The Autonomic Nervous System
Location of their ganglia.
Most parasympathetic ganglia are
located in the visceral eﬀector organs. Sympathetic ganglia
lie close to the spinal cord.
illustrates these and other key diﬀerences, which
are summarized in
We begin our detailed exploration of the ANS with the ana-
tomically simpler parasympathetic division.
Parasympathetic (Craniosacral) Division
Te parasympathetic division is also called the
because its preganglionic ﬁbers spring from opposite
ends of the CNS—the brain stem and the sacral region of the
. Te preganglionic axons extend
from the CNS nearly all the way to the structures they inner-
vate. Tere the axons synapse with postganglionic neurons lo-
that lie close to or within the target
organs. Very short postganglionic axons issue from the terminal
ganglia and synapse with eﬀector cells in their immediate area.
Dilates the bronchioles in the lungs, increasing ventilation
(and thus increasing oxygen delivery to body cells)
Causes the liver to release more glucose into the blood to ac-
commodate the increased energy needs of body cells
At the same time, the sympathetic division temporarily
damps nonessential activities, such as gastrointestinal tract mo-
tility. If you are running from a mugger, digesting lunch can
wait! It is far more important to give your muscles everything
they need to get you out of danger. In such active situations,
the sympathetic division generates a head of steam that enables
the body to cope with situations that threaten homeostasis. It
provides the optimal conditions for an appropriate response to
some threat, whether that response is to run, see distant objects
better, or think more clearly.
We have just looked at two extreme situations in which one
or the other branch of the ANS dominates. Tink of the para-
sympathetic division as the
division [digestion, defecation,
and diuresis (urination)], and the sympathetic division as the
division (exercise, excitement, emergency, embarrassment).
±able 14.4 (p. 536) presents a more detailed summary of how
each division aﬀects various organs.
Remember, however, that the two ANS divisions rarely work
in an all-or-none fashion as described above. A dynamic antag-
onism exists between the divisions, and both make continuous
ﬁne adjustments to maintain homeostasis.
Check Your Understanding
Name the three types of effectors of the autonomic nervous
Which relays instructions from the CNS to muscles more
quickly, the somatic nervous system or the ANS? Explain why.
Which branch of the ANS would predominate if you were
lying on the beach enjoying the sun and the sound of the
waves? Which branch would predominate if you were on a
surfboard and a shark appeared within a few feet of you?
For answers, see Appendix H.
For the parasympathetic and sympathetic divisions, describe
the site of CNS origin, locations of ganglia, and general
Anatomically, the sympathetic and parasympathetic divisions
Sites of origin.
Parasympathetic ﬁbers are craniosacral—
they originate in the brain (cranium) and sacral spinal cord.
Sympathetic ﬁbers are thoracolumbar—they originate in the
thoracic and lumbar regions of the spinal cord.
Relative lengths of their ﬁbers.
Te parasympathetic divi-
sion has long preganglionic and short postganglionic ﬁbers.
Te sympathetic division has the opposite condition—the
preganglionic ﬁbers are short and the postganglionic ﬁbers
The subdivisions of the ANS.
and sympathetic divisions differ anatomically in the (1) sites where
their nerves originate, (2) relative lengths of their preganglionic and
postganglionic ﬁbers, and (3) locations of their ganglia (indicated
here by synapse sites).
*Although sympathetic innervation to the skin is mapped to the cervical
region here, all nerves to the periphery carry postganglionic sympathetic