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Chapter 15
The Autonomic Nervous System
Principles of Human Anatomy and Physiology, 11e
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INTRODUCTION
• The autonomic nervous system (ANS) operates via reflex
arcs.
• Operation of the ANS to maintain homeostasis, however,
depends on a continual flow of sensory afferent input, from
receptors in organs, and efferent motor output to the same
effector organs.
• Structurally, the ANS includes autonomic sensory neurons,
integrating centers in the CNS, and autonomic motor
neurons.
• Functionally, the ANS usually operates without conscious
control.
• The ANS is regulated by the hypothalamus and brain stem.
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Chapter 15
The Autonomic Nervous System
• Regulate activity of smooth muscle, cardiac muscle &
certain glands
• Structures involved
– general visceral afferent neurons
– general visceral efferent neurons
– integration center within the brain
• Receives input from limbic system and other regions of
the cerebrum
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SOMATIC AND AUTONOMIC NERVOUS SYSTEMS
• The somatic nervous system contains both sensory and
motor neurons.
• The somatic sensory neurons receive input from receptors
of the special and somatic senses.
• These sensations are consciously perceived.
• Somatic motor neurons innervate skeletal muscle to
produce conscious, voluntary movements.
• The effect of a motor neuron is always excitation.
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SOMATIC AND AUTONOMIC NERVOUS SYSTEMS
• The autonomic nervous system contains both autonomic
sensory and motor neurons.
– Autonomic sensory input is not consciously perceived.
• The autonomic motor neurons regulate visceral activities by
either increasing (exciting) or decreasing (inhibiting)
ongoing activities of cardiac muscle, smooth muscle, and
glands.
– Most autonomic responses can not be consciously
altered or suppressed.
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SOMATIC vs AUTONOMIC NERVOUS SYSTEMS
• All somatic motor pathways consist of a single motor
neuron
• Autonomic motor pathways consists of two motor neurons
in series
– The first autonomic neuron motor has its cell body in the
CNS and its myelinated axon extends to an autonomic
ganglion.
• It may extend to the adrenal medullae rather than an
autonomic ganglion
– The second autonomic motor neuron has its cell body in
an autonomic ganglion; its nonmyelinated axon extends
to an effector.
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Somatic versus Autonomic NS
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Basic Anatomy of ANS
• Preganglionic neuron
– cell body in brain or spinal cord
– axon is myelinated fiber that extends to autonomic ganglion
• Postganglionic neuron
– cell body lies outside the CNS in an autonomic ganglion
– axon is unmyelinated fiber that terminates in a visceral effector
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Sympathetic vs. Parasympathetic NS
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AUTONOMIC NERVOUS SYSTEM
• The output (efferent) part of the ANS is divided into two
principal parts:
– the sympathetic division
– the parasympathetic division
– Organs that receive impulses from both sympathetic and
parasympathetic fibers are said to have dual innervation.
• Table 15.1 summarizes the similarities and differences
between the somatic and autonomic nervous systems.
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Sympathetic ANS vs. Parasympathetic ANS
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Divisions of the
ANS
• 2 major divisions
– parasympathetic
– sympathetic
• Dual innervation
– one speeds up organ
– one slows down organ
– Sympathetic NS
increases heart rate
– Parasympathetic NS
decreases heart rate
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Divisions of the
ANS
• 2 major divisions
– parasympathetic
– sympathetic
• Dual innervation
– one speeds up organ
– one slows down organ
– Sympathetic NS
increases heart rate
– Parasympathetic NS
decreases heart rate
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Sympathetic Ganglia
• These ganglia include the sympathetic trunk or vertebral
chain or paravertebral ganglia that lie in a vertical row on
either side of the vertebral column (Figures 15.2).
• Other sympathetic ganglia are the prevertebral or collateral
ganglia that lie anterior to the spinal column and close to
large abdominal arteries.
– celiac
– superior mesenteric
– inferior mesenteric ganglia
– (Figures 15.2 and 15.4).
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Parasympathetic Ganglia
• Parasympathetic ganglia are the terminal or intramural
ganglia that are located very close to or actually within the
wall of a visceral organ.
