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The Autonomic System
Ching-Liang Lu, M.D.Professor
Institute of Brain ScienceNational 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[General Somatic Afferent (GSA)]
visceral sensory[General Visceral Afferent (GVA)]
Motor
(Efferent)somatic motor[General Somatic Efferent (GSE)]
visceral motor[General Visceral Efferent (GVE)]
SomaticNervousSystem
AutonomicNervousSystem
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 ANSFunctional 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
Somatic vs. AutonomicNervous 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
Overview of the Autonomic Nervous SystemSimilarities 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
CNS ganglion
preganglionicneuron
postganglionicneuron
glands
smoothmuscle
cardiacmuscle
Overview of the Autonomic Nervous SystemDifferences between Sympathetic & Parasympathetic
Location of Preganglionic Cell Bodies
ThoracolumbarT1 – L2/L3 levels of the spinal cord
CraniosacralBrain: CN III, VII, IX,
XSpinal cord: S2 – S4
Sympathetic Parasympathetic
SympatheticCNS ganglion
short preganglionicneuron
long postganglionicneuron
target
ParasympatheticCNS ganglion
long preganglionicneuron
target
short postganglionicneuron
Overview of the Autonomic Nervous SystemDifferences between Sympathetic & Parasympathetic
Relative Lengths of Neurons
Parasympathetic
Overview of the Autonomic Nervous SystemDifferences between Sympathetic & Parasympathetic
Neurotransmitters
ACh, +
NE (ACh at sweat glands),+ / -, α & ß receptors
ACh, + / -muscarinic receptors
• All preganglionics release acetylcholine (ACh) & are excitatory (+)
• Symp. postgangl. — norepinephrine (NE) & are excitatory (+) or inhibitory (-)
• Parasymp. postgangl. — ACh & are excitatory (+) or inhibitory (-)
Sympathetic
• Excitation or inhibition is a receptor-dependent & receptor-mediated response
Potential for pharmacologicmodulation of autonomic responsesPotential for pharmacologicmodulation of autonomic responses
ACh, +
Overview of the Autonomic Nervous SystemDifferences between Sympathetic & Parasympathetic
Target Tissues
ParasympatheticSympathetic
• Organs of head, neck, trunk, & external genitalia
• Organs of head, neck, 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 rootdorsal rootganglion
ventral root
spinalnerve
dorsalramus
ventralramus
dorsalhorn
ventralhorn
somaticsensorynerve(GSA))
somaticmotornerve(GSE)
CNSinter-
neuron
CNSinter-
neuron
Mixed SpinalNerve
Mixed SpinalNerve
gray ramuscommunicans white ramus
communicans
sympatheticganglion
spinalnerve
dorsalramus
ventralramus
gray ramuscommunicans white ramus
communicans
sympatheticganglion
intermediolateralgray column
Structure of spinal nerves: Sympathetic pathways
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 thesympathetic 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
Paravertebralganglia
Prevertebral ganglia
• celiac ganglion• sup. mesent. g.• inf. mesent. g.
aorta
sympathetictrunk (chain)
1. Paravertebral ganglia• Located along sides of vertebrae• United by preganglionics into Sympathetic Trunk• Preganglionic neurons are thoracolumbar (T1–L2/L3) but postganglionic neurons are cervical to coccyx• Some preganglionics ascend or descend in trunk
synapse atsame level
ascend tosynapse at
higher level
descend tosynapse atlower level
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
Paravertebralganglia
Prevertebral ganglia
• celiac ganglion• sup. mesent. g.• inf. mesent. g.• inf. hypogastric
aorta
sympathetictrunk (chain)
2. Prevertebral (preaortic) ganglia• Located anterior to abdominal aorta, in plexuses surrounding its major branches• Preganglionics reach prevertebral ganglia via abdominopelvic splanchnic nerves
Moore’s COA5 2006
abdominopelvicsplanchnic
nerve
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
29
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)
Sympathetic System: Summary
Moore’s COA5 2006
T1
L2
somatic tissues(body wall, limbs)
visceral tissues(organs)
postganglionicsvia 31 spinal nervesto somatic tissues of neck, body wall, and
limbs
sympathetictrunk
prevertebralganglia
Cardiopulmonary Splanchnics: postganglionic fibers to thoracic
viscera
Abdominopelvic Splanchnics: preganglionic fibers to prevertebral ganglia,
postganglionic fibers to abdominopelvic viscera
Role of the Sympathetic Division
• The sympathetic division is the “fightor-flight” 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
ParasympatheticPathways
Moore’s COA5 2006
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
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
Somatic sensation:• conscious, sharp, well-localized• touch, pain, temperature, pressure, proprioception
Visceral sensation:• often unconscious; if conscious: dull, poorly-localized• distension, blood gas, blood pressure, cramping, irritants
dorsal root ganglion
Visceral sensory nerves [GVA]• run with sympathetic nerves• cell bodies in dorsal root ganglion• nerve ending in viscera
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
visceral afferent
convergence &“cross-talk”
Kandel et al. 2000
www.merck.com
Interactions of the AutonomicDivisions
• 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
• 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 short-lived, highly localized control
• The sympathetic division exerts long-lasting, diffuse effects
45
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
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• Autonomic nervous system
– Extrinsic set of nerves• Parasympathetic• Sympathetic
• Enteric nervous system (ENS)– Intrinsic set of nerves – ~108 neurons - similar to spinal cord “brain of
gut”– Neurons extending from esophagus to anus– 2 plexuses
• Myenteric plexus• Submucosal plexus
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 system1. Sensory neurons (5 types)
– Chemoreceptors sensitive to acid, glucose and amino acids have been demonstrated which, in essence, allows "tasting" of lumenal contents. Sensory receptors in muscle respond to stretch and tension
2. Motor neurons• Control GI motility and secretion, and possibly
absorption3. Interneurons
• Largely responsible for integrating information from sensory neurons and providing it to motor neurons