The Autonomic Nervous System A Comprehensive Exploration of its Role in Regulating Involuntary Body Functions and the Complex Interplay between Sympathetic and Parasympathetic Divisions
The Autonomic Nervous System
and Homeostasis
As you learned in Chapter 12, the motor (eff erent) division of the
peripheral nervous system (PNS) is divided into a somatic nervous
system (SNS) and autonomic nervous system (ANS). The ANS
usually operates without conscious control. However, centers in
the hypothalamus and brainstem do regulate ANS reflexes. In this
chapter, we compare structural and functional features of the somatic
and autonomic nervous systems. Then we discuss the anatomy of the
motor portion of the ANS and compare the organization and actions of
its two major parts, the sympathetic and parasympathetic divisions.
Q Did you ever wonder how some blood pressure medications
exert their effects through the autonomic nervous system ?
Comparison of Somatic
and Autonomic Nervous
Systems
OBJECTIVE
• Compare the structural and functional diff erences between the
somatic and autonomic parts of the nervous system.
Somatic Nervous System
The somatic nervous system consists of somatic motor neurons that
innervate the skeletal muscles of the body. When a somatic motor neu-
ron stimulates a skeletal muscle, it contracts; the eff ect is always exci-
tation. If somatic motor neurons cease to stimulate a skeletal muscle,
the result is a paralyzed, limp muscle that has no muscle tone.
The somatic nervous system usually operates under voluntary
(conscious) control. Voluntary control of movement involves motor
areas of the cerebral cortex that activate somatic motor neurons
whenever you have a desire to move. For example, if you want to per-
form a particular movement (kick a ball, turn a screwdriver, smile for
a picture, etc.), neural pathways from the primary motor area of the
cerebral cortex activate somatic motor neurons that cause the appro-
priate skeletal muscles to contract. The somatic nervous system is not
always under voluntary control, however. The somatic motor neurons
that innervate skeletal muscles involved in posture, balance, breath-
ing, and somatic reflexes (such as the flexor reflex) are involuntarily
controlled by integrating centers in the brainstem and spinal cord.
The somatic nervous system can also receive sensory input from
sensory neurons that convey information for somatic senses (tactile,
thermal, pain, and proprioceptive sensations; see Chapter 16) or the
special senses (sight, hearing, taste, smell, and equilibrium; see Chap-
ter 17). All of these sensations normally are consciously perceived. In
response to this sensory information, somatic motor neurons cause
the appropriate skeletal muscles of the body to contract.
Autonomic Nervous System
The autonomic nervous system (ANS) (aw′-tō-NOM-ik) is the part of
the nervous system that regulates cardiac muscle, smooth muscle, and
glands. These tissues are oft en referred to as visceral eff ectors because
they are usually associated with the viscera (internal organs) of the body.
The term autonomic is derived from the Latin words auto- = self and
-nomic = law because the ANS was once thought to be self-governing.
The autonomic nervous system consists of autonomic motor
neurons that regulate visceral activities by either increasing (exciting)
or decreasing (inhibiting) ongoing activities in their eff ector tissues
(cardiac muscle, smooth muscle, and glands). Changes in the diame-
ter of the pupils, dilation and constriction of blood vessels, and ad-
justment of the rate and force of the heartbeat are examples of auto-
nomic motor responses. Unlike skeletal muscle, tissues innervated by
the ANS oft en function to some extent even if their nerve supply is
damaged. The heart continues to beat when it is removed for trans-
plantation into another person, smooth muscle in the lining of the
gastrointestinal tract contracts rhythmically on its own, and glands
produce some secretions in the absence of ANS control.
The ANS usually operates without conscious control. For example,
you probably cannot voluntarily slow down your heart rate; instead,
your heart rate is subconsciously regulated. For this reason, some auto-
nomic responses are the basis for polygraph (“lie detector”) tests. How-
ever, practitioners of yoga or other techniques of meditation may learn
how to regulate at least some of their autonomic activities through long
practice. Biofeedback, in which monitoring devices display information
about a body function such as heart rate or blood pressure, enhances
the ability to learn such conscious control. (For more on biofeedback,
see the Medical Terminology section at the end of the chapter).
