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PowerPoint® Lecture Slides
prepared by
Janice Meeking,
Mount Royal College
CHAPTER
14
The Autonomic
Nervous System
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Autonomic Nervous System (ANS)
• The ANS consists of motor neurons that:
• Innervate smooth and cardiac muscle and glands
• Make adjustments to ensure optimal support for
body activities
• Operate via subconscious control
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ANS Versus Somatic Nervous System (SNS)
• The ANS differs from the SNS in the following three areas
• 1. Effectors
• The effectors of the SNS are skeletal muscles
• The effectors of the ANS are cardiac muscle, smooth
muscle, and glands
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ANS Versus Somatic Nervous System (SNS)
• 2. Efferent pathways
• In the SNS heavily myelinated axons of the somatic motor neurons
extend from the CNS to the effector (one neuron)
• Efferent pathways in the ANS are a two-neuron chain
• The preganglionic (first) neuron has a lightly myelinated axon
• The post-ganglionic (second) neuron extends to an effector
organ
Pre-ganglionic
Post-ganglionic
Ganglion
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ANS Versus Somatic Nervous System (SNS)
• 3. Target organ responses
• All somatic motor neurons release Acetylcholine (ACh),
which has an excitatory effect
• In the ANS:
• Preganglionic fibers release ACh
• Postganglionic fibers release norepinephrine or ACh and
the effect is either stimulatory or inhibitory
• ANS effect on the target organ is dependent
upon the neurotransmitter released and the
receptor type of the effector
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Characteristic
Somatic Nervous System
Autonomic Nervous System
Voluntary (skeletal) muscle
Smooth muscle, Cardiac
Muscle, Glands
Adjustment to external
environment.
Adjustment within the
internal environment
(homeostasis)
Number of
neurons from CNS
to effectors
1
2
Ganglia outside
the CNS
0
Chain ganglia, collateral
ganglia or terminal ganglia
Acetylcholine
Acetylcholine, adrenaline,
noradrenalin
Effectors
General function
Neurotransmitter
On NMJ - nicotinic receptors – ACh always excitatory
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Divisions of the ANS
Enteric nervous system
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http://www.sirinet.net/~jgjohnso/nervous.html
Divisions of the ANS
1. Sympathetic division
2. Parasympathetic division
• Dual innervation
• Almost all visceral organs are served by both
divisions, but they cause opposite effects
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Anatomy of ANS
Location
of
Ganglia
Origin of
Fibers
Length of
Fibers
Sympathetic
Thoraco lumbar region
of the spinal
cord
Short pre ganglionic
and long post
- ganglionic
Close to
the spinal
cord
Parasympathetic
Brain and
sacral spinal
cord
Long
preganglionic
and short
postganglioni
c
In the
visceral
effector
organs
Division
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ANS receptors
•
ANS receptors are divided into 2 groups:
•
Cholinergic – receive and respond to acetylcholine (ACh):
•
Two subgroups that are names of chemicals that mimic
some of the actions of Ach:
•
•
Muscarinic (chemical found in the mushroom Amanita
muscarina)
•
Nicotinic (chemical found in the tobacco plant – nicotina
tabacum)
Adrenergic – receive and respond to norepinephrin (NE) /
epinephrine (E)
•
Divided into alpha and beta
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Role of the Parasympathetic Division
• Concerned with keeping body energy use low
• Involves the D activities – digestion, defecation, and
diuresis
• Its activity is illustrated in a person who relaxes after a
meal
• Blood pressure, heart rate, and respiratory rates are
low
• Gastrointestinal tract activity is high
• The skin is warm and the pupils are constricted
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Parasympathetic Division Outflow
Cranial Outflow
Cranial Nerve
Ganglion
Effector Organ(s)
Occulomotor (III)
Ciliary
Eye
Facial (VII)
Pterygopalatin
Submandibular
Otic
Salivary, nasal, and
lacrimal glands
Parotid salivary glands
Located within the
walls of target organs
Located within the
walls of the target
organs
Heart, lungs, and most
visceral organs
Large intestine, urinary
bladder, ureters, and
reproductive organs
Glossopharyngeal
(IX)
Vagus (X)
Sacral Outflow
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S2-S4
Nicotinic Receptors
• Found on
• Motor end plates of skeletal muscle cells
• All ganglionic
parasympathetic)
neurons
(sympathetic
and
• Hormone-producing cells of the adrenal medulla
• Effect of ACh at nicotinic receptors is always direct
and stimulatory
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Direct effect – receptors are part of the ion channel
Nicotinic receptors
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Muscarinic Receptors
• Found on
• All effector cells stimulated by postganglionic
cholinergic fibers
• The effect of ACh at muscarinic receptors
• Can be either inhibitory or excitatory
• Depends on the receptor type of the target organ
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Indirect effect – through G-protein and 2nd messenger
Muscarinic (cholinergic) and all adrenergic receptors
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Parasympathetic pre and post-ganglionic receptors
(nACh)
(mACh)
(nACh) (mACh)
Copyright © The McGraw-Hill Companies, Inc
Copyright © 2010 Pearson Education, Inc.
