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The Autonomic Nervous System
Chapter 15
Introduction
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The Autonomic Nervous System (ANS) is
the system of motor neurons that
innervate the smooth muscle, cardiac
muscle, and glands of the body
By controlling these effectors, the ANS
regulates such visceral functions as …
–
–
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Heart rate
Blood pressure
Digestion
Urination
Introduction

The ANS is the general visceral motor
division of the peripheral nervous system
and is distinct from the general somatic
motor and brachial motor divisions
which innervate skeletal muscles
Introduction

The general visceral sensory system
continuously monitors the activities of the
visceral organs so that the autonomic
motor neurons can make adjustments as
necessary to ensure optimal performance
of the visceral organs
Introduction
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The stability of our internal environment
depends largely on the autonomic
nervous system
Autonomic nervous system(ANS) receives
signals from visceral organs
The ANS makes adjustments as necessary
to ensure optical support for body
systems
Comparison of ANS & PNS
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Recall that the somatic motor system
innervates skeletal muscles
Each somatic motor neuron runs from the
central nervous system all the way to the
muscle being innervated, and that each
motor unit consists of a single neuron plus
the skeletal muscle cells it innervates
Typical somatic motor axons are thick,
heavily myelinated fibers that conduct
nerve impulses rapidly
Comparison of ANS & PNS
Comparison of ANS & PNS

In the somatic system
– Cell bodies are within the central nervous
system
– Axons extend to the muscles they serve
– Somatic motor fibers are thick, heavily
myelinated Type A fibers that conduct
impulses very rapidly
Comparison of ANS & PNS
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In the autonomic nervous system
– The motor unit is a two neuron chain
– The cell body of the first neuron, the
preganglionic neuron, resides in the brain or
spinal cord
– Its axon, the preganglionic axon, synapses
with the second motor neuron, the postganglionic neuron, in an autonomic ganglion
outside the central nervous system
– The postganglionic axon then extends to the
effector organ
Comparison of ANS & PNS

Compare the one motor neuron of the somatic
motor division with the two neuron chain of the
autonomic nervous system
Efferent Pathways and Ganglia

Axons of most preganglionic neurons run from the
CNS to synapse in a peripheral autonomic ganglion
with a postganglionic neuron
Efferent Pathways and Ganglia

Axons of postganglionic neurons run from the
ganglion to the effectors (cardiac and smooth muscle
fibers and glands)
Efferent Pathways and Ganglia
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Preganglionic axons are lightly myelinated
thin fibers
Postganglionic axons are even thinner and
are unmyelinated
Conduction though the autonomic chain is
slower than through the somatic motor
Many pre and postganglionic fibers are
incorporated into spinal or cranial nerves
for most of their course
Efferent Pathways and Ganglia
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Remember that autonomic ganglion are
motor ganglia, containing the cell bodies
of motor neurons
They are sites of synapse and information
transmission from pre to postganglionic
neurons
Also note that the somatic motor division
lacks ganglia entirely
Neurotransmitter Effects

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All somatic motor neurons release
acetylcholine at their synapses with their
effectors, skeletal muscle fibers
The effect is always excitatory, and if
stimulation reaches threshold, the
skeletal muscle fibers contacts
Neurotransmitter Effects

Neurotransmitters released onto visceral
effector organs by postganglionic
autonomic fibers include
– Norepinephrine secreted by most sympathetic
fibers
– Acetylcholine released by parasympathetic
fibers

