174
Chapter 6
few neurons connected in sequence. Because the long path-
ways contain few synapses, there are fewer opportunities for
alteration in the information they transmit.
The cell bodies of neurons with similar functions are
often clustered together. Groups of neuron cell bodies in the
peripheral nervous system are called
ganglia
(singular,
gan-
glion
). In the central nervous system, they are called
nuclei
(singular,
nucleus
), not to be confused with cell nuclei.
Central Nervous System: Brain
During development, the central nervous system forms from
a long tube. As the anterior part of the tube, which becomes
the brain, folds during its continuing formation, four different
regions become apparent. These regions become the four subdi-
visions of the brain: the
cerebrum, diencephalon,
brainstem,
and
cerebellum
(
Figure 6–38
). The cerebrum and diencepha-
lon together constitute the
forebrain.
The brainstem consists
of the
midbrain,
pons,
and
medulla oblongata.
The brain
also contains four interconnected cavities, the
cerebral ven-
tricles,
which are fi lled with fl
uid.
Overviews of the brain subdivisions are included here
and in
Table 6–7
, but details of their functions are given more
fully in Chapters 7, 8, and 10.
Forebrain
The larger component of the forebrain, the cerebrum, consists
of the right and left
cerebral hemispheres
as well as certain
other structures on the underside of the brain. The central
core of the forebrain is formed by the diencephalon.
The cerebral hemispheres (
Figure 6–39
) consist of the
cerebral cortex,
an outer shell of
gray matter
composed pri-
marily of cell bodies that give the area a gray appearance, and an
inner layer of
white matter,
composed primarily of myelinated
fi ber tracts. This in turn overlies cell clusters, which are also gray
matter and are collectively termed the
subcortical nuclei.
The
fi ber tracts consist of the many nerve fi
bers that bring informa-
tion into the cerebrum, carry information out, and connect dif-
ferent areas within a hemisphere. The cortex layers of the left
and right cerebral hemispheres, although largely separated by a
deep longitudinal division, are connected by a massive bundle
of nerve fi bers known as the
corpus callosum.
The cortex of each cerebral hemisphere is divided into
four
lobes:
the
frontal,
parietal,
occipital,
and
temporal.
Although it averages only 3 mm in thickness, the cortex is
highly folded. This results in an area containing cortical neu-
rons that is four times larger than it would be if unfolded, yet
does not appreciably increase the volume of the brain. This
folding also results in the characteristic external appearance
of the human cerebrum, with its sinuous ridges called
gyri
(singular,
gyrus
) separated by grooves called
sulci
(singular,
sulcus
). The cells of the cerebral cortex are organized in six
layers. The cortical neurons are of two basic types: pyramidal
cells (named for the shape of their cell bodies) and nonpyra-
midal cells. The
pyramidal cells
form the major output cells
of the cortex, sending their axons to other parts of the cortex
and to other parts of the central nervous system.
The cerebral cortex is the most complex integrating area
of the nervous system. In the cerebral cortex, basic afferent
information is collected and processed into meaningful per-
ceptual images, and control over the systems that govern the
movement of the skeletal muscles is refi ned. Nerve fi bers enter
the cortex predominantly from the diencephalon, specifi cally
from a region known as the thalamus as well as from other
regions of the cortex and areas of the brainstem. Some of the
Table 6–7
Summary of Functions of the Major
Parts of the Brain
I
.
FOREBRAIN
A. Cerebral hemispheres
1. Contain the cerebral cortex, which participates in
perception (Chapter 7), the generation of skilled
movements (Chapter 10), reasoning, learning, and
memory (Chapter 8)
2. Contain subcortical nuclei, including those that
participate in coordination of skeletal muscle
activity (Chapter 10)
3. Contain interconnecting fi ber pathways
B. Thalamus
1. Acts as a synaptic relay station for sensory
pathways on their way to the cerebral cortex
(Chapter 7)
2. Participates in control of skeletal muscle
coordination (Chapter 10)
3. Plays a key role in awareness (Chapter 8)
C. Hypothalamus
1. Regulates anterior pituitary gland function
(Chapter 11)
2. Regulates water balance (Chapter 14)
3. Participates in regulation of autonomic nervous
system (Chapters 6 and 16)
4. Regulates eating and drinking behavior (Chapter
16)
5. Regulates reproductive system (Chapters 11 and 17)
6. Reinforces certain behaviors (Chapter 8)
7. Generates and regulates circadian rhythms
(Chapters 1, 7, 11, and 16)
8. Regulates body temperature (Chapter 16)
9. Participates in generation of emotional behavior
(Chapter 8)
D. Limbic system
1. Participates in generation of emotions and
emotional behavior (Chapter 8)
2. Plays essential role in most kinds of learning
(Chapter 8)
II
.
CEREBELLUM
A. Coordinates movements, including those for posture and
balance (Chapter 10)
B. Participates in some forms of learning (Chapter 8)
III
.
BRAINSTEM
A. Contains all the fi bers passing between the spinal cord,
forebrain, and cerebellum
B. Contains the reticular formation and its various
integrating centers, including those for cardiovascular
and respiratory activity (Chapters 12 and 13)
C. Contains nuclei for cranial nerves III through XII
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