The ANS can also receive sensory input from sensory neurons as-
sociated with interoceptors (IN-ter-ō-sep′-tors), sensory receptors
located in blood vessels, visceral organs, muscles, and the nervous
system that monitor conditions in the internal environment. Exam-
ples of interoceptors are chemoreceptors that monitor blood CO2
level and mechanoreceptors that detect the degree of stretch in the
walls of organs or blood vessels. Unlike those triggered by a flower’s
perfume, a beautiful painting, or a delicious meal, these sensory sig-
nals are not consciously perceived most of the time, although intense
activation of interoceptors may produce conscious sensations. Two
examples of perceived visceral sensations are pain sensations from
damaged viscera and angina pectoris (chest pain) from inadequate
blood flow to the heart. Signals from the somatic senses and special
senses, acting via the limbic system, also influence responses of au-
tonomic motor neurons. Seeing a bike about to hit you, hearing
squealing brakes of a nearby car, or being grabbed by an attacker
would all increase the rate and force of your heartbeat.
The ANS consists of two main division (branches): the sympathet-
ic nervous system and the parasympathetic nervous system. Most
organs receive nerves from both of these divisions, an arrangement
known as dual innervation. In general, one division stimulates the or-
gan to increase its activity (excitation), and the other division decreas-
es the organ’s activity (inhibition). For example, neurons of the sympa-
thetic nervous system increase heart rate, and neurons of the
parasympathetic nervous system slow it down. The sympathetic nerv-
ous system promotes the fight-or-flight response, which prepares the
body for emergency situations. By contrast, the parasympathetic nerv-
ous system enhances rest-and-digest activities, which conserve and
restore body energy during times of rest or digesting a meal. Although
both the sympathetic and parasympathetic divisions are concerned
with maintaining health, they do so in dramatically diff erent ways.
The ANS is also comprised of a third division known as the enteric
nervous system (ENS). The ENS consists of millions of neurons in
plexuses that extend most of the length of the gastrointestinal tract.
Its operation is involuntary. Although the neurons of the ENS can
function autonomously, they can also be regulated by the other divi-
sions of the ANS. The ENS contains sensory neurons, interneurons,
and motor neurons. Enteric sensory neurons monitor chemical
changes within the GI tract as well as the stretching of its walls. Enteric
interneurons integrate information from the sensory neurons and
provide input to motor neurons. Enteric motor neurons govern con-
traction of GI tract smooth muscle and secretion of GI tract glands.
The ENS is described in greater detail in the discussion of the digestive
system in Chapter 24. The rest of this chapter is devoted to the sympa-
thetic and parasympathetic divisions of the ANS.
Recall from Chapter 10 that the axon of a single, myelinated
somatic motor neuron extends from the CNS all the way to the skel-
etal muscle fibers in its motor unit (Figure15.1a). By contrast, most
autonomic motor pathways consist of two motor neurons in series;
that is, one following the other (Figure 15.1b). The first neuron
(preganglionic neuron) has its cell body in the CNS; its myelinated
axon extends from the CNS to an autonomic ganglion. (Recall that
a ganglion is a collection of neuronal cell bodies In the PNS.) The
cell body of the second neuron (postganglionic neuron) is also in
that same autonomic ganglion; its unmyelinated axon extends
directly from the ganglion to the eff ector (smooth muscle, cardiac
muscle, or a gland). Alternatively, in some autonomic pathways, the
first motor neuron extends to specialized cells called chromaffin
cells in the adrenal medullae (inner portion of the adrenal glands)
rather than an autonomic ganglion. Chromaff in cells secrete the
neurotransmitters epinephrine and norepinephrine (NE). All
somatic motor neurons release only acetylcholine (ACh) as their
Anatomy of Autonomic
Motor Pathways
OBJECTIVES
• Describe preganglionic and postganglionic neurons of the autonomic
nervous system.
• Compare the anatomical components of the sympathetic and
parasympathetic divisions of the autonomic nervous system.
Anatomical Components
Each division of the ANS has two motor neurons. The first of the
two motor neurons in any autonomic motor pathway is called a
preganglionic neuron (Figure 15.1b). Its cell body is in the brain or
spinal cord; its axon exits the CNS as part of a cranial or spinal nerve.
