Guyton Chapter 60 - The Autonomic Nervous System And Adrenal Medulla

Question 1

What is the autonomic system activated mainly by?

Answer 1

Centres located in the spinal cord, brain stem and hypothalamus

Question 2

In what ways are the sympathetic nerves different from skeletal motor fibres?

Answer 2

Each sympathetic pathway from the cord to the stimulated tissue is composed of 2 neurons: a preganglionic neuron and a postganglionic neuron

Question 3

Where does the cell body of each preganglionic neuron lie?

Answer 3

In the intermediolateral horn of the spinal cord. It’s fibre passes through an anterior root of the cord into the corresponding spinal nerve.

Question 4

Describe the sympathetic nerve endings in the adrenal medulla.

Answer 4

Preganglionic sympathetic nerve fibres pass without synapsing from the intermediolateral horn cells of the spinal cord through the sympathetic chains then through the splanchnic nerves and finally into the adrenal medulla. There they end directly on modified neuronal cells that secrete epinephrine and norepinephrine into the blood stream.

Question 5

Describe the segmental distribution of the sympathetic nerve fibers.

Answer 5

Sympathetic fibers from cord segment T1 generally pass up the sympathetic chain to terminate in the head.

From T2 to terminate in the neck

From T3, T4, T5 and T6 into the thorax

T7-T11 into the abdomen

T12, L1 and L2 into the legs

Question 6

Through which cranial nerves does the parasympathetic fibers leave?

Answer 6

Cranial nerves III, VII, IX and X

Question 7

Which 2 substances does the sympathetic and parasympathetic nerve fibres secrete?

Answer 7

Acetylcholine (cholinergic) or nepinephrine (adrenergic)

Question 8

Are the preganglionic cholinergic or adrenergic in the sympathetic and parasympathetic nervous system?

Answer 8

Cholinergic

Question 9

How will acetylcholine or acetylcholine like substances affect sympathetic and parasympathetic postganglionic neurons when applied to the ganglia?

Answer 9

It will excite both sympathetic and parasympathetic postganglionic neurons

Question 10

Are the postganglionic neurons of the parasympathetic system adrenergic or cholinergic?

Answer 10

Cholinergic

Question 11

Are the postganglionic sympathetic neurons adrenergic or cholinergic?

Answer 11

Adrenergic

Question 12

What does the terminal nerve endings of the parasympathetic system secrete?

Answer 12

Acetylcholine

Question 13

What does the sympathetic nerve endings secrete?

Answer 13

Norepinephrine

Question 14

What are varicosities?

Answer 14

Transmitted vesicles of acethylcholine and norepinephrine are synthesised and stored here.

Question 15

Describe the secretion of acethylcholine and norepinephrine by postganglionic nerve endings:

Answer 15

When an action potential spreads over the terminal fibers, the depolarisation process increases the permeability of the fiber membrane to calcium ions allowing these ions to diffuse into the nerve terminals or nerve varicosities. The calcium ions in turn cause the terminals or varicosities to empty their contents to the exterior and that transmitter substance is secreted.

Question 16

How is acethylcholine synthesised?

Answer 16

IT’s synthesised in the terminal endings and varicosities of the cholinergic nerve fibers where it is stored in vesicles in highly concentrated form until it is released. The basic chemical reaction is:

Acetyl-CoA + choline turns into acethylcholine via choline acetylcholine transferase

Question 17

Describe the destruction of Acethylcholine:

Answer 17

When it’s secreted into the tissue by a cholinergic nerve ending, it persists in the tissue for a few seconds while it performs it’s nerve signal transmitter.
Then it splits into an acetate ion and choline catalysed by the enzyme acethylcholinesterase that is bound with collagen and glycosaminoglycans in the local connective tissue.
The choline is then transported back into the terminal nerve ending.

Question 18

Describe the synthesis of norepinephrine:

Answer 18

Synthesis begins in the axoplasm of the terminal nerve endings of adrenergic nerve fibers but is completed inside the secretory vesicles. Basic steps:
1.Tyrosine becomes dopa (through hydroxylation)
2.Dopa becomes dopamine (through decarboxylation)
3. Transport of dopamine into the vesicles
4. Dopamine becomes norepinephrine (through hydroxylation)
5. Norepinephrine becomes epinephrine (through methylation)

Question 19

Describe the removal of norepinephrine from the secretory site (3 ways):

Answer 19

1. Reuptake into the adrenergic nerve endings by an active transport process - accounting for removal of 50-80% of the secreted norepinephrine.
2. Diffusion away from the nerve endings into the surrounding body fluids and then into the blood.
3. Destruction of small amounts by tissue enzymes (for eg monoamine oxidase and Cathecol-O-methyl transferase.

