426
UNIT THREE
pons, and medulla oblongata of the brainstem and from part
of the sacral region (S2–4) of the spinal cord
(f g. 11.39)
.
From there, they lead outward on cranial or sacral nerves
to ganglia near or in various organs (
terminal ganglia
).
The short postganglionic ±
bers continue from the ganglia
to speci± c muscles or glands in these organs
(f g. 11.40)
.
Parasympathetic preganglionic axons are usually myeli-
nated, and the parasympathetic postganglionic fibers are
unmyelinated.
The parasympathetic preganglionic fibers associated
with parts of the head are included in the oculomotor,
facial, and glossopharyngeal nerves. Those ± bers that inner-
vate organs of the thorax and upper abdomen are parts of
the vagus nerves. (The vagus nerves carry about 75% of all
parasympathetic ±
bers.) Preganglionic ±
bers arising from the
sacral region of the spinal cord lie in the branches of the sec-
ond through the fourth sacral spinal nerves, and they carry
impulses to viscera in the pelvic cavity (±
g. 11.39).
PRACTICE
41
What is the general function of the autonomic nervous system?
42
How are the divisions of the autonomic system distinguished?
43
Describe a sympathetic nerve pathway and a parasympathetic
nerve pathway.
collateral ganglia before they synapse. Typically, a pregan-
glionic axon will synapse with several other neurons in a
sympathetic ganglion (an example of divergence).
The axons of the second neurons in sympathetic path-
ways, the postganglionic ± bers, extend from the sympathetic
ganglia to visceral effectors. Those leaving paravertebral gan-
glia usually pass through branches called
gray rami
and return
to a spinal nerve before proceeding to an effector (± g. 11.37).
These branches appear gray because the postganglionic axons
generally are unmyelinated, whereas the preganglionic axons
in the white rami are nearly all myelinated.
An important exception to the usual arrangement of
sympathetic ±
bers is in a set of preganglionic ± bers that pass
through the sympathetic ganglia and extend to the medulla
of each adrenal gland. These ±
bers terminate in the glands
on special hormone-secreting cells that release
norepineph-
rine
(20%) and
epinephrine
(80%) when they are stimu-
lated. Chapter 13 (pp. 504–506) discusses the functions of
the adrenal medulla and its hormones.
Figure 11.38
shows
the sympathetic division.
Parasympathetic Division
The preganglionic fibers of the parasympathetic division
(craniosacral division) arise from neurons in the midbrain,
Spinal cord
Spinal
nerve
Visceral effector
(intestine)
To visceral effectors
(smooth muscle
of blood vessels,
arrector pili
muscles, and
sweat glands)
Ventral root
Dorsal root
Dorsal root
ganglion
Collateral
ganglion
Dorsal branch of
spinal nerve
Ventral branch of
spinal nerve
Sympathetic trunk
Paravertebral
sympathetic
ganglion
Gray
ramus
White
ramus
Posterior horn
Lateral horn
Anterior horn
Postganglionic
neuron
Preganglionic
neuron
FIGURE 11.37
±Sympathetic±F
bers leave the spinal cord in the ventral roots of spinal nerves, enter paravertebral ganglia, and synapse with other
neurons that extend to visceral e²
ectors.
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