Depending on the receptors present on the
target organ, its response to these neurotransmitters may be either excitation or
inhibition
Overlap of Somatic & Autonomic
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Higher brain centers regulate and
coordinate both somatic and visceral
motor activities
Nearby spinal nerves and many cranial
nerves contain both somatic and
autonomic fibers
Most of the body’s adaptations to
changing internal and external conditions
involve both skeletal activity and
enhanced response of visceral organs
Divisions of ANS
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There are two division of the ANS
– Parasympathetic
– Sympathetic
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Generally the two divisions have chains of
two motor neurons that innervate same
visceral organs but cause essentially
opposite effects
If one division stimulates certain smooth
muscle to contract or a gland to secrete,
the other division inhibits that action
Through this process of duel innervation
the two systems counterbalance each other
Divisions of ANS
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The sympathetic part mobilizes the body
during extreme situations (such as fear,
exercise or rage)
The parasympathetic division allows us to
unwind as it performs maintenance
activities and conserves body energy
Divisions of ANS
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Both the sympathetic
and parasympathetic
divisions issue from the
brain and spinal cord
Two neuron pathways
are shown for both
divisions
Solid lines indicate preganglionic axons while
broken lines indicate
post-ganglionic axons
Sympathetic Division
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The sympathetic division is responsible
for the “fight, flight, or fright” response
Its activity is evident during vigorous
exercise, excitement, or emergencies
Physiological changes like a pounding
heart, fast and deep breathing, dilated
eye pupils, and cold, sweaty skin are signs
of the mobilization of the sympathetic
division, which help us survive danger
Sympathetic Division
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Sympathetic responses prepare our
bodies to cope with physiological
stressors
While sympathetic response may
increases the capacities of some systems
they may in fact inhibit “non-essential”
functions such as digestion and urinary
tract motility
Sympathetic Division
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The sympathetic system also innervates
blood vessels, sending signals to the
smooth muscles in their walls
Even though sympathetic input causes
the smooth muscle in some vessels (in
skeletal muscle) to relax so that the vessel
dilates, the bulk of sympathetic input
signals smooth muscle in blood vessels to
contract, producing vasoconstriction
Sympathetic Division
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Vasoconstriction results in the narrowing
of vessel diameter which forces the heart
to work harder to pump blood around
the vascular circuit
As a result sympathetic activity results in
blood pressure to rise during excitement
and stress
Role of Sympathetic Division

During exercise the sympathetic division
also promotes physiological adjustments
– Visceral blood supply is diminished
– Blood is shunted to working musculature
– Bronchioles of the lungs dilate to increase
ventilation
– Liver releases more sugar into blood stream
to support metabolism
Role of Sympathetic Division
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Its activity is evident when we are excited or
find ourselves in emergency or threatening
situations (frightened)
Pounding heart; rapid, deep breathing; cold,
sweaty skin; and dilated eyes are signs
Also changes in brain wave patterns
Its function is to provide the optimal
conditions for an appropriate response to
some threat (run / see / think)
Parasympathetic Division
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The parasympathetic division is most
effective in non-stressful situations
This division is chiefly concerned with
keeping body energy use as low as
possible, even as it directs body processes
such as digestion and elimination
Resting and digesting division
Autonomic Homeostasis
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Autonomic homeostasis is the dynamic
counteraction between the two divisions
such that they balance each other during
times when we are neither highly excited
nor completed at rest
Divisions of ANS
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In addition to the functional differences
between the parasympathetic and
sympathetic divisions , there are also
anatomical and biochemical differences
Divisions of ANS
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The two divisions issue
from different regions
of the CNS
The sympathetic can
also be called the
thoracolumbar division
because its fibers
emerge from the
thoracic and lumbar
parts of the spinal cord
Divisions of ANS

The parasympathetic
division can also be
termed the craniospinal
division because its
fibers emerge from the
brain and spinal cord
(sacral)
Comparison of ANS & PNS
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A second difference between the two divisions is
that sympathetic pathways have short preganglionic fibers and long post-ganglionic fibers
Comparison of ANS & PNS

Parasympathetic pathways in contrast have long
pre-ganglionic fibers and short post-ganglionic
fibers
Divisions of ANS
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Therefore, all
sympathetic
ganglia lie near
the spinal cord
and vertebral
column, and all
parasympathetic
ganglia lie far
from the CNS, in
or near the
organs
innervated
Divisions of ANS

