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The Autonomic Nervous System
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Introduction
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
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The ANS is the general visceral motor
division of the peripheral nervous system
and is distinct from the general somatic
motor and branchial motor divisions
which innervate skeletal muscles
Introduction
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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 & SNS
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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
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In the somatic system
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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
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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
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Compare the one motor neuron of the somatic motor
division with the two neuron chain of the autonomic
nervous system
Efferent Pathways and Ganglia
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Axons of most preganglionic neurons run from
the CNS to synapse in a peripheral autonomic
ganglion with a postganglionic neuron
Efferent Pathways and Ganglia
Efferent Pathways and Ganglia
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Axons of postganglionic neurons run from
the ganglion to the visceral effectors
(cardiac and smooth muscle fibers and
glands)
Preganglionic axons are lightly myelinated
thin fibers
Postganglionic axons are even thinner and
are unmyelinated
Conduction through the autonomic chain is
slower than through the somatic motor
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 contracts
Neurotransmitter Effects
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Neurotransmitters released onto visceral
effector organs by postganglionic autonomic
fibers include
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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
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 postganglionic 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 cause
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
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During exercise the sympathetic division also
promotes physiological adjustments
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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
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The parasympathetic
division can also be
termed the 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 pre-ganglionic
fibers and long post-ganglionic fibers
Comparison of ANS & PNS
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Parasympathetic pathways in contrast have long preganglionic 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
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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
Anatomy of ANS
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The sympathetic and parasympathetic
divisions are distinguished by
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Unique sites of origin
Different lengths of their fibers
Location of their ganglia
Anatomy of ANS
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Unique origin sites
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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
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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
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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
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
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Receptor
Sensory neuron
Integration center
Motor neuron
Effector
Visceral Reflexes
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Visceral reflex arcs differ in that they have a twoneuron 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 gastro-intestinal tract
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Enteric nervous system
Controls gastrointestinal activity
Visceral Reflexes
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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
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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
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Since the same
spinal segments
(T1-T5) 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
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Additional
cutaneous areas to
which visceral pain
is referred
Overview of the ANS
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The autonomic nervous system differs in…
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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