Human Physiology, 14th edition (2016)

The Autonomic Nervous System 247

and an inner medulla. These two parts are really two function-

ally different glands with different embryonic origins, different

hormones, and different regulatory mechanisms. The adrenal

cortex secretes steroid hormones; the adrenal medulla secretes

the hormone epinephrine (adrenaline) and, to a lesser degree,

norepinephrine, when it is stimulated by the sympathetic system.

The adrenal medulla can be likened to a modified sympa-

thetic ganglion; its cells are derived from the same embryonic

tissue (the neural crest, chapter 8) that forms postganglionic

sympathetic neurons. Like a sympathetic ganglion, the cells

of the adrenal medulla are innervated by preganglionic sym-

pathetic fibers ( fig.  9.5 ). The adrenal medulla secretes epi-

nephrine into the blood in response to this neural stimulation.

The effects of epinephrine are complementary to those of the

neurotransmitter norepinephrine, which is released from post-

ganglionic sympathetic nerve endings. For this reason, and

because the adrenal medulla is stimulated as part of the mass

activation of the sympathetic system, the two are often grouped

together as a single sympathoadrenal system.

Parasympathetic Division

The parasympathetic division is also known as the craniosacral

division of the autonomic system. This is because its preganglionic

fibers originate in the brain (specifically, in the midbrain, pons,

and medulla oblongata) and in the second through fourth sacral

levels of the spinal column. These preganglionic parasympathetic

to the ganglia and the convergence of impulses within the gan-
glia can result in the mass activation of almost all of the post-
ganglionic sympathetic neurons. This mass activation allows
the entire sympathetic division to be tonically (constantly)
active to a certain degree and to increase its activity in response
to “fight-or-flight” situations (section 9.3). However, mass
activation does not always occur. In response to particular vis-
ceral stimuli (such as changes in blood pressure, blood vol-
ume, and plasma osmolality), the CNS can direct appropriate
increases or decreases in the activity of postganglionic sympa-
thetic axons to the heart and kidneys that allows these organs to
compensate for the changes and maintain homeostasis.

Collateral Ganglia

Many preganglionic fibers that exit the spinal cord below the
level of the diaphragm pass through the sympathetic chain of
ganglia without synapsing. Beyond the sympathetic chain,
these preganglionic fibers form splanchnic nerves. Pregan-
glionic fibers in the splanchnic nerves synapse in collateral,
or prevertebral, ganglia. These include the celiac, superior
mesenteric, and inferior mesenteric ganglia ( fig. 9.4 ). Postgan-
glionic fibers that arise from the collateral ganglia innervate
organs of the digestive, urinary, and reproductive systems.

Adrenal Glands

The paired adrenal glands are located above each kidney
( fig. 9.4 ). Each adrenal is composed of two parts: an outer cortex

Figure 9.3 The pathway of sympathetic neurons. The preganglionic neurons enter the sympathetic chain of ganglia on the white
ramus (one of the two rami communicantes). Some synapse there, and the postganglionic axon leaves on the gray ramus to rejoin a spinal
nerve. Others pass through the ganglia without synapsing. These ultimately synapse in a collateral ganglion, such as the celiac ganglion.

Preganglionic neuron

Postganglionic neuron

Dorsal

root

Dorsal root

ganglion

Visceral effectors:

Smooth muscle of

blood vessels, arrector

pili muscles, and

sweat glands

Sympathetic

chain

ganglion

Sympathetic chain

Splanchnic

nerve

Visceral effector:

intestine

Collateral

ganglion

(celiac ganglion)

Gray ramus

White

ramus

Spinal

nerve

Spinal cord

Ventral

root

1. Preganglionic axons
   synapse with
   postganglionic
   neurons
   2. Postganglionic
   axons innervate
   target organs