The Autonomic Nervous System

Question 1

Describe how the sympathetic nervous system exerts control over the trachea and bronchi.

Answer 1

The trachea and bronchi have no sympathetic nerve fibres. They are affected by the release of catecholamines from the adrenal medulla.

Question 2

Describe the response of blood vessels to skeletal muscle during a period of increased sympathetic discharge. What causes this response?

Answer 2

Blood vessels supplying the skeletal muscle dilate. This is caused by circulating catecholamines.

Question 3

Describe the role of the adrenal medulla in the sympathetic nervous system.

Answer 3

Adrenal medulla is innervated by sympathetic pre-ganglionic nerve fibres, which release acetyl choline that binds to the nicotinic acetyl choline receptors and causes release of catecholamines from the chromaffine cells.

Question 4

What is special about the sympathetic innervation of the sweat glands?

Answer 4

The sympathetic nerves to the sweat glands release acetylcholine instead of noradrenaline.

Question 5

What are the two outflows of the parasympathetic nervous system?

Answer 5

Craniosacral

Question 6

State the four cranial nerves that form part of the parasympathetic nervous system.

Answer 6

Oculomotor - 3
Facial - 7
Glossopharyngeal - 9
Vagus – 10

Question 7

Which parasympathetic nerves come out of the sacral region?

Answer 7

Pelvic spanchnic nerves

Question 8

Describe the outflow of the sympathetic nervous system. State the vertebral levels that define the limits of this outflow.

Answer 8

Thoracolumbar (T1 - L2)

Question 9

Describe the relative lengths of sympathetic and parasympathetic pre-ganglionic fibres.

Answer 9

Sympathetic nerves have a short preganglionic and a long post ganglionic
Parasympathetic has a long preganglionic and a short post ganglionic

Question 10

Describe the effect of the SNS on the heart.

Answer 10

The SNS has an inotropic (increase force of contraction) and chronotropic (increase heart rate) effect

Question 11

Describe the effect of increased sympathetic firing on the blood vessels.

Answer 11

Increased sympathetic firing causes constriction of most blood vessels (except those supplying the skeletal muscle, which dilate)

Question 12

How does the SNS affect arterial blood pressure?

Answer 12

SNS increase cardiac output via the chronotropic and inotropic effects and it increases TPR by causing vasoconstriction so it increases blood pressure

Question 13

What can cause vasodilation in various tissues?

Answer 13

Vasodilation can be caused by local mediators such as nitric oxide and histamines
Vasodilation in the penis is caused by increased parasympathetic stimulation
Vasodilation in the blood vessels supplying skeletal muscle is due to increased sympathetic activity (by circulating catecholamines)

Question 14

Describe how the SNS affects the GI tract.

Answer 14

SNS decreases gut motility and tone
SNS stimulates contraction of sphincter
SNS generally inhibits secretory activity

Question 15

Describe the effects of increased sympathetic activity on the lungs and airways.

Answer 15

Increased sympathetic activity dilates the airways

Question 16

Describe the effects of sympathetic and parasympathetic activity on the eye muscles.

Answer 16

Sympathetic - causes dilation of the pupil (contraction of radial muscles)
Parasympathetic - constriction of the pupil and ciliary muscle contracts (lens bulges allowing near vision)

Question 17

Describe the nervous control of the bladder.

Answer 17

The parasympathetic nervous system controls the muscles of the bladder wall
The sympathetic nervous system controls the internal sphincter
There is somatic motor control of the external sphincter

Question 18

What is the capacity of the bladder?

Answer 18

450-500 mL

Question 19

State the three nerves involved in micturition.

Answer 19

Pelvic - sensory input from the stretch receptors in the wall of the bladder
Hypogastric - sympathetic input to the internal sphincter
Pudendal - voluntary control of the external sphincter

Question 20

What is acetyl choline made from?

Answer 20

Acetyl CoA and Choline

Question 21

What are catecholamines derived from?

Answer 21

Tyrosine

Question 22

What structure is common to all catecholamines?

Answer 22

Hydroxylated phenyl ring

Question 23

If you block acetylcholine action in autonomic ganglia, how would it affect heart rate?

Answer 23

The effect on heart rate would be dependent on which branch of the autonomic nervous system was more dominant at the time. Because all autonomic ganglia have acetyl choline so blocking it would affect both sympathetic and parasympathetic pathways.

Question 24

What are the differences between nicotinic and muscarinic receptors?

Answer 24

Nicotinic receptors are ion channel linked receptors so they allow fast transmission
Muscarinic receptors are G protein linked receptors so their transmission is slower

Question 25

Describe the synthesis and breakdown of acetyl choline.

Answer 25

Acetyl choline is synthesised from Acetyl CoA and choline via the action of choline acetyl transferase. It is broken down in the synaptic cleft by acetyl cholinesterase

Question 26

What would happen if you inhibit acetylcholinesterase?

Answer 26

Inhibiting acetylcholinesterase would result in a build up of acetylcholine in the synaptic cleft and a decrease in the responsiveness of the post synaptic neurone receptors to acetylcholine and you will become paralysed. This how botulinum toxin works.

Question 27

Describe the synthesis of noradrenaline.

Answer 27

Tyrosine —> DOPA (tyrosine hydroxylase)
DOPA —> Dopamine (dopamine decarboxylase)
Dopamine —> Noradrenaline (Dopamine  hydroxylase)

Question 28

What can happen to the noradrenaline after it has had its effect?

Answer 28

It is taken up into pre-synaptic neurones and into extraneuronal tissue
In the pre-synaptic neurones it is broken down by MAO (monoamine oxidase). In extraneuronal tissue it is broken down by COMT (Catechol-o-methyl transferase)

Question 29

Which cells in the adrenal medulla produce catecholamines?

Answer 29

Chromaffine cells

Question 30

What extra step and enzyme is involved in the synthesis of adrenaline?

Answer 30

Noradrenaline leaves the vesicle and is converted to adrenaline (via PNMT - phenylethanolamine N-methyl transferase) and repackaged into vesicles.

Question 31

Describe the effect of cortisol on adrenaline secretion.

Answer 31

Cortisol increases adrenaline secretion by upregulating PNMT.

Question 32

What are the break down products of noradrenaline and adrenaline by COMT and MAO-A? What do measurements of these values in the urine give an indication of?

Answer 32

COMT - VMA
MAO-A - MOPEG
Levels of MOPEG in the urine given an indication of sympathetic activity in the brain
VMA give an idea of peripheral production of noradrenaline