• Examples of terminal ganglia include (Figure 15.3)
– ciliary,
– submandibular,
– otic ganglia
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Sympathetic ANS vs. Parasympathetic ANS
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Dual Innervation, Autonomic Ganglia
• Sympathetic (thoracolumbar)
division
– preganglionic cell bodies in
thoracic and first 2 lumbar
segments of spinal cord
• Ganglia
– trunk (chain) ganglia near
vertebral bodies
– prevertebral ganglia near
large blood vessel in gut
(celiac, superior mesenteric,
inferior mesenteric)
Principles of Human Anatomy and Physiology, 11e
• Parasympathetic
(craniosacral) division
– preganglionic cell
bodies in nuclei of 4
cranial nerves and the
sacral spinal cord
• Ganglia
– terminal ganglia in wall
of organ
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Structures of Sympathetic NS
• Preganglionic cell bodies at T1 to L2
• Postganglionic cell bodies
– sympathetic chain ganglia along the spinal column
– prevertebral ganglia at a distance from spinal cord
• celiac ganglion
• superior mesenteric ganglion
• inferior mesenteric ganglion
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Postganglionic Neurons:
Sympathetic vs. Parasympathetic
• Sympathetic preganglionic neurons pass to the sympathetic
trunk. They may connect to postganglionic neurons in the
following ways. (Figure 17.5).
– May synapse with postganglionic neurons in the ganglion
it first reaches.
– May ascend or descend to a higher of lower ganglion
before synapsing with postganglionic neurons.
– May continue, without synapsing, through the
sympathetic trunk ganglion to a prevertebral ganglion
where it synapses with the postganglionic neuron.
• Parasympathetic preganglionic neurons synapse with
postganglionic neurons in terminal ganglia (Figure 17.3).
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Pathways of Sympathetic Fibers
• Spinal nerve route
– out same level
• Sympathetic chain route
– up chain & out spinal
nerve
• Collateral ganglion route
– out splanchnic nerve to
collateral ganglion
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Organs Innervated by Sympathetic NS
• Structures innervated by each spinal nerve
– sweat glands, arrector pili mm., blood vessels to skin
& skeletal mm.
• Thoracic & cranial plexuses supply:
– heart, lungs, esophagus & thoracic blood vessels
– plexus around carotid artery to head structures
• Splanchnic nerves to prevertebral ganglia supply:
– GI tract from stomach to rectum, urinary &
reproductive organs
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Ganglia & Plexuses of Sympathetic NS
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Structure of the Parasympathetic Division
• Preganglionic axons extend from the brain stem in four
cranial nerves. (Figure 15.3).
– The cranial outflow consists of four pairs of ganglia and
the plexuses associated with the vagus (X) nerve.
• The sacral parasympathetic outflow consists of
preganglionic axons in the anterior roots of the second
through fourth sacral nerves and they form the pelvic
splanchnic nerve. (Figure15.3)
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Anatomy of Parasympathetic NS
• Preganglionic cell bodies
found in
– 4 cranial nerve nuclei in
brainstem
– S2 to S4 spinal cord
• Postganglionic cell bodies
very near or in the wall of
the target organ in a
terminal ganglia
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Parasympathetic Cranial Nerves
• Oculomotor nerve
– ciliary ganglion in orbit
– ciliary muscle & pupillary constrictor muscle inside eyeball
• Facial nerve
– pterygopalatine and submandibular ganglions
– supply tears, salivary & nasal secretions
• Glossopharyngeal
– otic ganglion supplies parotid salivary gland
• Vagus nerve
– supply heart, pulmonary and GI tract as far as the midpoint of
the colon
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Parasympathetic Sacral Nerve Fibers
• Form pelvic splanchnic
nerves
• Preganglionic fibers end
on terminal ganglia in
walls of target organs
• Innervate smooth muscle
and glands in colon,
ureters, bladder &
reproductive organs
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ANS Neurotransmitters
• Classified as either cholinergic or adrenergic neurons based
upon the neurotransmitter released
• Adrenergic
• Cholinergic
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Cholinergic Neurons and Receptors
• Cholinergic neurons release
acetylcholine
– all preganglionic neurons
– all parasympathetic
postganglionic neurons
– few sympathetic
postganglionic neurons (to
most sweat glands)
• Excitation or inhibition
depending upon receptor
subtype and organ involved.
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Cholinergic Neurons and Receptors
• The two types of cholinergic receptors are nicotinic and
muscarinic receptors (Figure 15.6 a , b).