The axon of a preganglionic neuron is a small-diameter, mye-
linated type B fiber that usually extends to an autonomic ganglion,
where it synapses with a postganglionic neuron, the second neuron
in the autonomic motor pathway. Note that the postganglionic
neuron lies entirely outside the CNS in the PNS. Its cell body and
dendrites are located in an autonomic ganglion, where it forms
synapses with one or more preganglionic axons. The axon of a post-
ganglionic neuron is a small-diameter, unmyelinated type C fiber that
terminates in a visceral eff ector. Thus, preganglionic neurons convey
nerve impulses from the CNS to autonomic ganglia, and postgangli-
onic neurons relay the impulses from autonomic ganglia to visceral
eff ectors.
Preganglionic Neurons In the sympathetic division, the
preganglionic neurons have their cell bodies in the lateral horns
of the gray matter in the 12 thoracic segments and the first two
(and sometimes three) lumbar segments of the spinal cord (Figure
15.2). For this reason, the sympathetic division is also called the
thoracolumbar division (thōr′-a-kō-LUM-bar), and the axons of the
sympathetic preganglionic neurons are known as the thoracolumbar
outflow.
Cell bodies of preganglionic neurons of the parasympathetic
division are located in the nuclei of four cranial nerves in the
brainstem (III, VII, IX, and X) and in the lateral gray matter of the
second through fourth sacral segments of the spinal cord
(Figure 15.3). Hence, the parasympathetic division is also known
as the craniosacral division (krā′-nē-ō-SĀK-ral), and the axons of
the parasympathetic preganglionic neurons are referred to as the
craniosacral outflow.
Autonomic Ganglia There are two major groups of autonomic
ganglia: (1) sympathetic ganglia, which are components of the
sympathetic division of the ANS, and (2) parasympathetic ganglia,
which are components of the parasympathetic division of the ANS.
SYMPATHETIC GANGLIA The sympathetic ganglia are the sites of
synapses between sympathetic preganglionic and postganglionic
neurons. There are two major types of sympathetic ganglia: sympa-
thetic trunk ganglia and prevertebral ganglia. Sympathetic trunk
ganglia (also called vertebral chain ganglia or paravertebral ganglia)
lie in a vertical row on either side of the vertebral column. These ganglia
extend from the base of the skull to the coccyx (Figure 15.2). Postgan-
glionic axons from sympathetic trunk ganglia primarily innervate
organs above the diaphragm, such as the head, neck, shoulders, and
heart. Sympathetic trunk ganglia in the neck have specific names.
They are the superior, middle, and inferior cervical ganglia. The
remaining sympathetic trunk ganglia do not have individual names.
Because the sympathetic trunk ganglia are near the spinal cord, most
sympathetic preganglionic axons are short and most sympathetic
postganglionic axons are long.
The second group of sympathetic ganglia, the prevertebral
(collateral) ganglia, lies anterior to the vertebral column and close to
the large abdominal arteries. In general, postganglionic axons from
prevertebral ganglia innervate organs below the diaphragm. There are
five major prevertebral ganglia (Figure 15.2; see also Figure 15.5):
(1) The celiac ganglion (SĒ-lē-ak) is on either side of the celiac trunk,
an artery that is just inferior to the diaphragm. (2) The superior mesen-
teric ganglion (MEZ-en-ter′-ik) is near the beginning of the superior
mesenteric artery in the upper abdomen. (3) The inferior mesenteric
ganglion is near the beginning of the inferior mesenteric artery in the
middle of the abdomen. (4) The aorticorenal ganglion (ā-or′-ti-kō-RĒ-
nal) and (5) the renal ganglion are near the renal artery of each kidney.
PARASYMPATHETIC GANGLIA Preganglionic axons of the parasympa-
thetic division synapse with postganglionic neurons in terminal
(intramural) ganglia. Most of these ganglia are located close to or
actually within the wall of a visceral organ. Terminal ganglia in the
head have specific names. They are the ciliary ganglion, pterygo-
palatine ganglion (ter′-i-gō-PAL-a-tīn), submandibular ganglion,
and otic ganglion (Figure 15.3). The remaining terminal ganglia do
not have specific names. Because terminal ganglia are located either
close to or in the wall of the visceral organ, parasympathetic pregan-
glionic axons are long, in contrast to parasympathetic postganglionic
axons, which are short.
