424
UNIT THREE
B
irth injuries, dislocations, vertebral frac-
tures, stabs, gunshot wounds, and pres-
sure from tumors can all injure spinal
nerves. Suddenly bending the neck, called whip-
lash, can compress the nerves of the cervical plex-
uses, causing persistent headache and pain in the
neck and skin, which the cervical nerves supply.
If a broken or dislocated vertebra severs or dam-
ages the phrenic nerves associated with the cer-
vical plexuses, partial or complete paralysis of the
diaphragm may result.
Intermittent or constant pain in the neck,
shoulder, or upper limb may result from pro-
longed abduction of the upper limb, as in paint-
ing or typing. This is due to too much pressure
on the brachial plexus. Called
thoracic outlet syn-
drome,
this condition may also result from a con-
genital skeletal malformation that compresses
the plexus during upper limb and shoulder
movements.
Degenerative changes may compress an
intervertebral disc in the lumbar region, produc-
ing
sciatica,
which causes pain in the lower back
and gluteal region that can radiate to the thigh,
calf, ankle, and foot. Sciatica is most common in
middle-aged people, particularly distance run-
ners. It usually compresses spinal nerve roots
between L2 and S1, some of which contain F
bers
of the sciatic nerve. Rest, drugs, or surgery are
used to treat sciatica.
In
carpal tunnel syndrome,
repeated hand
movements, such as typing, or weeding, in±
ame
the tendons that pass through the carpal tunnel,
a space between bones in the wrist. The swol-
len tendons compress the median nerve in the
wrist, sending pain up the upper limb. Surgery or
avoiding repetitive hand movements can relieve
symptoms.
11.7
CLINICAL APPLICATION
Spinal Nerve Injuries
one set of F
bers may activate an organ, and impulses on the
other set inhibit it. Thus, the divisions may function antag-
onistically, regulating the actions of some organs by alter-
nately activating or inhibiting them.
The functions of the autonomic divisions are varied;
each activates some organs and inhibits others. This reveals
that the divisions have important functional differences. The
sympathetic division primarily prepares the body for energy-
expending, stressful, or emergency situations. Conversely,
the parasympathetic division is most active under ordinary,
restful conditions. It also counterbalances the effects of
the sympathetic division and restores the body to a resting
state following a stressful experience. ±or example, during
an emergency, the sympathetic division increases heart and
breathing rates; following the emergency, the parasympa-
thetic division decreases these activities.
Autonomic Nerve Fibers
The neurons of the autonomic nervous system are efferent,
or motor, neurons. In the motor pathways of the somatic
nervous system, a single neuron typically links the CNS and
a skeletal muscle. In the autonomic system, motor pathways
include two neurons, as
f gure 11.35
shows. The cell body
of one neuron is in the brain or spinal cord. Its axon, the
preganglionic
(pre
gang-gle-on
ik)
fiber,
leaves the CNS
and synapses with one or more neurons whose cell bodies
are within an autonomic ganglion. The axon of such a sec-
ond neuron is called a
postganglionic
(po
ˉst
gang-gle-on
ik)
±
ber,
and it extends to a visceral effector.
Sympathetic Division
In the sympathetic division (thoracolumbar division), the
preganglionic fibers originate from neurons in the lateral
11.7
AUTONOMIC NERVOUS
SYSTEM
The autonomic nervous system is the part of the PNS that
functions independently (autonomously) and continuously,
without conscious effort. This system controls visceral activ-
ities by regulating the actions of smooth muscles, cardiac
muscles, and various glands. It oversees heart rate, blood
pressure, breathing rate, body temperature, and other vis-
ceral activities that aid in maintaining homeostasis. Portions
of the autonomic nervous system also respond during
times of emotional stress and prepare the body to meet the
demands of strenuous physical activity.
General Characteristics
Refl
exes in which sensory signals originate from receptors
in the viscera and the skin regulate autonomic activities.
Afferent nerve fibers transmit these signals to nerve cen-
ters in the brain or spinal cord. In response, motor impulses
leave these centers on efferent nerve F
bers in cranial and
spinal nerves. Typically, these efferent F bers lead to ganglia
outside the CNS. The impulses they carry are integrated in
the ganglia and are relayed to various organs (muscles or
glands) that respond by contracting, secreting, or being
inhibited. The integrative function of the ganglia provides
the autonomic nervous system with some degree of indepen-
dence from the brain and spinal cord, and the visceral effer-
ent nerve F bers associated with these ganglia comprise the
autonomic nervous system.
The autonomic nervous system includes two interact-
ing divisions, called the
sympathetic
(sim
pah-thet
ik) and
parasympathetic
(par
ah-sim
pah-thet
ik)
divisions.
Many
organs have nerve F bers from each division. Impulses on
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