The third
anatomical
difference
between the two
divisions is that
sympathetic
axons branch
profusely, while
parasympathetic
fibers do not
Divisions of ANS
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Extensive branching allows each
sympathetic neuron to influence a
number of different visceral organs,
enabling many organs to mobilize
simultaneously during the “fight, flight or
fright” response
Parasympathetic effects, by contrast are
more localized and discrete
Divisions of ANS
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The main
biochemical
difference between
the two divisions
involves the
neurotransmitter
release by the
postganglionic
axons
Divisions of ANS
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In the sympathetic division, most
postganglionic axons release norepinephine
(also called noradrenaline) these fibers are
termed adrenergic
The postganglionic neurotransmitter in the
parasympathetic division is acetycholine
(Ach) these fibers are termed cholinergic
The preganglionic axon terminals of both
divisions always release acetylcholine
Divisions of ANS
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The main anatomical and physiological
differences between the parasympathetic
and sympathetic divisions are
summarized in Table 15.1
Anatomy of ANS
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The sympathetic and parasympathetic
divisions are distinguished by
– Unique sites of origin
– Different lengths of their fibers
– Location of their ganglia
Anatomy of ANS
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Unique origin sites
– Parasympathetic fibers emerge from the
brain and from the spinal cord at the sacral
level
– Sympathetic fibers originate from the
thoracic and lumbar regions of the spinal
cord
Anatomy of ANS
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Different Lengths of their Fibers
– Parasympathetic division has long
preganglionic and short postganglionic
fibers
– Sympathetic is the opposite with short
preganglionic and long postganglionic fibers
Anatomy of ANS
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Length of their Ganglia
– Most parasympathetic ganglia are located in
the visceral effector organs
– Sympathetic ganglia lie close to the spinal
cord
Parasympathetic Division
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The parasympathetic emerge from
opposite ends of the central nervous
system
The preganglionic axons extend from the
CNS nearly all the way to the structures
to be innervated
Parasympathetic Division
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The preganglionic
neurons synapse with
the ganglionic
neurons located in
terminal ganglia
Very short post
ganglionic axons issue
from the terminal
ganglia and synapse
with effector cells in
their immediate area
Parasympathetic Division
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Several cranial nerves
contain outflow of the
parasympathetic
Preganglionic fibers run
in the oculomotor,
facial, glossopharyngeal,
and vagus nerve
Cranial Outflow
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Oculomotor nerve III
– The parasympathetic fibers of the oculomotor
nerves innervate smooth muscles of the eye
• Constrictor muscles of iris cause pupil to constrict
• Ciliary muscle within the orbits of the eye controls
lense shape for visual focusing
• Allow the eye to focus on close objects in the visual
field
Cranial Outflow
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Facial Nerves VII
– The parasympathetic fibers of the facial
nerves stimulate the secretory activity of
many large glands of the head
• The pathway activates the nasal glands and the
lacrimal glands of the eyes
• The preganglionic fibers then run to synapse
with ganglionic neurons in the pterygopalatine
ganglia stimulating the submandibular and
sublingual salivary glands
Cranial Outflow
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Glossopharyngeal
(IX)
– The
parasympathetic
nerves originate in
the medulla and
activate the parotid
salivary gland
Cranial Outflow
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Vagus nerves (X)
– The major portion of the parasympathetic
cranial outflow is via the vagus nerves
– The two vagus nerves account for an
estimated 90% of all preganglionic
parasympathetic fibers in the body
– They provide fibers to the neck and
contribute to nerve plexuses that serve
virtually every organ in the thoracic and
abdominal cavity
Cranial Outflow
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The vagus nerve
fibers arise from
the dorsal motor of
the medulla and
terminate by
synapsing in
terminal ganglia
that are usually
located in the walls
of the target organ
Cranial Outflow
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Most of the terminal ganglia are not
individually named; instead they are
collectively called intramural ganglia,
literally ganglia “within the walls”
As the vagus nerves passes into the thorax,
they send branches to autonomic plexuses
– Cardiac plexuses
– Pulmonary plexuses
– Esophageal plexuses
Cranial Outflow
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When the vagus
nerves reach the
esophagus, their
fibers intermingle
to form the anterior
and posterior vagal
trunks
Each trunk carries
fibers from both
vargus nerves
Cranial Outflow
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The vagal trunks
ride the esophagus
down to enter the
abdominal cavity
They send fibers to
form the aortic
plexuses (formed by
the celiac, superior
mesenteric and
hypogastric)
Cranial Outflow
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Abdominal organs
which receive vagal
innervation include
the liver, gallbladder,
stomach, small
intestine, kidneys,
pancreas, and the
proximal half of the
large intestine
The rest of the cavity
are innervated by the
sacral outflow
Sacral Outflow
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The sacral outflow
arises from neurons
located in the lateral
horn of the spinal
cord at S2 - S4
The axons of these
neurons run in the
ventral roots of the
spinal nerves to the
ventral rami
Sacral Outflow
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From the ventral
rami the neurons
branch to form the
pelvic splanchnic
nerves
Most neurons
synapse in the
intramural ganglia
located in the walls of
the distal large
intestine, urinary
bladder and
reproductive organs
Sympathetic Division
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The sympathetic division innervates more
organs
It supplies not only the visceral organs in
the internal body cavities, but also the
visceral structures in the superficial part
of the body
– Sweat glands
– Arrector pili
– Arteries and veins
Sympathetic Division
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All preganglionic
fibers in the
sympathetic
division arise from
cell bodies of
preganglionic
neurons located in
spinal cord
segments from T1
through L2
It is also called the
thoracolumbar
Sympathetic Division
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After leaving the cord
via the ventral root,
the preganglionic
sympathetic fibers
pass through a white
ramus communicans
to enter the adjoining
chain (paravertebral)
ganglion forming
part of the
sympathetic trunk or
chain
Sympathetic Division
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The sympathetic
trunks flank each side
of the vertebral
column and appear as
strands of white
beads
Sympathetic Division
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Although the
sympathetic
trunks extend
from the neck to
the pelvis,
sympathetic fibers
arise only from the
thoracic and
lumbar spinal cord
segments
Sympathetic Division
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The ganglia vary in size, position, and
number, but there are typically 23 ganglia
in each sympathetic chain…
–
–
–
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3 cervical
11 thoracic
4 lumbar
4 sacral
1 coccygeal
Sympathetic Division
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Once a preganglionic
axon reaches a
paravertebral ganglion
one of three things can
happen to it
Sympathetic Division
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Once a preganglionic
axon reaches a
paravertebral ganglion
one of three things can
happen to it…
1. It can synapse with a
ganglionic neuron within
the same chain ganglion
Sympathetic Division

