Similarly, descending tracts pass through the spinal
cord in different ways. The axons of some motor neurons
cross the midline at the medulla, allowing the right hemi-
sphere of your cerebrum to control motor functions on the
left side of your body, and vice versa. The fibers of some
tracts do not cross over so some functions are controlled by
the same side of the brain. The descending tracts have a
two-neuron pathway that leads from the brain to the mus-
cle. The upper motor neuron conducts the message from
the brain to the lower motor neuron, which then relays
that message to the muscle.
CONCEPT CHECK
1.
Cerebrospinal
fluid flows between which layers
of the meninges?
2.
Where
is cerebrospinal fluid produced?
3.
What
cerebral lobe is primarily responsible for
processing visual information?
4.
Where
are the cell bodies of motor neurons
located?
5.
Which
spinal tracts contain the upper motor
neurons?
The Autonomic Nervous System Controls
the Activities of Smooth Muscle,
Cardiac Muscle, and Glands
LEARNING OBJECTIVES
1.
Outline
the parts of the autonomic nervous
3.
Describe
the functions of the sympathetic and
system.
parasympathetic divisions of the autonomic
2.
Compare
the main structures and functions of
nervous system.
the somatic and autonomic nervous systems.
W
hile crossing the street to go to your car after
lunch, a bus careens around the corner, barely
missing you. Your heart starts rac-
ing, you are breathing rapidly, you
begin to sweat, and your lunch feels like a lump in
your stomach. What is going on here? These and
many other involuntary functions are controlled
by your
autonomic nervous system (ANS)
.
nervous
neurons.
The ANS Uses a Two-Neuron
Pathway to Communicate
with the Effectors
The ANS is divided into two divisions, sympa-
thetic and parasympathetic. Like the somatic
autonomic ner-
vous system (ANS)
An automatic or
self-regulating system
of autonomic motor
neurons, both sympa-
thetic and parasym-
pathetic, that conduct
nerve impulses from
the central nervous sys-
tem to smooth muscle,
cardiac muscle, and
glands.
system (SNS), the ANS has sensory and motor
The sensory neurons are located in the walls of
organs and blood vessels. The ANS has mo-
tor neurons that regulate cardiac muscles,
smooth muscles, and glands. However, unlike
the somatic nervous system, the ANS has two
neurons that link the CNS to the peripheral
effector (Figure 7.14). The cell bodies for the
second motor neurons in ANS motor pathways
are located in ganglia. These ganglia are out-
side the spinal cord, either in the walls of the
organs or organized into chains. The chains
can run alongside the vertebral column or be
positioned near the large abdominal arteries
(see Table 7.1 on page 210).
208 CHAPTER 7
The Nervous System
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