Human A&P I - Autonomic nervous system

I. What does the autonomic nervous system control?

A. involuntary or visceral motor nervous system: controls internal

homeostasis
1. motor fibers to smooth and cardiac muscles and glands
2. efferent pathway to effectors is 2 neuron chain
a. preganglionic (myelinated) neuron cell body in CNS,

synapses in an autonomic ganglion with …

b. postganglionic neuron cell body in a ganglion in PNS
c. many are incorporated into spinal or cranial nerves for

most of their course
3. lightly myelinated or none for slow conduction
4. acetylcholine and norepinephrine may cause excitation or

inhibition

5. Usually functions even if nerve supply is interrupted

6. Has input from autonomic sensory neurons, usually

interoceptors (e.g. those that monitor carbon dioxide levels and

stretch of blood vessels)

B. overlap of somatic (innervates skeletal muscles) and autonomic

function
1. spinal and some cranial nerves carry both fibers

C. Motor part of ANS has two principal parts

1. Parasympathetic: "rest and digest" (Craniosacral division)

a. conserves body energy and maintains basal levels

b. pupil constriction, gland secretion
c. increased digestive tract mobility, smooth muscle motility

leading to elimination of wastes, both feces and urine
2. Sympathetic nervous system: "fight or flight", emergency

conditions (Thoracolumbar division)

a. dilate pupils
b. increase heart rate, increased blood pressure
c. increased blood glucose levels, sweating
d. increased respiratory rates, bronchioles dilate
e. shunts blood from the skin and digestive viscera to heart,

brain and skeletal muscles
f. vasoconstriction during exercise, vessels of heart and

muscles dilated
g. thermoregluatory responses
h. renin release by kidneys, metabolism

3. Most organs have dual innervation; fibers from both division

supply the organ

II. Anatomy of Autonomic Nervous System

A. Sympathetic & parasympathetic divisions are distinguished by…

1. Origin site of nerves

2. Different lengths of the axons

3. Location of the ganglia

B. Autonomic Ganglia – there are 3 types

1. Sympathetic trunk (chain) ganglia

a. Lie in a vertical row just lateral to vertebral column on

both sides

b. Run from base of skull to coccyx

c. Sympathetic preganglionic axons are short

d. Usually innervate regions above diaphragm

2. Prevertebral ganglia

a. Lie anterior to spinal column & near abdominal arteries

b. Innervate regions below diaphragm

3. Parasympathetic ganglia

a. Preganglionic fibers of parasympathetic division synapse

with postganglionic neurons in terminal ganglia

b. Located close to or on effector organ àparasympathetic

preganglionic axons are longer than sympathetic

preganglionic neurons

C. Parasympathetic nervous system: craniosacral division
1. long preganglionic neurons from brain stem and sacral spinal

cord
2. short postganglionic fibers
3. synapse in terminal ganglia near or within visceral effector

organs
4. preganglionic fibers from cranial nerves III, VII, IX, and

postganglionic fibers with cranial nerve X
a. oculomotor nerves (III) to smooth muscles to eye for pupil

constriction and adjust lens for focusing
b. facial nerves (VII) to nasal, lacrimal, and salivary glands
c. glossopharyngeal nerves (IX) to parotid salivary glands
d. vagus nerve (X) serves all organs of thoracic and

abdominal cavities
5. sacral fibers (S₂-S₄) go through ventral roots of spinal nerves to

ventral rami to form pelvic splanchnic nerves that go to pelvic

viscera

D. Sympathetic nervous system: thoracolumbar division
1. preganglionic fibers from lateral horn of spinal cord (T₁ to L₂),

leave by ventral root through white rami communicantes

2. preganglionic fibers are short, postganglionic fibers are long
3. postganglionic fibers through the gray rami to skin and blood

vessels
4. may synapse with paravertebral chain ganglion, near spinal cord

at the same or a different level
a. serves skin and blood vessels
b. dilates eyes
c. inhibits nasal and salivary glands
d. carotid body and sinus
e. larynx and pharynx
f. cardiac nerves to heart
5. may form splanchnic nerves (thoracic, lumbar, sacral)
a. synapse in prevertebral ganglia, goes through chain

ganglia without synapsing
b. most postganglionic fibers serve abdominal viscera
c. some synapse with cells of adrenal medulla