Once a preganglionic
axon reaches a
paravertebral ganglion
one of three things can
happen to it…
2. It can ascend or descend
the sympathetic chain to
synapse in another chain
ganglion
Sympathetic Division
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Once a preganglionic
axon reaches a
paravertebral ganglion
one of three things can
happen to it…
3. It can pass through the
chain ganglion and
emerge from the
sympathetic chain
without synapsing
Sympathetic Division
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Preganglionic fibers
which emerge from the
sympathetic chain
without synapsing help
to form the splanchnic
nerves which synapse
with prevertebral or
collateral ganglia
Sympathetic Division
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The prevertebral
ganglia are located
anterior to the
vertebral column
Sympathetic Division
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Unlike the paravertebral ganglia the
prevertebral ganglia . . .
– Are neither paired nor segmentally arranged
– They occur only in the abdomen and pelvis
Sympathetic Division
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Note:
Regardless of where the synapse occurs,
all sympathetic ganglia lie close to the
spinal cord
The postganglionic fibers which run from
the ganglion to the organs are typically
much longer than the preganglionic
fibers
Visceral Reflexes
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The visceral sensory neurons are the first
link in the autonomic reflexes
These neurons send information
concerning chemical changes, stretch,
and irritation of the viscera
Visceral Reflexes
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Visceral reflex arcs have essentially the
same components as somatic reflex arcs
–
–
–
–
–
Receptor
Sensory neuron
Integration center
Motor neuron
Effector
Visceral Reflexes
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Visceral reflex arcs differ in that they have a
two-neuron chain
Visceral Reflexes
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Nearly all sympathetic and parasympathetic
fibers are accompanied by afferent fibers
conducting sensory impulses from glands or
muscular structures
Thus, peripheral processes of visceral
sensory neurons are found in cranial nerves,
VII, IX, and X, the splanchnic nerves, and
the sympathetic trunk, as well as the spinal
nerves
Visceral Reflexes
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Like sensory neurons serving somatic
structures (skeletal muscles and skin)
The cell bodies of visceral sensory
neurons are located in the sensory
ganglia of associated cranial nerves or in
the dorsal root ganglia of the spinal cord
Visceral Reflexes
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Visceral sensory reflexes are also found
within sympathetic ganglia where synapses
with preganglionic neurons occur
Complete three-neuron reflex arcs
(sensory, motor, and intrinsic neurons)
exist within the walls of the gastrointestinal tract
– Enteric nervous system
– Controls gastrointestinal activity
Visceral Reflexes

The fact that
visceral pain travels
along the same
pathways as
somatic pain fibers
helps to explain the
phenomenon of
referred pain in
which pain stimuli
arising in the
viscera is perceived
as somatic in origin
Visceral Reflexes

A heart attach may
produce a sensation
of pain that radiates
to the superior
thoracic wall and
along the medial
aspect of the left
arm
Visceral Reflexes

Since the same
spinal segments (T1T5) innervate both
the heart and the
regions to which
pain signals from
heart tissue are
referred, the brain
interprets most such
inputs as coming
from the somatic
pathway
Visceral Reflexes

Additional
cutaneous areas to
which visceral pain
is referred
Overview of the ANS

The autonomic nervous system differs in…
– Its effectors
– Its efferent pathways
– Its target organs
Effectors of ANS

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The somatic nervous system stimulates
skeletal muscles
The ANS innervates cardiac and smooth
muscles and glands