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The Autonomic System
Ching-Liang Lu, M.D.
Professor
Institute of Brain Science
National Yang-Ming
University
Autonomic Nervous System
(ANS)
• Innervate smooth and cardiac muscle and
glands
• Make adjustments to ensure optimal support
for body activities
• Operate via subconscious control
• Have viscera as most of their effectors
Somatic vs. Visceral
attribute
Somatic System
Visceral System
embryological
origin of tissue
“body wall:” somatic (parietal)
mesoderm (dermatome,
myotome)
“organs:” splanchnic
(visceral) mesoderm,
endoderm
examples of
adult tissues
dermis of skin, skeletal muscles,
connective tissues
glands, cardiac muscle,
smooth muscle
perception
conscious, voluntary
unconscious, involuntary
Sensory/Motor + Somatic/Visceral
Somatic
Visceral
Sensory
(Afferent)
somatic sensory
visceral sensory
[General Somatic
Afferent (GSA)]
[General Visceral
Afferent (GVA)]
Motor
(Efferent)
somatic motor
visceral motor
[General Somatic
Efferent (GSE)]
[General Visceral
Efferent (GVE)]
Somatic
Nervous
System
Autonomic
Nervous
System
Somatic vs. Autonomic
Nervous Systems
• The ANS differs from the SNS in the
following three areas
– Effectors
– Efferent pathways
– Target organ responses
Somatic vs. Autonomic Systems:
Effector
• The effectors of the SNS are skeletal
muscles
• The effectors of the ANS are cardiac
muscle, smooth muscle, and glands
Somatic vs. Autonomic Systems:
Efferent Pathways
• Heavily myelinated axons of the somatic motor
neurons extend from the CNS to the effector
• Axons of the ANS are a two-neuron chain
– The preganglionic (first) neuron has a lightly
myelinated axon
– The ganglionic (second) neuron extends to an
effector organ
Somatic vs. Autonomic
Systems
Divisions of the ANS
• Sympathetic division (thoracolumbar,“fight or flight”)
– Thoracic and lumbar segments
• Parasympathetic division
(craniosacral, “rest and repose”)
– Preganglionic fibers leaving the brain and sacral segments
•
Enteric nervous system (ENS)
– May work independently
Sympathetic and Parasympathetic
• Often they have opposing effects
• May work independently
• May work together each one controlling one
stage of the process
Overview of ANS
Functional Differences
Sympathetic
• “Fight or flight”
• Catabolic (expend energy)
Parasympathetic
• “Feed & breed”, “rest
digest”
• Homeostasis
&
» Dual innervation of many
organs — having a brake and an
accelerator provides more
control
Overview of the Autonomic Nervous System
Similarities between Sympathetic & Parasympathetic
• Both are efferent (motor) systems: “visceromotor”
• Both involve regulation of the“internal”environment generally outside of our
conscious control: “autonomous”
• Both involve 2 neurons that synapse in a peripheral ganglion
• Innervate glands, smooth muscle, cardiac muscle
glands
ganglion
CNS
smooth
muscle
preganglionic
neuron
postganglionic
neuron
cardiac
muscle
Overview of the Autonomic Nervous System
Differences between Sympathetic & Parasympathetic
Location of Preganglionic Cell Bodies
Sympathetic
Parasympathetic
Thoracolumbar
Craniosacral
T1 – L2/L3 levels
of the spinal cord
Brain: CN III, VII, IX, X
Spinal cord: S2 – S4
Overview of the Autonomic Nervous System
Differences between Sympathetic & Parasympathetic
Relative Lengths of Neurons
Sympathetic
CNS
target
ganglion
short preganglionic
neuron
long postganglionic
neuron
Parasympathetic
ganglion
CNS
long preganglionic
neuron
target
short postganglionic
neuron
Overview of the Autonomic Nervous System
Differences between Sympathetic & Parasympathetic
Neurotransmitters
NE (ACh at sweat glands),
Sympathetic
ACh, +
+ / -, α & ß receptors
• All preganglionics release acetylcholine (ACh) & are excitatory (+)
• Symp. postgangl. — norepinephrine (NE) & are excitatory (+) or inhibitory (-)
• Parasymp. postgangl. — ACh & are excitatory (+) or inhibitory (-)