– Activation of nicotinic receptors causes excitation of the
postsynaptic cell.
• Nicotinic receptors are found on dendrites & cell
bodies of autonomic NS cells (and at NMJ.)
– Activation of muscarinic receptors can cause either
excitation or inhibition depending on the cell that bears
the receptors.
• Muscarinic receptors are found on plasma
membranes of all parasympathetic effectors
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Adrenergic Neurons and Receptors
• Adrenergic neurons release
norepinephrine (NE)
– from postganglionic
sympathetic neurons only
• Excites or inhibits organs
depending on receptors
• NE lingers at the synapse until
enzymatically inactivated by
monoamine oxidase (MAO) or
catechol-O-methyltransferase
(COMT)
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Adrenergic Neurons and Receptors
• The main types of adrenergic receptors are alpha and beta
receptors. These receptors are further classified into
subtypes.
– Alpha1 and Beta1 receptors produce excitation
– Alpha2 and Beta2 receptors cause inhibition
– Beta3 receptors (brown fat) increase thermogenesis
• Effects triggered by adrenergic neurons typically are longer
lasting than those triggered by cholinergic neurons.
• Table 15.2 describes the location of the subtypes of
cholinergic and adrenergic receptors and summarizes the
responses that occur when each type of receptor is
activated.
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Receptor Agonists and Antagonists
• An agonist is a substance that binds to and activates a
receptor, mimicking the effect of a natural neurotransmitter
or hormone.
• An antagonist is a substance that binds to and blocks a
receptor, preventing a natural neurotransmitter or hormone
from exerting its effect.
• Drugs can serve as agonists or antagonists to selectively
activate or block ANS receptors.
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Physiological Effects of the ANS
• Most body organs receive dual innervation
– innervation by both sympathetic & parasympathetic
• Hypothalamus regulates balance (tone) between
sympathetic and parasympathetic activity levels
• Some organs have only sympathetic innervation
– sweat glands, adrenal medulla, arrector pili mm & many
blood vessels
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Sympathetic Responses
• Dominance by the sympathetic system is caused by physical
or emotional stress -- “E situations”
– emergency, embarrassment, excitement, exercise
• Alarm reaction = flight or fight response
– dilation of pupils
– increase of heart rate, force of contraction & BP
– decrease in blood flow to nonessential organs
– increase in blood flow to skeletal & cardiac muscle
– airways dilate & respiratory rate increases
– blood glucose level increase
• Long lasting due to lingering of NE in synaptic gap and
release of norepinephrine by the adrenal gland
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Parasympathetic Responses
• Enhance “rest-and-digest” activities
• Mechanisms that help conserve and restore body energy
during times of rest
• Normally dominate over sympathetic impulses
• SLUDD type responses = salivation, lacrimation, urination, digestion
& defecation and 3 “decreases”--- decreased HR, diameter of airways and
diameter of pupil
• Paradoxical fear when there is no escape route or no way to win
– causes massive activation of parasympathetic division
– loss of control over urination and defecation
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Autonomic or Visceral Reflexes
• A visceral autonomic reflex adjusts the activity of a
visceral effector, often unconsciously.
– changes in blood pressure, digestive functions etc
– filling & emptying of bladder or defecation
• Autonomic reflexes occur over autonomic reflex arcs.
Components of that reflex arc:
– sensory receptor
– sensory neuron
– integrating center
– pre & postganglionic motor neurons
– visceral effectors
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Control of Autonomic NS
• Not aware of autonomic responses because control
center is in lower regions of the brain
• Hypothalamus is major control center
– input: emotions and visceral sensory information
• smell, taste, temperature, osmolarity of blood, etc
– output: to nuclei in brainstem and spinal cord
– posterior & lateral portions control sympathetic NS
• increase heart rate, inhibition GI tract, increase
temperature
– anterior & medial portions control parasympathetic NS
• decrease in heart rate, lower blood pressure,
increased GI tract secretion and mobility
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Autonomic versus Somatic NS - Review
• Somatic nervous system
– consciously perceived sensations
– excitation of skeletal muscle
– one neuron connects CNS to organ
• Autonomic nervous system
– unconsciously perceived visceral sensations
– involuntary inhibition or excitation of smooth muscle,
cardiac muscle or glandular secretion
– two neurons needed to connect CNS to organ
• preganglionic and postganglionic neurons
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