements, control of
micturition, and a variety of more specific functions such
as salivation and pupillary diameter.
Midbrain
Pituitary gland
Pons
Spinal cord
Central canal
Cerebellum
Cerebral
aqueduct
Pineal body
Occipital lobe
Third ventricle
Fourth
ventricle
Interventricular
foramen
Frontal lobe
Corpus callosum
Anterior commissure
Medulla
oblongata
Forebrain
Hindbrain
FIGURE 34-14.
Midsagittal section
showing structures of the hindbrain,
midbrain, and forebrain.