• Excitation or inhibition is a receptor-dependent & receptor-mediated response
Parasympathetic
Potential for pharmacologic
modulation of autonomic responses
ACh, +
ACh, + / muscarinic receptors
Overview of the Autonomic Nervous System
Differences between Sympathetic & Parasympathetic
Target Tissues
Sympathetic
Parasympathetic
• Organs of head, neck,
• Organs of head, neck,
trunk, & external genitalia
trunk, & external genitalia
• Adrenal medulla
• Sweat glands in skin
• Arrector muscles of hair
• ALL vascular smooth muscle
» Sympathetic system is distributed to essentially all
tissues (because of vascular smooth muscle)
» Parasympathetic system never reaches limbs or body wall
(except for external genitalia)
Sympathetic division anatomy
• Preganglionic neurons between segments T1 and L2 –
lateral gray horn of spinal cord
• Preganglionic fibers
– Short
– Travel in the ventral root and spinal nerve
• Ganglionic neurons in ganglia near vertebral column
– Specialized neurons in adrenal glands
• Postganglionic fibers
– Long fibers
Sympathetic ganglia
• Sympathetic chain ganglia
(paravertebral ganglia)
– Typically there are 23 ganglia – 3 cervical, 11
thoracic, 4 lumbar, 4 sacral,
and 1 coccygeal
• Collateral ganglia
(prevertebral ganglia)
• Adrenal medulla
Structure of spinal nerves: Somatic pathways
dorsal
ramus
dorsal root
ganglion
dorsal root
spinal
nerve
dorsal
horn
somatic
sensory
nerve
CNS
interneuron
(GSA)
ventral
horn
ventral
ramus
ventral root
Mixed Spinal
Nerve
gray ramus
communicans
sympathetic
ganglion
somatic
motor
nerve
(GSE)
white ramus
communicans
Structure of spinal nerves: Sympathetic pathways
dorsal
ramus
intermediolateral
gray column
spinal
nerve
ventral
ramus
gray ramus
communicans
sympathetic
ganglion
white ramus
communicans
Organization and anatomy of the
sympathetic division
• Segments T1-L2, ventral roots give rise to
myelinated white ramus
• Leads to sympathetic chain ganglia
Postganglionic fibers of the
sympathetic ganglia
• Some fibers will return to the spinal nerve
through a gray ramus and will innervate skin,
blood vessels, sweat glands, adipose tissue,
arrector pili muscle
• Some fibers will form sympathetic nerves that
will innervate thoracic organs
– Go directly to innervate the thoracic organs
Sympathetic System: Postganglionic Cell Bodies
1. Paravertebral ganglia
• Located along sides of vertebrae
• United by preganglionics into Sympathetic Trunk
• Preganglionic neurons are thoracolumbar (T1–L2/L3)
postganglionic neurons are cervical to coccyx
• Some preganglionics ascend or descend in trunk
but
Paravertebral
ganglia
sympathetic
trunk (chain)
synapse at
same level
Prevertebral
ganglia
• celiac ganglion
• sup. mesent. g.
• inf. mesent. g.
ascend to
synapse at
higher level
descend to
synapse at
lower level
aorta
Moore’s COA5 2006
Collateral (prevertebra) ganglia
• Preganglionic fibers will pass through the
sympathetic chain without synapsing
• Preganglionic fibers will synapse within
collateral ganglia (prevertebra ganglia)
– Splanchnic nerves will synapse on one of the 4
collateral ganglions
Collateral (prevertebra) ganglia
• Celiac ganglion
– Postganglionic fibers innervates stomach, liver, gall bladder,
pancreas, spleen
• Superior mesenteric ganglion
– Posganglinic fibers innervates small intestine and initial portion of
large intestine
• Inferior mesenteric ganglion
– Postganglionic fibers innervate the final portion of large intestine
• Inferior hypogastric
– Posganglionic fibers innervates urinary bladder , sex organs
Sympathetic System: Postganglionic Cell Bodies
2. Prevertebral (preaortic) ganglia
• Located anterior to abdominal aorta, in plexuses
surrounding its major branches
• Preganglionics reach prevertebral ganglia via
abdominopelvic splanchnic nerves
Paravertebral
ganglia
sympathetic
trunk (chain)
Prevertebral
ganglia
abdominopelvic
splanchnic
nerve
• celiac ganglion
• sup. mesent. g.
