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Chapter 14
Autonomic
Nervous
System
© Annie Leibovitz/Contact Press Images
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Autonomic Nervous System
• Automatic nervous system (ANS)
consists of motor neurons that:
– Innervate smooth muscles, cardiac muscle,
and glands
– Make adjustments to ensure optimal support
for body activities
– Operate via subconscious control
• Also called involuntary nervous system
or general visceral motor system
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Figure 14.1 Place of the ANS in the structural organization of the nervous system.
Central nervous system (CNS)
Peripheral nervous system (PNS)
Sensory (afferent)
division
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Motor (efferent) division
Somatic nervous
system
Autonomic nervous
system (ANS)
Sympathetic
division
Parasympathetic
division
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14.1 ANS versus Somatic Nervous System
• Both have motor fibers but differ in:
– Effectors
– Efferent pathways and ganglia
– Target organ responses to
neurotransmitters
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Figure 14.2 Comparison of motor neurons in the somatic and autonomic nervous systems.
SOMATIC
NERVOUS
SYSTEM
Cell bodies in central
nervous system
Neurotransmitter
at effector
Peripheral nervous system
Effector
organs
Effect
Single neuron from CNS to effector organs
ACh
Stimulatory
Heavily myelinated axon
Skeletal muscle
Two-neuron chain from CNS to effector organs
NE
SYMPATHETIC
Acetylcholine (ACh)
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Lightly myelinated
preganglionic axons
Nonmyelinated
postganglionic axon
Ganglion
ACh
Epinephrine and
norepinephrine
Adrenal medulla
PARASYMPATHETIC
AUTONOMIC NERVOUS SYSTEM
ACh
Blood vessel
ACh
ACh
Lightly myelinated
preganglionic axon
Ganglion
Nonmyelinated
postganglionic
axon
Smooth muscle
(e.g., in gut), glands,
cardiac muscle
Stimulatory
or inhibitory,
depending
on neurotransmitter
and receptors
on effector
organs
Norepinephrine (NE)
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Role of the Parasympathetic Division – D
activities
• Keeps body energy use as low as possible, even
while carrying out maintenance activities
– Directs digestion, diuresis, defecation
• Referred to as “rest-and-digest” system
• Example: person relaxing and reading after a
meal
– Blood pressure, heart rate, and respiratory
rates are low
– Gastrointestinal tract activity is high
– Pupils constricted, lenses accommodated for
close vision
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Role of the Sympathetic Division – E
activities
• Mobilizes body during activity
• Referred to as “fight-or-flight” system
• Exercise, excitement, emergency,
embarrassment activates sympathetic system
– Increased heart rate; dry mouth; cold, sweaty
skin; dilated pupils
• During vigorous physical activity:
– Shunts blood to skeletal muscles and heart
– Dilates bronchioles
– Causes liver to release glucose
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Figure 14.3 Key anatomical differences between ANS divisions.
Sympathetic
Parasympathetic
Eye
Salivary
glands
Eye
Brain stem
Skin*
Cranial
Sympathetic
ganglia
Heart
Lungs
Stomach
1 Fibers originate
in the brain stem
(cranial fibers) or
sacral spinal cord.
1 Fibers originate
in the thoracic and
lumbar spinal cord.
2a Preganglionic
2a Preganglionic
Liver and
gallbladder
Bladder
Genitals
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Lungs
Heart
fibers are long.
fibers are short.
Stomach
2b Postganglionic
2b Postganglionic
fibers are long.
Pancreas
fibers are short.
Pancreas
T1
Salivary
glands
3 Ganglia are
within or near
visceral effector
organs.
Sacral
3 Ganglia are
close to spinal
cord.
L1
Liver
and gallbladder
Adrenal
gland
Bladder
Genitals
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Application
• Work as a group on chart
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14.6 Neurotransmitters
• Major neurotransmitters of ANS are
acetylcholine (ACh) and norepinephrine (NE)
– Ach (same as ACh used by somatic motor
neuron) is released by cholinergic fibers at:
• All ANS preganglionic axons and
• All parasympathetic postganglionic axons
– NE is released by adrenergic fibers at:
• Almost all sympathetic postganglionic axons,
except those at sweat glands (release ACh)
• Effects of neurotransmitter depends on whether
it binds to cholinergic receptor or adrenergic
receptor
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Figure 14.2 Comparison of motor neurons in the somatic and autonomic nervous systems.
