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

Somatic vs. Autonomic Systems

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

Parasympathetic DivisionOutflow

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

Grant’s Atlas 11 2005

Visceral Afferents and Referred Pain

Maps of Referred Pain

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

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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

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 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

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