Autonomic Nervous System

Nestor T. Hilvano, M.D., M.P.H.

Learning Objectives

1. State the divisions of the ANS and give its importance.

2. Compare the organization of ANS and SNS.3. Discuss the sympathetic division of ANS noting

structures and functions, and describe the neurotransmitter release and its effects on target organs.

4. Discuss the parasympathetic division of ANS noting structures and functions, and describe the neurotransmitter release and its effects on target organs.

5. Define a visceral reflex and its importance.

Somatic versus Autonomic N.S.

• Somatic N.S.- conscious control - effectors: skeletal m. - 1 motor neuron from CNS

• Autonomic N.S. - subconscious control - effectors: viscera - 2 motor neuron from CNS

Division of Autonomic N. S. • Sympathetic

– prepares body for physical activity– increases alertness, metabolic rate, and muscular

abilities– Preganglionic fibers arising from Lateral Gray Horns of

T1- L2 segment of spinal cord: ________– Preganglionic fibers are short (ACh); postganglionic

fibers are long (NE, sometimes NO or Ach) • Parasympathetic

– calms body functions and assists in bodily maintenance – reduces metabolic rate and promotes digestion; anabolic

system – Preganglionic fibers originate from brain stem and sacral

segment of spinal cord: ___________– Preganglionic fibers are long (Ach); postganglionic fibers

are short (Ach)

Sympathetic Nervous System• Preganglionic neurons = anterior gray horns of SC. • Ganglionic neurons occurs in 3 locations

1. Sympathetic chain ganglia

= also called paravertebral

= target organs in _______ cavity

2. Collateral ganglia

= or prevertebral

= via splanchnic nerves

= target organs in _____ cavity

3. Adrenal medulla

= release neurotransmitters into

the ____________.a. Abdominopelvic b. thoracic c. blood circulation

Sympathetic: Adrenergic Receptors • NE binds to 2 classes of receptors

– alpha adrenergic receptors – beta adrenergic receptors

• Exceptions - existence of subclasses of each receptor type

• alpha 1 excitatory (vasoconstriction; closure of sphincter along digestive tract and urinary bladder)

• alpha 2 inhibitory (neuromuscular and neuroglandular junction)

• beta 1 stimulates metabolic activity (increase HR), beta 2 causes inhibition (bronchodilatation), and beta 3 leads to lipolysis.

• Nitrous oxide – b.v. in skeletal muscles and the brain (vasodilatation and increase blood flow)

Parasympathetic Nervous System

• Origin of preganglionic fibers– pons and medulla (cranial nerve nuclei)

and sacral spinal cord segments S2-S4

• Pathways of preganglionic fibers– cranial nerves III, VII, IX and X– arising from sacral spinal cord

• pelvic splanchnic nerves and inferior hypogastric plexus

• Terminal ganglia in or near target organs – long preganglionic, short postganglionic fibers

Cholinergic Receptors• ACH binds to 2 classes of receptors

1.nicotinic receptors• on all ANS postganglionic neurons (autonomic

ganglia), in the adrenal medulla, and neuromuscular junctions of somatic n.s.

• excitatory when ACh binding occurs – Na+ channel open (ex: muscular contraction)

2.muscarinic receptors• on all gland, smooth muscle and cardiac muscle

cells that receives cholinergic innervation (neuromuscular or neuroglandular junctions)

• excitatory or inhibitory (variable response) – K+ channel open (ex: bronchoconstriction, decrease HR)

Dual Innervation• Viscera receive nerve fibers from both

parasympathetic and sympathetic divisions• Antagonistic effects

- pupils constict (PS) and dilate (S)

• Cooperative effects seen when 2 divisions act on different effectors to produce a unified effect- erection (PS) and ejaculation (S)

cont. Innervation & Autonomic Tone

• Some effectors receive only sympathetic– adrenal medulla, arrector pili muscles, sweat

glands and many blood vessels (dilatation to skin, skeletal m, heart, lungs, brain; and constriction to digestive viscera, kidneys)

• Autonomic tone – Autonomic motor neurons show a resting

level of spontaneous activity– vasomotor tone provides partial constriction

• increase in firing frequency = vasoconstriction• decrease in firing frequency = vasodilation• can shift blood flow from one organ to another as

needed

Functional Comparison of ANS

Organ SYMPATHETIC PARASYMPATHETIC

Pupil dilatation constriction

Heart (HR/BP) increase decrease

RR/passages / dilatation ↓ / constriction

Digestive (glands, motility)

Inhibition, decrease stimulation, increase

Urinary bladder inhibitory stimulatory

Penis ejaculation erection

Sphincter constriction relaxation

* Refer to Table in textbook

Visceral Reflexes

• Unconscious, automatic responses to stimulation

1. Receptors – detect internal stimuli -- stretch, blood chemicals, etc.

2. Afferent neurons – connect to interneurons in the CNS

3. Efferent neurons – carry motor signals to effectors

4. Effectors – glands, smooth or cardiac muscle

Homework (Self Review) 1. Define key terminologies: lateral gray horn, collateral

ganglia, sympathetic chain ganglia, intramural ganglia, nicotinic receptor, muscarinic receptor, adrenergic receptor, antagonistic effect, cooperative effect

2. Differentiate sympathetic from parasympathetic division of the ANS as to structure and function.

3. Discuss why a beta blocker drug is given to person with hypertension to correct the condition.

4. State the given responses if it is a sympathetic or parasympathetic effect. a) stimulate sweating, b) constriction of pupils c) stimulate urination and defecation, d) increase heart rate and respiratory rate, e) vasodilatation of skin, f) bronchoconstriction of respiratory tract, g) increase metabolic rate and alertness, h) increase salivary and digestive glands secretion