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

attribute

embryological origin of tissue examples of adult tissues perception Somatic vs. Visceral

Somatic System

“body wall:” somatic (parietal) mesoderm (dermatome, myotome) dermis of skin, skeletal muscles, connective tissues conscious, voluntary

Visceral System

“organs:” splanchnic (visceral) mesoderm, endoderm glands, cardiac muscle, smooth muscle unconscious, involuntary

Sensory (Afferent) Motor (Efferent)

Sensory/Motor + Somatic/Visceral

Somatic

somatic sensory

[General Somatic Afferent (GSA)]

somatic motor

[General Somatic Efferent (GSE)]

Visceral

visceral sensory

[General Visceral Afferent (GVA)]

visceral motor

[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

CNS ganglion preganglionic neuron postganglionic neuron

glands smooth muscle cardiac muscle

Overview of the Autonomic Nervous System Differences between Sympathetic & Parasympathetic

Location of Preganglionic Cell Bodies Sympathetic

Thoracolumbar T1 – L2/L3 levels of the spinal cord

Parasympathetic

Craniosacral 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 ganglion target

short preganglionic neuron

Parasympathetic CNS

long postganglionic neuron

ganglion target

long preganglionic neuron short postganglionic neuron

Overview of the Autonomic Nervous System Differences between Sympathetic & Parasympathetic

Neurotransmitters Sympathetic ACh, + NE (ACh at sweat glands) , + / -, 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 ACh, +

Potential for pharmacologic modulation of autonomic responses

ACh, + / muscarinic receptors

Overview of the Autonomic Nervous System Differences between Sympathetic & Parasympathetic

Target Tissues Sympathetic

• Organs of head, neck, trunk, & external genitalia • • Adrenal medulla • Sweat glands in skin • Arrector muscles of hair

ALL

vascular smooth muscle

Parasympathetic

• Organs of head, neck, trunk, & external genitalia » 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 root dorsal root ganglion dorsal ramus dorsal horn spinal nerve

CNS inter neuron somatic sensory nerve (GSA)

ventral horn ventral root

Mixed Spinal Nerve

gray ramus communicans sympathetic ganglion ventral ramus

somatic motor nerve (GSE)

white ramus communicans

Structure of spinal nerves: Sympathetic pathways dorsal ramus intermediolateral gray column spinal nerve gray ramus communicans sympathetic ganglion ventral ramus 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) abdominopelvic splanchnic nerve • Prevertebral ganglia • 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

somatic tissues

(body wall, limbs) postganglionics via 31 spinal nerves to somatic tissues of neck, body wall, and limbs sympathetic trunk

T1 L2

Sympathetic System: Summary

visceral tissues

(organs) Cardiopulmonary Splanchnics: postganglionic fibers to thoracic viscera Abdominopelvic Splanchnics: preganglionic fibers to prevertebral ganglia, postganglionic fibers to abdominopelvic viscera prevertebral ganglia

Moore’s COA5

2006

Role of the Sympathetic Division

• The sympathetic division is the “fight or 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 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

www.merck.com

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” 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 short lived, highly localized control • The sympathetic division exerts long lasting, 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) – ~10 8 neurons - similar to spinal cord – 2 plexuses • Myenteric plexus • Submucosal plexus  “brain of gut” – Neurons extending from esophagus to anus

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

– –

Chemoreceptor acids "tasting

s

(fibers

) sensitive to

acid, glucose and amino

have been demonstrated which, in essence, allows " 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

200

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m 5-10

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