94 Neuroanatomy: Draw It to Know It
Autonomic Fiber Arrangements
Here, we will draw the motor fi ber arrangements for the
parasympathetic and sympathetic divisions of the auto-
nomic nervous system. First, indicate that the parasym-
pathetic nervous system brings the body into a rest state
whereas the sympathetic nervous system is activated in
states of physical and psychological stress: it produces
the so-called “fi ght-or-fl ight” response. Next, draw rep-
resentative preganglionic neurons for both the parasym-
pathetic and sympathetic nervous systems. Indicate that
the visceral neurons of the parasympathetic nervous
system lie within cranial nerve nuclei and the sacral
intermediolateral cell column of the spinal cord, from S2
to S4, and that the visceral neurons of the sympathetic
nervous system lie within the thoracolumbar interme-
diolateral cell column, from T1 to L2. Next, in the para-
sympathetic arrangement show a long preganglionic
axon synapse on a ganglion within its eff ector tissue: cra-
nial and sacral parasympathetic ganglia lie either very
close to or within the wall of their target organ. Th en,
show a postganglionic parasympathetic fi ber project
deep into the target organ.
Now, let’s draw the axons of the sympathetic nervous
system. Show a preganglionic sympathetic axon synapse
on a nearby peripheral ganglion. Other than the pregan-
glionic sympathetic fi bers that travel to the adrenal
gland, sympathetic preganglionic axons are short and
synapse close to their site of origin: either in the paraver-
tebral chain or one of the prevertebral ganglia. Next,
show a postganglionic sympathetic fi ber travel a long
distance to its target organ and also to the body walls
and limbs, where the sympathetic nervous system inner-
vates sweat glands, hair fi bers, and blood vessels of skel-
etal muscle and skin.
Now, let’s label the relevant neurotransmitters
involved in these autonomic fi ber pathways. As a class,
neurotransmitters are small molecules with transient
eff ects. Indicate that both the preganglionic parasympa-
thetic and sympathetic neurons release acetylcholine.
Th en, indicate that all postganglionic parasympathetic
fi bers release acetylcholine and also that most post-
ganglionic sympathetic fi bers release norepinephrine
(noradrenaline). Th e exceptions are the postganglionic
sympathetic fi bers to sweat glands, which release acetyl-
choline, and the adrenal medullary cells, which mostly
release epinephrine (adrenaline).^1
In addition to the neurotransmitters, neuropeptides
also exist within autonomic neurons. In comparison to
the neurotransmitters, neuropeptides are generally pack-
aged into larger vesicles and have more wide-reaching
and long-lasting eff ects. Th e neuropeptides are organized
into many diff erent classes, such as the calcitonin family,
hypothalamic hormones, hypothalamic- releasing and
-inhibiting hormones, neuropeptide Y family, opioid
peptides, pituitary hormones, tachykinins, VIP-glucagon
family, and additional peptides that do not fall into any
of these categories.^2 , 3
Lastly, consider that the digestive tract also contains
its own autonomic system, called the enteric nervous
system. Th e enteric nervous system comprises numerous
neurons distributed in myenteric and submucosal plex-
uses. Th e enteric nervous system and the pacemaker
cells of the digestive system wall (the interstitial cells of
Cajal) generate and propagate patterns of depolarization
that result in waves of peristaltic muscle contraction.
Food ingestion triggers the peristaltic refl ex, which pro-
pels food through the digestive tract, and the enteric
neural circuits adjust intestinal blood fl ow and secreto-
motor activity for absorption. Notably, psychopharma-
cologic drugs oft en aff ect the neurotransmitters and
neuromodulators of the enteric nervous system. For
instance, acetylcholine is an important peristaltic pro-
moter, so cholinesterase inhibitors, which increase circu-
lating levels of acetylcholine, promote gastrointestinal
activity and can result in diarrhea. In contrast, tricyclic
antidepressants contain anticholinergic properties and,
as a result, can cause constipation.^4 – 8