SOMATIC
NERVOUS
SYSTEM
Cell bodies in central
nervous system
Neurotransmitter
at effector
Peripheral nervous system
Effector
organs
Effect
Single neuron from CNS to effector organs
ACh
Stimulatory
Heavily myelinated axon
Skeletal muscle
Two-neuron chain from CNS to effector organs
NE
SYMPATHETIC
Acetylcholine (ACh)
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Lightly myelinated
preganglionic axons
Nonmyelinated
postganglionic axon
Ganglion
ACh
Epinephrine and
norepinephrine
Adrenal medulla
PARASYMPATHETIC
AUTONOMIC NERVOUS SYSTEM
ACh
Blood vessel
ACh
ACh
Lightly myelinated
preganglionic axon
Ganglion
Nonmyelinated
postganglionic
axon
Smooth muscle
(e.g., in gut), glands,
cardiac muscle
Stimulatory
or inhibitory,
depending
on neurotransmitter
and receptors
on effector
organs
Norepinephrine (NE)
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Cholinergic Receptors
• Two types of cholinergic receptors bind
ACh
1. Nicotinic receptors
2. Muscarinic receptors
• Named after drugs that bind to them and
mimic ACh effects: nicotine and muscarine
(mushroom poison)
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Adrenergic Receptors
• Two major classes that respond to NE or
epinephrine
– Alpha () receptors
• Divided into subclasses: 1, 2
– Beta () receptors
• Divided into subclasses: 1, 2, 3
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Effects of Drugs
• Epinephrine (adrenaline)
– Hormone released or drug
– Sympathomimetic effect
– Hormone is released during “flight or fight”
– Given as drug during heart attacks and
anaphylactic shock - improve blood pressure
and circulation
– Binds to all adrenergic receptors (1, 2, 1,
2 , 3) – mimicks effect of NE
– Active ingredient in OTC inhalers
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Effects of Drugs
• Albuterol – Proventil or Ventolin
– Sympathomimetic effects – selective
– Binds to 2 receptors – dilation of bronchioles
– Given to asthma patients – relieves
bronchospasms
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Effects of Drugs
• Propanolol (Inderal)
– Sympatholytic effects
– Rx as a “beta blocker to control hypertension
– Vessels of skin and viscera receive constant impulses
from sympathetic neurons – sympathetic tone
– Action - decrease the number of sympathetic
impulses from medulla to blood vessels – vasodilation
and reduced blood pressure
– Action – Binds and blocks NE receptors to heart –
decreases heart rate to reduce cardiac output and BP
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Effects of Drugs
• Atropine
– Anticholinergic; blocks muscarinic ACh
receptors
– Used to prevent salivation during surgery, and
to dilate pupils for examination
• Neostigmine
– Inhibits acetylcholinesterase that breaks
down ACh
– Used to treat myasthenia gravis
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Effects of Drugs
• Over-the-counter drugs for colds, allergies,
and nasal congestion
– Stimulate -adrenergic receptors
• Beta-blockers
– Drugs that attach to 2 receptors to dilate lung
bronchioles in asthmatics; other uses
• See Tables 14.3 and 14.4 p. 53
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Sympathetic Tone
• Almost all blood vessel smooth muscle is
entirely innervated by sympathetic fibers
only, so this division controls blood
pressure, even at rest
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Sympathetic Tone (cont.)
• Sympathetic tone (vasomotor tone):
continual state of partial constriction of
blood vessels
– If blood pressure drops, sympathetic fibers fire faster
than normal to increase constriction of blood vessels
and cause blood pressure to rise
– If blood pressure rises, sympathetic fibers fire less
than normal, causing less constriction (dilation) of
vessels, which leads to decrease in blood pressure
– Allows sympathetic system to shunt blood where
needed
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Parasympathetic Tone
• Parasympathetic division normally dominates
heart and smooth muscle of digestive and
urinary tract organs, and it activates most
glands except for adrenal and sweat glands
– Slows the heart and dictates normal activity levels of digestive and
urinary tracts
– These organs also exhibit parasympathetic tone where they are
always slightly activated
• The sympathetic division can override these
effects during times of stress
• Drugs that block parasympathetic responses
increase heart rate and cause fecal and
urinary retention
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14.8 Control of ANS Function
• ANS is under control of CNS centers in:
– Brain stem and spinal cord, hypothalamus,
and cerebral cortex
– Hypothalamus is generally main integrative
center of ANS activity
• Cerebral input may modify ANS but does
so subconsciously
– Works through limbic system structures on
hypothalamic centers
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Figure 14.9 Levels of ANS control.
Communication at
subconscious level
Cerebral cortex
(frontal lobe)
Limbic system
(emotional input)
Hypothalamus
The “boss”: Overall
integration of ANS
Brain stem
(reticular formation, etc.)
Regulates pupil size, heart,
blood pressure, airflow,
salivation, etc.
Spinal cord
Reflexes for urination,
defecation, erection,
and ejaculation
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14.9 Disorders of the ANS
• Many ANS disorders involve deficient
control of smooth muscle activity
– Hypertension (high blood pressure)
• Overactive sympathetic vasoconstrictor response
to stress
• Heart must work harder, and artery walls are
subject to increased wear and tear
• Can be treated with adrenergic receptor-blocking
drugs
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14.9 Disorders of the ANS
– Raynaud’s disease
• Painful, exaggerated vasoconstriction in fingers
and toes
– Digits turn pale, then cyanotic
– Treated with vasodilators
– Autonomic dysreflexia
• Life-threatening, uncontrolled activation of
autonomic neurons in quadriplegics and people
with spinal cord injuries above T6
• Blood pressure skyrockets, posing increased risk
for stroke
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Clinical – Homeostatic Imbalance 14.1
• Autonomic neuropathy: damage to
autonomic nerves that is a common
complication of diabetes mellitus
• Early signs include sexual dysfunction
• Other frequent symptoms include
dizziness after standing suddenly (poor
blood pressure control), urinary
incontinence, sluggish eye pupil reactions,
and impaired sweating
• Best way to prevent diabetic neuropathy is
to maintain good blood glucose levels
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Developmental Aspects of the Autonomic
Nervous System
• ANS efficiency declines in old age,
partially because of structural changes at
preganglionic axon terminals
• Effects of age on ANS
– Constipation
– Dry eyes and frequent eye infections
– Orthostatic hypotension
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• Low blood pressure after position change
• Pressure receptors are less responsive to blood
pressure changes
• Cardiovascular centers fail to maintain healthy
blood pressure
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