Postganglionic Neurons Once axons of sympathetic pre-
ganglionic neurons pass to sympathetic trunk ganglia, they may
connect with postganglionic neurons in one of the following ways
(Figure 15.4):
1 An axon may synapse with postganglionic neurons in the ganglion
it first reaches.
2 An axon may ascend or descend to a higher or lower ganglion
before synapsing with postganglionic neurons. The axons of
incoming sympathetic preganglionic neurons pass up or down
the sympathetic trunk from ganglion to ganglion.
3 An axon may continue, without synapsing, through the sympa-
thetic trunk ganglion to end at a prevertebral ganglion and synapse
with postganglionic neurons there.
4 An axon may also pass, without synapsing, through the sym-
pathetic trunk ganglion and a prevertebral ganglion and then
extend to chromaff in cells of the adrenal medullae that are
functionally similar to sympathetic postganglionic neurons.
A single sympathetic preganglionic fiber has many axon collaterals
(branches) and may synapse with 20 or more postganglionic neurons.
This pattern of projection is an example of divergence and helps
explain why many sympathetic responses aff ect almost the entire body
simultaneously. Aft er exiting their ganglia, the postganglionic axons
typically terminate in several visceral eff ectors (see Figure 15.2).
Axons of preganglionic neurons of the parasympathetic division
pass to terminal ganglia near or within a visceral eff ector (see Figure
15.3). In the ganglion, the presynaptic neuron usually synapses with
only four or five postsynaptic neurons, all of which supply a single
visceral eff ector, allowing parasympathetic responses to be localized
to a single eff ector.
Autonomic Plexuses In the thorax, abdomen, and pelvis,
axons of both sympathetic and parasympathetic neurons form tangled
networks called autonomic plexuses, many of which lie along major
arteries. The autonomic plexuses also may contain sympathetic ganglia
and axons of autonomic neurons. The major plexuses in the thorax
are the cardiac plexus, which supplies the heart, and the pulmonary
plexus, which supplies the bronchial tree (Figure 15.5).
The abdomen and pelvis also contain major autonomic plexuses
(Figure 15.5), and oft en the plexuses are named aft er the artery along
which they are distributed. The celiac (solar) plexus is the largest
autonomic plexus and surrounds the celiac trunk. It contains two
large celiac ganglia, two aorticorenal ganglia, and a dense network of
autonomic axons and is distributed to the stomach, spleen, pancreas,
liver, gallbladder, kidneys, adrenal medullae, testes, and ovaries. The
superior mesenteric plexus contains the superior mesenteric
ganglion and supplies the small and large intestines. The inferior
mesenteric plexus contains the inferior mesenteric ganglion, which
innervates the large intestine. Axons of some sympathetic postgangli-
onic neurons from the inferior mesenteric ganglion also extend through
the hypogastric plexus, which is anterior to the fift h lumbar vertebra,
to supply the pelvic viscera. The renal plexus contains the renal gan-
glion and supplies the renal arteries within the kidneys and ureters.
With this background in mind, we can now examine some of the
specific structural features of the sympathetic and parasympathetic
divisions of the ANS in more detail.
Structure of the Sympathetic Division
Pathway from Spinal Cord to Sympathetic Trunk
Ganglia Cell bodies of sympathetic preganglionic neurons are
part of the lateral gray horns of all thoracic segments and of the
first two lumbar segments of the spinal cord (see Figure 15.2). The
preganglionic axons leave the spinal cord along with the somatic
motor neurons at the same segmental level. Aft er exiting through the
intervertebral foramina, the myelinated preganglionic sympathetic
axons pass into the anterior root of a spinal nerve and enter a short
pathway called a white ramus (RĀ-mus) before passing to the nearest
sympathetic trunk ganglion on the same side (see Figure 15.4). Col-
lectively, the white rami are called the white rami communicantes
(kō-mū-ni-KAN-tēz; singular is ramus communicans). Thus, white
rami communicantes are structures containing sympathetic pregan-
glionic axons that connect the anterior ramus of the spinal nerve with
the ganglia of the sympathetic trunk. The “white” in their name indi-
cates that they contain myelinated axons. Only the thoracic and first
two or three lumbar nerves have white rami communicantes.
