Human Anatomy & PhysiologyNinth Edition

C H A P T E R 14

© 2013 Pearson Education, Inc.© Annie Leibovitz/Contact Press Images

The AutonomicNervous System

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Autonomic Nervous System (ANS)

• 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

• 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

Motor (efferent) division

Somatic nervoussystem

Autonomic nervoussystem (ANS)

Sympatheticdivision

Parasympatheticdivision

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Somatic Versus Autonomic Nervous Systems

• Both have motor fibers

• 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.

Cell bodies in centralnervous system Peripheral nervous system

Neurotransmitterat effector

Effectororgans Effect

SO

MA

TIC

NER

VO

US

S

YS

TEM

AU

TO

NO

MIC

NER

VO

US

SYS

TEM

SYM

PA

TH

ETIC

PA

RA

SYM

PA

TH

ETIC

Single neuron from CNS to effector organs

Heavily myelinated axon

ACh

Skeletal muscle

Stimulatory

Two-neuron chain from CNS to effector organs

Lightly myelinated preganglionic axons

Ganglion

Nonmyelinatedpostganglionic axon

Epinephrine andnorepinephrine

ACh

ACh

Adrenal medulla Blood vessel

Lightly myelinated preganglionic axon Ganglion

ACh

Nonmyelinatedpostganglionic axon

AChSmooth muscle(e.g., in gut), glands,cardiac muscle

Stimulatoryor inhibitory,dependingon neuro-transmitterand receptorson effectororgans

Acetylcholine (ACh) Norepinephrine (NE)

NE

+

+–

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Role of the Parasympathetic Division – D activities

• Promotes maintenance activities and conserves body energy– Directs digestion, diuresis, defecation

• As in person relaxing and reading after a meal– Blood pressure, heart rate, and respiratory

rates are low– Gastrointestinal tract activity high– Pupils constricted; lenses accommodated for

close vision

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Role of the Sympathetic Division – E activities

• Mobilizes body during activity; "fight-or-flight" system

• Exercise, excitement, emergency, embarrassment– 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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Parasympathetic Sympathetic

Eye Eye

Salivaryglands

Brain stem

Cranial

Sympatheticganglia

Salivaryglands

Heart Cervical

Lungs

T1

Lungs

Heart

StomachThoracic

PancreasStomach

PancreasLiverand gall-bladderL1

Liver and gall-bladder

LumbarAdrenalgland

Bladder Bladder

Genitals Sacral Genitals

Skin*

Figure 14.3 The subdivisions of the ANS.

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Application

• Work as a group on chart

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Neurotransmitters

• Cholinergic fibers release neurotransmitter ACh– All ANS preganglionic axons– All parasympathetic postganglionic axons at

effector synapse

• Adrenergic fibers release neurotransmitter NE– Most sympathetic postganglionic axons– Exception: sympathetic postganglionic fibers

secrete ACh at sweat glands

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Figure 14.2 Comparison of motor neurons in the somatic and autonomic nervous systems.

Cell bodies in centralnervous system Peripheral nervous system

Neurotransmitterat effector

Effectororgans Effect

SO

MA

TIC

NER

VO

US

S

YS

TEM

AU

TO

NO

MIC

NER

VO

US

SYS

TEM

SYM

PA

TH

ETIC

PA

RA

SYM

PA

TH

ETIC

Single neuron from CNS to effector organs

Heavily myelinated axon

ACh

Skeletal muscle

Stimulatory

Two-neuron chain from CNS to effector organs

Lightly myelinated preganglionic axons

Ganglion

Nonmyelinatedpostganglionic axon

Epinephrine andnorepinephrine

ACh

ACh

Adrenal medulla Blood vessel

Lightly myelinated preganglionic axon Ganglion

ACh

Nonmyelinatedpostganglionic axon

AChSmooth muscle(e.g., in gut), glands,cardiac muscle

Stimulatoryor inhibitory,dependingon neuro-transmitterand receptorson effectororgans

Acetylcholine (ACh) Norepinephrine (NE)

NE

+

+–

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Cholinergic Receptors

• Two types of receptors bind ACh1. Nicotinic – always stimulatory

2. Muscarinic – both stimulatory and inhibitory depending on the organ

• Named after drugs that bind to them and mimic ACh effects

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Adrenergic Receptors

• Two major classes– Alpha () (subtypes 1, 2)

– Beta () (subtypes 1, 2 , 3)

• Effects of NE depend on which subclass of receptor predominates on target organ

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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 Table 14.3 p. 535

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Sympathetic Tone

• Sympathetic division controls blood pressure, even at rest

• Vascular system ~ entirely innervated by sympathetic fibers

• Sympathetic tone (vasomotor tone)– Keeps blood vessels in continual state of

partial constriction

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Sympathetic Tone

• Sympathetic fibers fire more rapidly to constrict blood vessels and cause blood pressure to rise

• Sympathetic fibers fire less rapidly to prompt vessels to dilate to decrease blood pressure

• Alpha-blocker drugs interfere with vasomotor fibers– Used to treat hypertension

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Parasympathetic Tone

• Parasympathetic division normally dominates heart, smooth muscle of digestive and urinary tract organs, activate most glands except for adrenal and sweat glands– Slows the heart– Dictates normal activity levels of digestive and urinary

tracts

• 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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Control of ANS Function

• Hypothalamus—main integrative center of ANS activity

• Subconscious cerebral input via limbic system structures on hypothalamic centers

• Other controls come from cerebral cortex, reticular formation, and spinal cord

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Communication atsubconscious level

Cerebral cortex(frontal lobe)

Limbic system(emotional input)

HypothalamusThe “boss”: Overall integration of ANS

Brain stem(reticular formation, etc.)Regulates pupil size, heart,

blood pressure, airflow, salivation, etc.

Spinal cordReflexes for urination, defecation, erection,

and ejaculation

Figure 14.8 Levels of ANS control.

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Homeostatic Imbalances of the ANS

• Hypertension (high blood pressure)– Overactive sympathetic vasoconstrictor

response to stress– Treated with adrenergic receptor - blocking

drugs

• Raynaud's disease– Exaggerated vasoconstriction in fingers and

toes• Pale, then cyanotic and painful• Treated with vasodilators

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Homeostatic Imbalances of the ANS

• Autonomic dysreflexia– Uncontrolled activation of autonomic neurons

in quadriplegics and those with spinal cord injuries above T6

– Blood pressure skyrockets– Life-threatening

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Developmental Aspects of the ANS

• Effects of age on ANS– Constipation– Dry eyes– Frequent eye infections– Orthostatic hypotension

• Low blood pressure after position change• Pressure receptors less responsive to blood

pressure changes• Cardiovascular centers fail to maintain healthy

blood pressure

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