Somatic and Autonomic Nervous System Assignment

15.1 COMPARISON OF SOMATIC AND
AUTONOMIC NERVOUS SYSTEMS
OBJECTIVE
• Compare the structural and functional differences between the somatic and autonomic parts of the
nervous system.
Somatic Nervous System
The somatic nervous system includes both sensory and motor neurons. Sensory neurons convey input from
receptors for somatic senses (tactile, thermal, pain, and proprioceptive sensations; see Chapter 16) and from
receptors for the special senses (sight, hearing, taste, smell, and equilibrium; see Chapter 17). All these
sensations normally are consciously perceived. In turn, somatic motor neurons innervate skeletal muscles—the
effectors of the somatic nervous system—and produce both reflexive and voluntary movements. When a
somatic motor neuron stimulates the muscle, it contracts; the effect always is excitation. If somatic motor
neurons cease to stimulate a muscle, the result is a paralyzed, limp muscle that has no muscle tone. Although
we are generally not conscious of breathing, the muscles that generate respiratory movements are also skeletal
muscles controlled by somatic motor neurons. If the respiratory motor neurons become inactive, breathing
stops. A few skeletal muscles, such as those in the middle ear, are controlled by reflexes and cannot be
contracted voluntarily.
Autonomic Nervous System
The main input to the ANS comes from autonomic (visceral) sensory neurons. Mostly, these neurons are
associated with interoceptors (IN ter ō sep′ tors), which are sensory receptors located in blood vessels,‐ ‐ ‐ ‐
visceral organs, muscles, and the nervous system that monitor conditions in the internal environment.
Examples 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 signals 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. Input that influences the ANS also includes some sensations monitored by somatic
sensory and special sensory neurons. For example, pain can produce dramatic changes in some autonomic
activities.
Autonomic motor neurons regulate visceral activities by either increasing (exciting) or decreasing
(inhibiting) ongoing activities in their effector tissues (cardiac muscle, smooth muscle, and glands). Changes in
the diameter of the pupils, dilation and constriction of blood vessels, and adjustment of the rate and force of the
heartbeat are examples of autonomic motor responses. Unlike skeletal muscle, tissues innervated by the ANS
often function to some extent even if their nerve supply is damaged. For example, the heart continues to beat
when it is removed for transplantation 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.
Most autonomic responses cannot be consciously altered to any great degree. You probably cannot voluntarily
slow your heartbeat to half its normal rate. For this reason, some autonomic responses are the basis for
polygraph (“lie detector”) tests. However, 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. Signals from the general somatic and special senses, acting via the
limbic system, also influence responses of autonomic 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.
Examples
Anatomy Overview: Nervous System: Organization of the ANS
Comparison of Somatic and Autonomic Motor Neurons
Recall from Chapter 10 that the axon of a single, myelinated somatic motor neuron extends from the central
nervous system (CNS) all the way to the skeletal muscle fibers in its motor unit (Figure 15.1a). 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 effector (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 portions of the adrenal glands) rather than an
autonomic ganglion. In addition, all somatic motor neurons release only acetylcholine (ACh) as their
neurotransmitter, but autonomic motor neurons release either ACh or norepinephrine (NE).
Figure 15.1
Motor neuron pathways in the (a) somatic nervous system and (b) autonomic
nervous system (ANS).
Note that autonomic motor neurons release either acetylcholine (ACh) or norepinephrine (NE); somatic motor
neurons release ACh.
Somatic nervous system stimulation always excites its effectors (skeletal muscle fibers); stimulation by the
autonomic nervous system either excites or inhibits visceral effectors.
Kevin Somerville/Imagineering
What does dual innervation mean?
Unlike somatic output (motor), the output part of the ANS has two divisions: the sympathetic division and
the parasympathetic division. Most organs have dual innervation; that is, they receive impulses from both
sympathetic and parasympathetic neurons. In some organs, nerve impulses from one division of the ANS
stimulate the organ to increase its activity (excitation), and impulses from the other division decrease the
organ's activity (inhibition). For example, an increased rate of nerve impulses from the sympathetic division
increases heart rate, and an increased rate of nerve impulses from the parasympathetic division decreases heart
rate. The sympathetic division is often called the fight or flight division‐ ‐ . Sympathetic activities result in
increased alertness and metabolic activities in order to prepare the body for an emergency situation. Responses
to such situations, which may occur during physical activity or emotional stress, include a rapid heart rate,
faster breathing rate, dilation of the pupils, dry mouth, sweaty but cool skin, dilation of blood vessels to organs

