Physiology of ANS Lecture 3 by Dr. Mudassar Ali Roomi

U N I T XI

Textbook of Medical Physiology, 11th edition

GUYTON & HALL

Chapter 60: lecture3The Autonomic Nervous System (ANS);

The Adrenal Medulla

ByDr. Mudassar Ali Roomi (MBBS, M.Phil)

Assistant Professor Physiology

Function of the Adrenal Medulla• Adrenal medulla is a large sympathetic ganglion• when stimulated releases epinephrine (80%) and norepinephrine (20%) into the blood

stream• causes prolonged activity of the substances (2-4 minutes)• helps the body deal with stress• Difference b/w action of epinephrine and nor-epinephrine:

– Epinephrine has greater effect on cardiac stimulation– epinephrine raises the arterial pressure to a lesser extent but increases the cardiac output more– Epinephrine has 5 to 10 times as great a metabolic effect as norepinephrine

• Importance of adrenal medulla: 1. the dual mechanism of sympathetic stimulation provides a safety factor, one mechanism

substituting for the other if it is missing.2. to stimulate structures of the body that are not innervated by direct sympathetic fibers

Sympathetic and Parasympathetic “Tone”

• the basal rate of activity of each system• Importance: this background activity allows for an

increase or decrease in activity of an organ by a single system – sympathetic tone normally causes about a 50 %

vasoconstriction (so, SANS has more effect on vessels)• increasing or decreasing “tone” can change vessel diameter

– parasympathetic tone provides background G.I. activity (so, PANS has more effect on GIT)

• Mechanism of Denervation Supersensitivity: “up-regulation” of the receptors

Autonomic Reflexes

• Cardiovascular Autonomic Reflexes• Gastrointestinal Autonomic Reflexes• Other Autonomic Reflexes

• Sympathetic System Often Responds by Mass Discharge

• Parasympathetic System Usually Causes Specific Localized Responses.

Stress Response (fight or flight response)• mass sympathetic discharge

– increase in arterial pressure, heart rate and contractility, blood flow to muscles, blood glucose, metabolic rate, muscle strength, mental activity, blood coagulation

• prepares the body for vigorous activity need to deal with a life-threatening situation

1. Increased arterial pressure2. Increased blood flow to active muscles concurrent with decreased blood flow to

organs such as the gastrointestinal tract and the kidneys that are not needed for rapid motor activity

3. Increased rates of cellular metabolism throughout the body4. Increased blood glucose concentration5. Increased glycolysis in the liver and in muscle6. Increased muscle strength7. Increased mental activity8. Increased rate of blood coagulation

Pharmacology of the Sympathetic Nervous System

• adrenergic or sympathomimetic drugs act like norepi and epi

• these drugs have an effect which is much more prolonged than that of either norepi or epi– phenylephrine stimulates alpha receptors– isoproterenol stimulates both beta1 and beta2 receptors

– albuterol stimulates only beta2 receptors

• some drugs act indirectly by increasing the release of norepi from its storage terminals– ephedrine, tyramine, and amphetamine

Pharmacology of the Sympathetic Nervous System

• drugs that block the effect of norepi and epi• synthesis and storage

– reserpine• release from the nerve terminal

– guanethidine• alpha blockers

– phentolamine and phenoxybenzamine• beta blockers

– beta1 and 2 - propranolol, beta1 - metoprolol• ganglionic blockers

– hexamethonium

Pharmacology of the Parasympathetic Nervous System

• parasympathomimetic drugs– nicotine

• activates nicotinic receptors (nicotinic agonist)– pilocarpine and methacholine (muscarinic agonist)

• activates muscarinic receptors, cause profuse sweating• cholinesterase enzyme inhibitors

– neostigmine, pyridostigmine,and ambenonium• potentiates the effect of acetylcholine

• antimuscarinic drugs– atropine and scopolamine (muscarinic antagonist)

• blocks the effect of acetylcholine on effector cells

Some Disorders of the Autonomic Nervous System

Horner syndrome :• Chronic unilateral pupillary constriction (miosis), sagging of the eyelid (ptosis),

withdrawal of the eye into the orbit, flushing of the skin, and lack of facial perspiration,

• resulting from lesions in the cervical ganglia, upper thoracic spinal cord, or brainstem that interrupt sympathetic innervation of the head.

Raynaud disease : • Intermittent attacks of paleness, cyanosis, and pain in the fingers and toes, caused

when cold or emotional stress triggers excessive vasoconstriction in the digits; • most common in young women. • In extreme cases, causes gangrene and may require amputation. Sometimes

treated by cutting sympathetic nerves to the affected regions.