Neuronal Signaling and the Structure of the Nervous System
177
viscera, and the parts of the brain involved in control of move-
ment. Although the cerebellum’s function is almost exclusively
motor, it is implicated in some forms of learning.
Brainstem
All the nerve fi bers that relay signals between the forebrain,
cerebellum, and spinal cord pass through the brainstem.
Running through the core of the brainstem and consisting of
loosely arranged neuron cell bodies intermingled with bundles
of axons is the
reticular formation,
the one part of the brain
absolutely essential for life. It receives and integrates input from
all regions of the central nervous system and processes a great
deal of neural information. The reticular formation is involved
in motor functions, cardiovascular and respiratory control, and
the mechanisms that regulate sleep and wakefulness and that
focus attention. Most of the biogenic amine neurotransmitters
are released from the axons of cells in the reticular formation
and, because of the far-reaching projections of these cells, these
neurotransmitters affect all levels of the nervous system.
Some reticular formation neurons send axons for con-
siderable distances up or down the brainstem and beyond,
to most regions of the brain and spinal cord. This pattern
explains the very large scope of infl uence that the reticular
formation has over other parts of the central nervous system
and explains the widespread effects of the biogenic amines.
The pathways that convey information from the reticular
formation to the upper portions of the brain stimulate arousal
and wakefulness. They also direct attention to specifi c events by
selectively stimulating neurons in some areas of the brain while
inhibiting others. The fi
bers that descend from the reticular for-
mation to the spinal cord infl uence activity in both efferent and
afferent neurons. Considerable interaction takes place between
the reticular pathways that go up to the forebrain, down to the
spinal cord, and to the cerebellum. For example, all three com-
ponents function in controlling muscle activity.
The reticular formation encompasses a large portion of
the brainstem, and many areas within the reticular formation
serve distinct functions. For example, some reticular forma-
tion neurons are clustered together, forming brainstem nuclei
and integrating centers. These include the cardiovascular,
respiratory, swallowing, and
vomiting centers, all of which we
will discuss in later chapters. The reticular formation also has
nuclei important in eye-movement control and the refl ex ori-
entation of the body in space.
In addition, the brainstem contains nuclei involved
in processing information for 10 of the 12 pairs of
cranial
nerves.
These are the peripheral nerves that connect directly
with the brain and innervate the muscles, glands, and sensory
receptors of the head, as well as many organs in the thoracic
and abdominal cavities.
Central Nervous System:
Spinal Cord
The spinal cord lies within the bony vertebral column (
Figure
6–41
). It is a slender cylinder of soft tissue about as big around
as the little fi nger. The central butterfl y-shaped area (in cross
section) of gray matter is composed of interneurons, the cell
Ventral
horn
Gray matter
Dorsal
root
ganglion
Dorsal
root
Ventral
root
Spinal cord
Spinal
nerve
White matter
Dorsal
horn
Vertebra
Figure 6–41
Section of the spinal cord, ventral view. The arrows indicate the
direction of transmission of neural activity.
bodies and dendrites of efferent neurons, the entering axons
of afferent neurons, and glial cells. The regions of gray matter
projecting toward the back of the body are called the
dorsal
horns,
whereas those oriented toward the front are the
ven-
tral horns.
The gray matter is surrounded by white matter, which
consists of groups of myelinated axons. These groups of fi ber
tracts run longitudinally through the cord, some descending
to relay information
from
the brain to the spinal cord, oth-
ers ascending to transmit information
to
the brain. Pathways
also transmit information between different levels of the spi-
nal cord.
Groups of afferent fi bers that enter the spinal cord from
the peripheral nerves enter on the dorsal side of the cord via
the
dorsal roots
(see Figure 6–41). Small bumps on the dor-
sal roots, the
dorsal root ganglia,
contain the cell bodies of
these afferent neurons. The axons of efferent neurons leave the
spinal cord on the ventral side via the
ventral roots.
A short
distance from the cord, the dorsal and ventral roots from the
same level combine to form a
spinal nerve,
one on each side
of the spinal cord.
Peripheral Nervous System
Neurons in the peripheral nervous system transmit signals
between the central nervous system and receptors and effec-
tors in all other parts of the body. As noted earlier, the axons
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