ACh (cholinergic) receptors
Nicotinic receptors (nACh)
Pre-ganglionic
Direct mechanism – open
Na+ channels
(depolarization)
Fast excitatory effect
Muscarinic receptors (mACh)
Post-ganglionic
Indirect mechanism – use of G-protein
and 2nd messenger system
Increase
intracellular
Ca2+ release
Slow excitatory
M1, M3, M5
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Inhibition of
adenylate
cyclase
Slow inhibitory
M2, M4
Cholinergic receptors function
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http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/P/PNS.html
Drugs affecting the parasympathetic nervous system
• Nicotinic receptors:
• Agonists at muscles will cause contraction
• Antagonists at muscles will cause paralysis.
• Muscarinic receptors:
• Agonists will cause increased bowel and bladder
function, decreased HR, bronchoconstriction, and
miosis.
• Agonists also help in cognition and memory.
• Antagonists will cause constipation, urinary retention,
increased heart rate, bronchodilation, and dilated pupils
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Break down of ACh
•
•
The effect of ACh is short-lived
•
Inactivated in the synapse
acetylcholinesterase (AChE)
by
the
enzyme
•
Inactivated in the surrounding tissues cholinesterase
The effect of the parasympathetic division is localized and
lasts about 20 msec.
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Role of the Sympathetic Division
• The sympathetic division is the “fight-or-flight”
system
• Involves E activities – exercise,
emergency, and embarrassment
excitement,
• Promotes adjustments during exercise – blood flow to
organs is reduced, flow to muscles is increased
• Its activity is illustrated by a person who is threatened
• Heart rate increases, and breathing is rapid and
deep
• The skin is cold and sweaty, and the pupils dilate
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Sympathetic Outflow
• Sympathetic nerves originate inside the vertebral column toward the
middle of the spinal cord in the lateral gray horn
• Arises from spinal cord segments T1 through L2 (thoracolumbar
outflow).
• Axons of these nerves leave the spinal cord in the ventral rami of the
spinal nerves, and then separate out as 'white rami'
• The white rami connect to the chain ganglia (paravertebral) extending
alongside the vertebral column on the left and right.
• There are 23 ganglia – 3 cervical, 11 thoracic, 4 lumbar, 4 sacral,
and 1 coccygeal
• The chain ganglia are interconnected by preganglionic fibers
• All preganglionic fibers go through the chain ganglia.
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Sympathetic Trunks and Pathways
A preganglionic fiber follows one
of three pathways upon entering the
paravertebral ganglia
1. Synapse with the ganglionic
neuron within the same ganglion
2. Ascend or descend the
sympathetic chain to synapse in
another chain ganglion
3. Pass through the chain ganglion
and emerge without synapsing
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Figure 14.6
http://www.wisc-online.com/objects/index_tj.asp?objID=AP2704
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Pathways with Synapses in the Adrenal Medulla
• Some preganglionic fibers pass directly to the adrenal
medulla without synapsing
• Upon
stimulation,
medullary
cells
secrete
norepinephrine and epinephrine into the blood
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Visceral Reflexes
• Visceral reflex arcs have the same components as
somatic reflexes
• Main difference: visceral reflex arc has two neurons in
the motor pathway
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Referred Pain
• Visceral pain afferents travel along the same pathway
as somatic pain fibers
• Pain stimuli arising in the viscera are perceived as
somatic in origin
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Sympathetic pre and post-ganglionic receptors
(nACh)
a
b
a
b
(nACh)
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Adrenergic Receptors
• Two types
• Alpha (a) (subtypes a1, a2)
• Beta (b) (subtypes b1, b2 , b3)
• Effects of NE depend on which subclass of receptor
predominates on the target organ
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NE (adrenergic) receptors - all indirect through G-protein
a1
a2
Increase
intracellular
Ca2+ release
Inhibition of
adenylate
cyclase
Slow excitation
Slow inhibition
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b1
b3
b2
2nd messenger – cAMP
Inhibition or activation of
adenylate cyclase
slow
excitation
Slow
inhibition
Lypolysis
(excitation)
Drugs affecting the sympathetic nervous system
• Alpha 1 receptors –
• agonists will raise BP,
• antagonists will lower BP
• Beta 1 receptors –
• agonists will increase heart rate and strength of contraction,
• antagonists will decrease heart rate and BP
• Beta 2 receptors –
• agonists will increase respiratory airflow, will increase blood
flow to skeletal muscles (via blood vessels dilation)
• antagonists will constrict respiratory flow
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Interactions of the Autonomic Divisions
• Most visceral organs have dual innervation
• Dynamic antagonism allows for precise control of
visceral activity
• Sympathetic division increases heart and
respiratory rates, and inhibits digestion and
elimination
• Parasympathetic division decreases heart and
respiratory rates, and allows for digestion and the
discarding of wastes
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Sympathetic Tone
• Sympathetic division controls blood pressure, even at
rest
• Sympathetic tone (vasomotor tone)
• Keeps the blood vessels in a continual state of
partial constriction
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Parasympathetic Tone
• Parasympathetic division normally dominates the
heart and smooth muscle of digestive and urinary tract
organs
• Slows the heart
• Dictates normal activity levels of the digestive and
urinary tracts
• The sympathetic division can override these effects
during times of stress
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Unique Roles of the Sympathetic Division
• only sympathetic fibers are found on:
• The adrenal medulla, sweat glands, arrector pili muscles,
kidneys, and most blood vessels
• The sympathetic division controls
• Thermoregulatory responses to heat
• Release of renin from the kidneys
• Metabolic effects
• Increases metabolic rates of cells
• Raises blood glucose levels
• Mobilizes fats for use as fuels
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