Question 20

How is norepinephrine and epinephrine that is secreted taken care of?

Answer 20

Norepinephrine and epinephrine secreted into the blood by the adrenal medulla remain active until they diffuse into some tissue where they can be destroyed by catechol-O-methyl transferase (this occurs mainly in the liver). They remain active for 10-30 seconds when secreted into the blood but their activity declines to extinction over 1-several minutes.

Question 21

Describe the binding of acethylcholine, norepinephrine and epinephrine to the effector organ:

Answer 21

It must first bind with specific receptors on the effector cells. The receptor is on the outside of the cell membrane, bound as a prosthetic group to a protein molecule that penetrates all the way through the cell membrane. When the transmitter substance binds with the receptor - this causes a conformational change in the structure of the protein molecule. In turn the altered protein molecule excites or inhibits the cell most often by 1. Causing a change in cell membrane permeability to one or more ions or 2. Activating or inactivating an enzyme attached to the other end of the receptor protein where it protrudes into the interior of the cell.

Question 22

Which enzyme activity is increased when norepinephrine binds to its receptor?

Answer 22

Adenylyl Cyclase on the inside of the cell and this causes formation of cyclic adenosine monophosphate (cAMP)

Question 23

Which two types of receptors does acethylcholine mainly activate?

Answer 23

Muscarinic receptors and nicotinic receptors

Question 24

Where are muscarinic receptors found?

Answer 24

On all effector cells that are stimulated by the postganglionic cholinergic neurons either the parasympathetic or sympathetic system.

Question 25

Where are nicotinic receptors found?

Answer 25

In the autonomic ganglia at the synapses between the preganglionic and postganglionic neurons of both the parasympathetic and sympathetic systems.

Question 26

What major types of adrenergic receptors are there?

Answer 26

Alpha receptors and Beta receptors

Question 27

Which of alpha and beta receptors does norepinephrine excite?

Answer 27

Norepinephrine excites mainly alpha receptors but excites beta receptors to a lesser extent as well.

Epinephrine excites both types of receptors approximately equally.

Question 28

Which two functions are controlled by the autonomic nervous system?

Answer 28

1. The pupillary opening
2. The focus of the lens

Sympathetic stimulation contracts the meridional fibers of the iris that dilate the pupil.
Parasympathetic stimulation contracts the circular muscle of the iris to constrict the pupil.

Question 29

Which system is focusing of the lens controlled by?

Answer 29

The parasympathetic nervous system

Question 30

How does parasympathetic excitation affect the ciliary muscle?

Answer 30

It contracts the ciliary muscle

Question 31

How does norepinephrine compared to epinephrine affect the heart and blood vessels?

Answer 31

Epinephrine (because of its greater ability to affect beta receptors) has a greater effect on cardiac stimulation than does norepinephrine.

Epinephrine causes only a weak constriction of the blood vessels in the muscles compared to much stronger constriction caused by norepinephrine.

Epinephrine has 5-10 times as great a metabolic effect as norepinephrine.

Question 32

What is mass discharge?

Answer 32

When all portions of the sympathetic nervous system discharge simultaneously as a complete unit. This frequently occurs when the hypothalamus is activated by fright or fear.

Question 33

Describe the effects that are seen in mass discharge:

Answer 33

1. Increased arterial pressure
2. Increased blood flow
   3.increased rates of cellular metabolism
3. Increased blood glucose
4. Increased glycolysis in the liver and muscle
5. Increased muscle strength
6. Increased mental activity
7. Increased rate of blood coagulation

Question 34

What do the drugs neostigmin, pyridostigmine and ambenonium do?

Answer 34

These drugs inhibit acetylcholineesterase thus preventing rapid destruction of the acetylcholine liberated at parasympathetic nerve endings.

Question 35

What does the drug atropine do?

Answer 35

Block the action of Acethylcholine on the muscarinic type of cholinergic effector organs. These drugs do not affect the nicotinic action of Acethylcholine on the postganglionic neurons or on skeletal muscle.