Organization of Sympathetic Trunk Ganglia The
paired sympathetic trunk ganglia are arranged anterior and lateral
to the vertebral column, one on either side. Typically, there are 3
cervical, 11 or 12 thoracic, 4 or 5 lumbar, 4 or 5 sacral sympathetic
trunk ganglia, and 1 coccygeal ganglion. The right and left coccygeal
ganglia are fused together and usually lie at the midline. Although the
sympathetic trunk ganglia extend inferiorly from the neck, chest, and
abdomen to the coccyx, they receive preganglionic axons only from
the thoracic and lumbar segments of the spinal cord (see Figure 15.2).
The cervical portion of each sympathetic trunk is located in the
neck and is subdivided into superior, middle, and inferior ganglia (see
Figure 15.2). Postganglionic neurons leaving the superior cervical
ganglion serve the head and heart. They are distributed to the sweat
glands, smooth muscle of the eye, blood vessels of the face, lacrimal
glands, pineal gland, nasal mucosa, salivary glands (which include
the submandibular, sublingual, and parotid glands), and heart. Post-
ganglionic neurons leaving the middle cervical ganglion and the
inferior cervical ganglion innervate the heart and blood vessels of
the neck, shoulder, and upper limb.
The thoracic portion of each sympathetic trunk lies anterior to
the necks of the corresponding ribs. This region of the sympathetic
trunk receives most of the sympathetic preganglionic axons. Postgan-
glionic neurons from the thoracic sympathetic trunk innervate the
heart, lungs, bronchi, and other thoracic viscera. In the skin, these
neurons also innervate sweat glands, blood vessels, and arrector pili
muscles of hair follicles. The lumbar portion of each sympathetic
trunk lies lateral to the corresponding lumbar vertebrae. The sacral
region of the sympathetic trunk lies in the pelvic cavity on the medial
side of the anterior sacral foramina.
Pathways from Sympathetic Trunk Ganglia to
Visceral Effectors Axons leave the sympathetic trunk in four
possible ways: (1) They can enter spinal nerves; (2) they can form
cephalic periarterial nerves; (3) they can form sympathetic nerves;
and (4) they can form splanchnic nerves.
SPINAL NERVES Recall that some of the incoming sympathetic
preganglionic neurons synapse with postganglionic neurons in the
sympathetic trunk, either in the ganglion at the level of entry or in a
ganglion farther up or down the sympathetic trunk. The axons of
some of these postganglionic neurons leave the sympathetic trunk by
entering a short pathway called a gray ramus and then merge with
the anterior ramus of a spinal nerve. Therefore, gray rami communi-
cantes are structures containing sympathetic postganglionic axons
that connect the ganglia of the sympathetic trunk to spinal nerves
(see Figure 15.4). The “gray” in their name indicates that they contain
unmyelinated axons. Gray rami communicantes outnumber the white
rami because there is a gray ramus leading to each of the 31 pairs of
spinal nerves. The axons of the postganglionic neurons that leave the
sympathetic trunk to enter spinal nerves provide sympathetic inner-
vation to the visceral eff ectors in the skin of the neck, trunk, and
limbs, including sweat glands, smooth muscle in blood vessels, and
arrector pili muscles of hair follicles.
CEPHALIC PERIARTERIAL NERVES Some sympathetic preganglionic
neurons that enter the sympathetic trunk ascend to the superior cer-
vical ganglion, where they synapse with postganglionic neurons. The
axons of some of these postganglionic neurons leave the sympathetic
trunk by forming cephalic periarterial nerves (per′-ē-ar-TĒ-rē-al),
nerves that extend to the head by wrapping around and following the
course of various arteries (such as the carotid arteries) that pass from
the neck to the head (see Figure 15.4). Cephalic periarterial nerves
provide sympathetic innervation to visceral eff ectors in the skin of the
face (sweat glands, smooth muscle of blood vessels, and arrector pili
muscles of hair follicles), as well as other visceral eff ectors of the head
(smooth muscle of the eye, lacrimal glands, pineal gland, nasal
mucosa, and salivary glands).
SYMPATHETIC NERVES Some of the incoming sympathetic pregangli-
onic neurons synapse with postganglionic neurons in one or more
ganglia of the sympathetic trunk. Then the axons of the postgan-
glionic neurons leave the trunk by forming sympathetic nerves that
extend to visceral eff ectors in the thoracic cavity (Figure 15.4). Sympa-
thetic nerves provide sympathetic innervation to the heart and lungs.