• inf. mesent. g.
• inf. hypogastric
aorta
Moore’s COA5 2006
Adrenal medulla
• Preganglionic fibers will pass through
sympathetic ganglia without synapsing
• Preganglionic fibers will synapse on adrenal
medulla
• Adrenal medulla will secrete
– Epinephrine
– Norepinephrine
Adrenal medulla
• Neurotransmitter will go into general
circulation
– Their effects last longer than those produced
by direct sympathetic innervation
Adrenal gland is exception
• Synapse in gland
• Can cause body-wide release of
epinephrine (adernalin) and
norepinephrine in an extreme
emergency
(adrenaline “rush” or surge)
29
Sympathetic System: Summary
visceral tissues
(organs)
Cardiopulmonary Splanchnics:
postganglionic fibers to thoracic
viscera
somatic tissues
(body wall, limbs)
T1
postganglionics
via 31 spinal nerves
to somatic tissues of
neck, body wall, and
limbs
sympathetic
trunk
Moore’s COA5 2006
Abdominopelvic Splanchnics:
preganglionic fibers to
prevertebral ganglia,
postganglionic fibers to
abdominopelvic viscera
L2
prevertebral
ganglia
Role of the
Sympathetic Division
• The sympathetic division is the “fight orflight” system
• Involves E activities – exercise, excitement,
emergency, and embarrassment
• Promotes adjustments during exercise–
blood flow to organs is reduced, flow to
muscles is increased
Role of the
Sympathetic Division
• 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
Parasympathetic division
(craniosacral division)
• Preganglionic neurons in the brainstem(nuclei of
cranial nerves III, VII, IX, X) and sacral segments
of spinal cord (S2-S4)
• Ganglionic neurons in peripheral ganglia located
within or near target Organs
– Terminal ganglion
– Intramural ganglion
Parasympathetic Division
Outflow
Parasympathetic
Pathways
Cranial outflow
• CN III, VII, IX, X
• Four ganglia in head
• Vagus nerve (CN X) is major
preganglionic parasymp.
supply to thorax & abdomen
• Synapse in ganglia within wall of the
target organs (e.g.,enteric plexus of
GI tract)
Sacral outflow
• S2–S4 via pelvic splanchnics
• Hindgut, pelvic viscera, and
external genitalia
Clinical Relevance
» Surgery for colorectal cancer
puts pelvic splanchnics at risk
» Damage causes bladder & sexual
dysfunction
Moore’s COA5 2006
Parasympathetic activation
• Effects produced by the parasympathetic division
–
–
–
–
–
–
–
Relaxation
food processing
energy absorption
Pupil constriction
Constriction of respiratory passageway
Decrease heart rate and blood pressure
Stimulates defecation and urination
Referred Pain
• Pain stimuli arising from the viscera are
perceived as somatic in origin
• This may be due to the fact that visceral
pain afferents travel along the same
pathways as somatic pain fibers
Visceral Afferents and Referred Pain
dorsal root ganglion
Visceral sensory nerves [GVA]
• run with sympathetic nerves
• cell bodies in dorsal root ganglion
• nerve ending in viscera
Somatic sensation:
• conscious, sharp, well-localized
• touch, pain, temperature, pressure, proprioception
Visceral sensation:
• 5-8% of the total afferent input to spinal cord
• often unconscious; if conscious: dull, poorly-localized
• distension, blood gas, blood pressure, cramping, irritants
Visceral Afferents and Referred Pain
Referred Pain:
• Pain originating in a visceral structure
perceived as being from an area of skin
innervated by the same segmental level as the
visceral afferent
• Results from convergence of somatic &
visceral afferents on the same segmental level
of the spinal cord
• “Cross-talk” in the dorsal horn
somatic afferent
convergence &
“cross-talk”
www.merck.com
visceral afferent
Kandel et al. 2000
Visceral Afferents and Referred Pain
Maps of Referred Pain
Grant’s Atlas 11 2005
Interactions of the Autonomic
Divisions
• Most visceral organs are dual-innervated
– both sympathetic and parasympathetic fibers
 dynamic antagonisms that precisely control visceral
activity
• Sympathetic fibers increase heart and respiratory rates, and
inhibit digestion and elimination.
