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U N I T 1 0
Nervous System
and decreased activity results in vasodilatation caused
by decreased tone. In structures such as the sinoatrial
node and atrioventricular node of the heart, which are
innervated by both divisions of the ANS, one division
predominates in controlling tone. In this case, the toni-
cally active parasympathetic nervous system exerts a
constraining or braking effect on heart rate, and when
parasympathetic outflow is withdrawn, similar to
releasing a brake, the heart rate increases. The increase
in heart rate that occurs with vagal withdrawal can be
further augmented by sympathetic stimulation.
Autonomic Efferent Pathways
The outflow of both divisions of the ANS follows a two-
neuron efferent pathway (Fig. 34-22). The cell body of the
first motor neuron, called the
preganglionic neuron
, lies in
the brain stemor the spinal cord. The secondmotor neuron,
called the
postganglionic neuron
, synapses with a pregangli-
onic neuron in an autonomic ganglion located in the PNS.
The two divisions of the ANS differ in terms of location of
preganglionic cell bodies, relative length of preganglionic
fibers, general function, nature of peripheral responses,
and preganglionic and postganglionic neurotransmitters
and their postsynaptic receptors (Table 34-3).
Most visceral organs are innervated by both sympa-
thetic and parasympathetic fibers. Exceptions include
structures such as blood vessels and sweat glands that
have input from only sympathetic division of the ANS.
The fibers of the sympathetic nervous system are distrib-
uted to effectors throughout the body, and as a result,
sympathetic actions tend to be more diffuse than those
of the parasympathetic nervous system, in which there
is a more localized distribution of fibers. The pregangli-
onic fibers of the sympathetic nervous system may tra-
verse a considerable distance and pass through several
ganglia before synapsing with postganglionic neurons,
and their terminals make contact with a large num-
ber of postganglionic fibers. In some ganglia, the ratio
of preganglionic to postganglionic cells may be 1:20;
because of this, the effects of sympathetic stimulation are
diffuse. There is considerable overlap, and one ganglion
cell may be supplied by several preganglionic fibers. In
contrast to the sympathetic nervous system, the para-
sympathetic nervous system has its postganglionic neu-
rons located very near or in the organ of innervation.
Because the ratio of preganglionic to postganglionic
communication often is 1:1, the effects of the parasym-
pathetic nervous system are much more circumscribed.
Sympathetic Nervous System
The preganglionic neurons of the sympathetic nervous
system are located primarily in the thoracic and upper
lumbar segments (T1 to L2) of the spinal cord; thus, the
Parasympathetic
Sympathetic
T1
C5
III
VII
X
IX
C6
C7
C8
T2
T3
T4
T5
T6
T7
T8
T9
T10
T11
T12
L1
L2
L3
L4
L5
S1
S2
S3
S4
S5
Iris
Iris
Ciliary ganglion
Celiac
ganglion
Superior
mesenteric
ganglion
Inferior
mesenteric
ganglion
Paravertebral
chain of
sympathetic
ganglia
Sphenopalatine ganglion
Submandibular ganglion
Otic ganglion
Lacrimal gland
Lacrimal gland
Carotid
artery
Salivary
glands
Superior,
middle, and
inferior
cervical ganglia
Submandibular
and sublingual
glands
Parotid
gland
Heart
Lungs
Stomach
Pancreas
Small
bowel
Liver
Colon
Splenic flexure
of colon
Pelvic
splanchnic
nerve
Urinary
bladder
Gonad
Adrenal
gland
Small
bowel
Liver
Colon
Urinary
bladder
Gonad
Heart
Lungs
Stomach
Pancreas
FIGURE 34-22.
The autonomic nervous
system with the parasympathetic division
(craniosacral) indicated in red on the
right and the sympathetic division
(thoracolumbar) indicated in blue on
the left. (Adapted from DeMyer W.
Neuroanatomy, 2nd ed. Baltimore, MD:
Williams &Wilkins; 1998.)
