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Chapter 17
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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Autonomic versus Somatic NS
• 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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Autonomic versus Somatic NS
• Notice that the ANS pathway is a 2 neuron pathway
while the Somatic NS only contains one neuron.
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Basic Anatomy of ANS
• Preganglionic neuron
– cell body in brain or spinal cord
– axon is myelinated type B fiber that extends to autonomic
ganglion
• Postganglionic neuron
– cell body lies outside the CNS in an autonomic ganglion
– axon is unmyelinated type C fiber that terminates in a visceral
effector
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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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Sources of Dual Innervation
• Sympathetic
(thoracolumbar) division
– preganglionic cell bodies
in thoracic and first 2
lumbar segments of spinal
cord
• Parasympathetic
(craniosacral) division
– preganglionic cell bodies in
nuclei of 4 cranial nerves
and the sacral spinal cord
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Locations of Autonomic Ganglia
• Sympathetic Ganglia
– trunk (chain) ganglia near
vertebral bodies
– prevertebral ganglia near
large blood vessel in gut
• celiac
• superior mesenteric
• inferior mesenteric
• Parasympathetic Ganglia
– terminal ganglia in wall of
organ
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Autonomic Plexuses
•
•
•
•
•
•
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Cardiac plexus
Pulmonary plexus
Celiac (solar) plexus
Superior mesenteric
Inferior mesenteric
Hypogastric
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Structures of Sympathetic NS
• Preganglionic cell bodies at T1 to L2
• Rami communicantes
– white ramus = myelinated = preganglionic fibers
– gray ramus = unmyelinated = postganglionic fibers
• 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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Ganglia & Plexuses of Sympathetic NS
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Pathways of Sympathetic Fibers
• Spinal nerve route
– out same level
• Sympathetic chain route
– up chain & out spinal n
• Collateral ganglion route
– out splanchnic n 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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Circuitry of Sympathetic NS
• Divergence = each preganglionic cell synapses
on many postganglionic cells
• Mass activation due to divergence
– multiple target organs
– fight or flight response explained
• Adrenal gland
– modified cluster of postganglionic cell bodies that
release epinephrine & norepinephrine into blood
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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
– many brs 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 from
preganglionic neurons & from parasympathetic
postganglionic neurons
• Excites or inhibits depending upon receptor type
and organ involved
• Nicotinic receptors are found on dendrites & cell
bodies of autonomic NS cells and at NMJ
• 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
Alpha1 and Beta1 receptors produce excitation
Alpha2 and Beta2 receptors cause inhibition
Beta3 receptors(brown fat) increase thermogenesis
• NE lingers at the synapse until enzymatically
inactivated by monoamine oxidase (MAO) or
catechol-O-methyltransferase (COMT)
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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
– controlled by regulation of the “tone” of the
sympathetic system
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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 17-21
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
• Autonomic reflexes occur over autonomic reflex
arcs. Components of that reflex arc:
–
–
–
–
–
sensory receptor
sensory neuron
integrating center
pre & postganglionic motor neurons
visceral effectors
• Unconscious sensations and responses
– changes in blood pressure, digestive functions etc
– filling & emptying of bladder or defecation
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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 Dysreflexia
• Exaggerated response of sympathetic NS in cases
of spinal cord injury above T6
• Certain sensory impulses trigger mass stimulation
of sympathetic nerves below the injury
• Result
– vasoconstriction which elevates blood pressure
– parasympathetic NS tries to compensate by slowing
heart rate & dilating blood vessels above the injury
– pounding headaches, sweating warm skin above the
injury and cool dry skin below
– can cause seizures, strokes & heart attacks
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