• Parasympathetic fibers decrease heart and respiratory rates,
and allow for digestion and the discarding of wastes
Cooperative Effects of
Symp. and Parasym.
• Example: control of external genitalia
– Parasympathetic fibers:
• vasodilation  erection of the penis and clitoris
– Sympathetic fibers
• cause ejaculation of semen in males and reflex
contraction of a female vagina
Unique Roles of the
Sympathetic Division
• Regulates many functions not subject to parasympathetic
influence
• These include the activity of 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
• Raises blood glucose levels
• Mobilizes fat as a food source
• Stimulates the reticular activating system (RAS) of the brain,
increasing mental alertness
Localized Versus Diffuse Effects
• The parasympathetic division exerts shortlived, highly localized control
• The sympathetic division exerts longlasting, diffuse effects
Central control of the
Autonomic NS
Amygdala: main limbic
region for emotions
-Stimulates sympathetic activity,
especially previously learned
fear-related behavior
-Can be voluntary when decide to
recall frightful experience cerebral cortex acts through
amygdala
-Some people can regulate some
autonomic activities by gaining
extraordinary control over their
emotions
Hypothalamus: main
integration center
Reticular formation: most
direct influence over
autonomic function
45
Hypothalamic Control
• Centers of the hypothalamus control:
– Heart activity and blood pressure
– Body temperature, water balance,and endocrine
activity
– Emotional stages (rage, pleasure) and biological
drives (hunger, thirst, sex)
– Reactions to “fear” and the “fight or-flight”
system
Neural innervation of bowel
• Extrinsic set of nerves: Autonomic nervous system
• Parasympathetic
• Sympathetic
•
–
–
–
Intrinsic set of nerves: Enteric nervous system (ENS)
~108 neurons - similar to spinal cord  “brain of gut”
Neurons extending from esophagus to anus
2 plexuses
• Myenteric plexus
• Submucosal plexus
Neuroanatomy of ENS
Intrinsic Nervous System
• Myenteric plexus (Auerbach)
– Located between the longitudinal and circular layers of muscle in
the tunica muscularis
– Controls tonic and rhythmic contractions
– Exerts control primarily over digestive tract motility
• Submucosal plexus (Meissner)
–
–
–
–
Buried in the submucosa
Senses the environment within the lumen
Regulates GI blood flow
Controls epithelial cell function (local intestinal secretion and
absorption)
– May be sparse or missing in some parts of GI tract
Intrinsic Nervous System
•
3 types of neurons in enteric system (80-100
million= spinal cord)
1. Sensory neurons
–
–
Chemoreceptors (fibers) sensitive to acid, glucose and amino
acids have been demonstrated which, in essence, allows
"tasting" of lumenal contents.
Sensory receptors (fibers) in muscle respond to stretch and
tension
2. Motor neurons
• Control GI motility and secretion, and possibly
absorption
3. Interneurons
• Largely responsible for integrating information
from sensory neurons and providing it to motor
neurons
Interstitial cell of Cajal (ICC) :
Pacemaker cell of GI tract
• Network within the muscularis
propria
• Specialized mesenchymal cells
• Generate spontaneous electrical
activity
J Physiol 2001; 531: 827
Slow Waves in GI smooth muscle
Resting
Acetylcholine
spikes
-3/min in stomach,
-12/min small intestine
Gap junction
5-10m
200m