III. visceral reflex arcs

A. same components as somatic reflexes, except two efferent neurons are

involved

B. Used to send information to the brain about chemical changes, stretch,

irritation of the viscera

C. referred pain: visceral pain afferents travel with somatic pain fibers
pain in viscera may be perceived as somatic, ex) heart attack

IV. Physiology of Autonomic Nervous system

A. neurotransmitters
1. cholinergic neurons release acetylcholine (ACh)
a. released by all preganglionic fibers and all

parasympathetic postganglionic fibers
b. local effect, short lived
2. adrenergic neurons release norepinephrine (NE)
a. released by most sympathetic postganglionic fibers
b. overall effects, longer acting

B. neuro receptors (transmembrane proteins on effector cell or

postsynaptic neuron)
1. Cholinergic (ACh) receptors
a. nicotinic: skeletal muscle, all postganglionic neurons,

adrenal medulla, always excites
b. muscarinic: all postganglionic effectors inhibits heart,

increases GI tract motility

2. Adrenergic (NE) receptors
a. alpha 1 and 2: stimulates
b. beta 1, 2, and 3: inhibits (except beta cardiac stimulates)

C. effects of drugs
1. mimic: ex) nicotine and muscatine mimic acetylcholine
2. enhance (agonists)
a. neostigmine (anti-AChase) prevents breakdown of ACh,

used to treat myasthenia gravis
b. tricyclic antidepressants (elavil) prolong activity of NE
3. inhibit (antagonists)

a. atropine (anti-Ach) blocks parasympathetic secretions,

used to dilate pupils
b. beta blockers reduce heart rate and prevent arrhythmias
c. alpha blockers treat high blood pressure

D. interactions of Autonomic divisions
1. most visceral organs are innervated by both sympathetic and

parasympathetic systems
2. some have only sympathetic function
a. most blood vessels are innervated only by sympathetic

fibers, vasomotor tone
b. sweat glands, adrenal medulla, arrector pilli, kidneys
c. body temperature control
d. release renin from kidneys to promote increase blood

pressure, maintains fluid balance
e. enhance metabolic rate
f. increase blood glucose levels
g. mobilize fat for fuel
h. increased alertness from RAS
i. contract skeletal muscle for stronger, quicker contractions
3. parasympathetic activity dominates heart and muscles of GI tract

and glands
4. both systems affect external genitalia
a. parasympathetic for erection
b. sympathetic for ejaculation
5. sympathetic: wide spread, long lasting effects
6. parasympathetic: localized, short-lived responses

E. control of autonomic functioning
1. CNS controls reflex activity at subconscious level
2. hypothalamus: coordinated ANS, Somatic and endocrine

responses
3. cortical centers influence ANS by connections with limbic system
4. conscious control of ANS by meditation and biofeedback

IV. Homeostatic imbalances of ANS

A. problems with smooth muscle control

B. abnormalities in vascular control
1. hypertension (high blood pressure): Overactive sympathetic

vasoconstrictor response

2. Raynaud's disease:
a. intermittent attacks where skin of fingers lose circulation

due to cold or emotional stress

b. Fingers become pale, cyanotic, & painful
c. treatment: sever preganglionic sympathetic nerves,

restores circulation

3. mass reflex reaction: with quadriplegia, epilepsy of the spinal

cord, episodes of mass reflex (a.k.a. Autonomic Dysreflexia)

a. Life-threatening condition caused by painful stimulus to

organs or skin

b. Uncontrolled activation of both autonomic & somatic

motor neurons common in spinal injuries above T6

c. Increased sweating & blood pressure (+220 mm Hg),

emptying of bowel & bladder

C. Hirschsprung's disease (Congenital megacolon)

1. Parasympathetic innervation of the distal part of the large

intestine fails to develop normally

2. Feces accumulates proximal to inactive bowel, becomes

distended, corrected by surgical removal

D. efficiency of ANS declines in old age
1. decreased glandular secretions
2. decreased gastrointestinal motility
3. slowed sympathetic vasomotor responses to changes in position