• Sympathetic nerves to the heart. Sympathetic innervation of the
heart consists of axons of preganglionic neurons that enter the
sympathetic trunk and then form synapses with postganglionic
neurons in the superior, middle, and inferior cervical ganglia and
first through fourth thoracic ganglia (T1–T4). From these ganglia,
axons of postganglionic neurons exit the sympathetic trunk by
forming sympathetic nerves that enter the cardiac plexus to supply
the heart (see Figure 15.2).
• Sympathetic nerves to the lungs. Sympathetic innervation of the
lungs consists of axons of preganglionic neurons that enter the sym-
pathetic trunk and then form synapses with postganglionic neurons
in the second through fourth thoracic ganglia (T2–T4). From these
ganglia, axons of sympathetic postganglionic neurons exit the trunk
by forming sympathetic nerves that enter the pulmonary plexus to
supply the smooth muscle of the bronchi and bronchioles of the
lungs (see Figure 15.2).
SPLANCHNIC NERVES Recall that some sympathetic preganglionic
axons pass through the sympathetic trunk without terminating in it.
Beyond the trunk, they form nerves known as splanchnic nerves
(SPLANGK-nik; see Figures 15.2 and 15.4), which extend to outlying
prevertebral ganglia.
• Splanchnic nerves to abdominopelvic organs. Most sympathetic
preganglionic axons that enter splanchnic nerves are destined to
synapse with sympathetic postganglionic neurons in the preverte-
bral ganglia that supply the organs of the abdominopelvic cavity.
Preganglionic axons from the fift h through ninth or tenth thoracic
ganglia (T5–T9 or T10) form the greater splanchnic nerve. It pierces
the diaphragm and enters the celiac ganglion of the celiac plexus.
From there, postganglionic neurons follow and innervate blood
vessels to the stomach, spleen, liver, kidneys, and small intestine.
Preganglionic axons from the tenth and eleventh thoracic ganglia
(T10–T11) form the lesser splanchnic nerve. It pierces the dia-
phragm and passes through the celiac plexus to enter the aorticore-
nal ganglion and superior mesenteric ganglion of the superior mes-
enteric plexus. Postganglionic neurons from the superior mesenteric
ganglion follow and innervate blood vessels of the small intestine
and proximal colon. The least (lowest) splanchnic nerve, which
is not always present, is formed by preganglionic axons from the
twelft h thoracic ganglia (T12) or a branch of the lesser splanchnic
nerve. It pierces the diaphragm and enters the renal plexus near the
kidney. Postganglionic neurons from the renal plexus supply kidney
arterioles and the ureters. Preganglionic axons that form the lumbar
splanchnic nerve from the first through fourth lumbar ganglia
(L1–L4) enter the inferior mesenteric plexus and terminate in the
inferior mesenteric ganglion, where they synapse with postgangli-
onic neurons. Axons of postganglionic neurons extend through the
inferior mesenteric plexus to supply the distal colon and rectum;
they also extend through the hypogastric plexus to supply blood
vessels of the distal colon, rectum, urinary bladder, and genital or-
gans. Postganglionic axons leaving the prevertebral ganglia follow the
course of various arteries to abdominal and pelvic visceral eff ectors.
• Splanchnic nerves to the adrenal medulla. Some sympathetic
preganglionic axons pass, without synapsing, through the sympa-
thetic trunk, greater splanchnic nerves, and celiac ganglion, and then
extend to chromaff in cells in the adrenal medullae of the adrenal
glands (see Figures 15.1 and 15.4). Developmentally, the adrenal
medullae and sympathetic ganglia are derived from the same tissue,
the neural crest (see Figure 14.27b). The adrenal medullae are mod-
ified sympathetic ganglia, and the chromaff in cells are similar to
sympathetic postganglionic neurons, except they lack dendrites and
axons. Rather than extending to another organ, however, these cells
release hormones into the blood. On stimulation by sympathetic
preganglionic neurons, the chromaff in cells of the adrenal medullae
release a mixture of catecholamine hormones—about 80% epineph-
rine, 20% norepinephrine, and a trace amount of dopamine. These
hormones circulate throughout the body and intensify responses
elicited by sympathetic postganglionic neurons.