involved in combating stress (such as the heart and skeletal muscles), constriction of blood vessels to organs
not involved in combating stress (for example, the gastrointestinal tract and kidneys), and release of glucose
from the liver.
The parasympathetic division is often referred to as the rest and digest division‐ ‐ because its activities conserve
and restore body energy during times of rest or digesting a meal; the majority of its output is directed to the
smooth muscle and glandular tissue of the gastrointestinal and respiratory tracts. The parasympathetic division
conserves energy and replenishes nutrient stores. Although both the sympathetic and parasympathetic divisions
are concerned with maintaining homeostasis, they do so in dramatically different ways.
Table 15.1 compares the somatic and autonomic nervous systems.
Table 15.1 Comparison of the Somatic and Autonomic Nervous Systems
Somatic Nervous System Autonomic Nervous System
Sensory input From somatic senses and
special senses.
Mainly from interoceptors; some from somatic senses
and special senses.
Control of motor
output
Voluntary control from
cerebral cortex, with
contributions from basal
ganglia, cerebellum,
brain stem, and spinal
cord.
Involuntary control from hypothalamus, limbic
system, brain stem, and spinal cord; limited control
from cerebral cortex.
Motor neuron
pathway
One neuron pathway:‐
Somatic motor neurons
extending from CNS
synapse directly with
effector.
Usually two neuron pathway: Preganglionic neurons‐
extending from CNS synapse with postganglionic
neurons in autonomic ganglion, and postganglionic
neurons extending from ganglion synapse with
visceral effector. Alternatively, preganglionic neurons
may extend from CNS to synapse with chromaffin
cells of adrenal medullae.
Neurotransmitters
and hormones
All somatic motor
neurons release ACh.
All sympathetic and parasympathetic preganglionic
neurons release ACh. Most sympathetic
postganglionic neurons release NE; those to most
sweat glands release ACh. All parasympathetic
postganglionic neurons release ACh. Chromaffin cells
of adrenal medullae release epinephrine and
norepinephrine.
Effectors Skeletal muscle. Smooth muscle, cardiac muscle, and glands.
Responses Contraction of skeletal
muscle.
Contraction or relaxation of smooth muscle;
increased or decreased rate and force of contraction
of cardiac muscle; increased or decreased secretions
of glands.
15.2 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,‐
myelinated 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 postganglionic neuron
is a small diameter, unmyelinated type C fiber that terminates in a visceral effector. Thus, preganglionic‐
neurons convey nerve impulses from the CNS to autonomic ganglia, and postganglionic neurons relay the
impulses from autonomic ganglia to visceral effectors.
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.
Figure 15.2 Structure of the sympathetic division of the autonomic nervous system.
Solid lines represent preganglionic axons; dashed lines represent postganglionic axons. Although the innervated
structures are shown for only one side of the body for diagrammatic purposes, the sympathetic division actually
innervates tissues and organs on both sides.
Cell bodies of sympathetic preganglionic neurons are located in the lateral horns of gray matter in the 12
thoracic and first two lumbar segments of the spinal cord.
Imagineering
Which division, sympathetic or parasympathetic, has longer preganglionic axons?
Why?
Cell bodies of preganglionic neurons of the parasympathetic division are located in the nuclei of four cranial
nerves in the brain stem (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.

Figure 15.3 Structure of the parasympathetic division of the autonomic nervous system.
Solid lines represent preganglionic axons; dashed lines represent postganglionic axons. Although the innervated
structures are shown only for one side of the body for diagrammatic purposes, the parasympathetic division actually
innervates tissues and organs on both sides.
Cell bodies of parasympathetic preganglionic neurons are located in brain stem nuclei and in the lateral gray
matter in the second through fourth sacral segments of the spinal cord.
Imagineering
Which ganglia are associated with the parasympathetic division? Sympathetic division?
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: sympathetic 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). Postganglionic 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 mesenteric 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 parasympathetic 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, pterygopalatine
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 preganglionic axons are long, in contrast to parasympathetic
postganglionic axons, which are short.
Postganglionic Neurons
Once axons of sympathetic preganglionic 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 that pass up or down the
sympathetic trunk collectively form the sympathetic chains, the fibers on which the ganglia are strung.
3. An axon may continue, without synapsing, through the sympathetic trunk ganglion to end at a
prevertebral ganglion and synapse with postganglionic neurons there.
4. An axon may also pass, without synapsing, through the sympathetic trunk ganglion and a prevertebral
ganglion and then extend to chromaffin cells of the adrenal medullae that are functionally similar to
sympathetic postganglionic neurons.
Figure 15.4 Types of connections between ganglia and postganglionic neurons in the
sympathetic division of the ANS.
Numbers correspond to descriptions in the text. Also illustrated are the gray and white rami communicantes.
Sympathetic ganglia lie in two chains on either side of the vertebral column (sympathetic trunk ganglia) and
near large abdominal arteries anterior to the vertebral column (prevertebral ganglia).
Kevin Somerville/Imagineering
What is the significance of the sympathetic trunk ganglia?
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 affect almost the entire body simultaneously. After exiting their ganglia, the
postganglionic axons typically terminate in several visceral effectors (see Figure 15.2).
Axons of preganglionic neurons of the parasympathetic division pass to terminal ganglia near or within a
visceral effector (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 effector, allowing parasympathetic responses to
be localized to a single effector.
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 sensory 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).
Figure 15.5 Autonomic plexuses in the thorax, abdomen, and pelvis.
An autonomic plexus is a network of sympathetic and parasympathetic axons that sometimes also includes
autonomic sensory axons and sympathetic ganglia. Kevin Somerville

Which is the largest autonomic plexus?
The abdomen and pelvis also contain major autonomic plexuses (Figure 15.5), and often the plexuses are
named after 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 postganglionic
neurons from the inferior mesenteric ganglion also extend through the hypogastric plexus, which is anterior to
the fifth lumbar vertebra, to supply the pelvic viscera. The renal plexus contains the renal ganglion 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.
Examples
Animation: ANS: Motor Pathways
Anatomy Overview: Visceral Receptors
Anatomy Overview: Visceral Effectors
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. After 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). Collectively, 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 preganglionic axons that connect the anterior ramus of the spinal nerve with
the ganglia of the sympathetic trunk. The “white” in their name indicates 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. Postganglionic neurons leaving the middle cervical ganglion and
the inferior cervical ganglion innervate the heart.
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. Postganglionic 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 communicantes 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 innervation to the visceral effectors
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 cervical
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 effectors 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 effectors of the head (smooth muscle of the eye,
lacrimal glands, pineal gland, nasal mucosa, and salivary glands).
Sympathetic Nerves
Some of the incoming sympathetic preganglionic neurons synapse with postganglionic neurons in one or more
ganglia of the sympathetic trunk. Then, the axons of the postganglionic neurons leave the trunk by
forming sympathetic nerves that extend to visceral effectors in the thoracic cavity (Figure 15.4). Sympathetic
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 sympathetic trunk and then form synapses with postganglionic