26) Drugs Affecting Gut Motility

Question 1

Describe the myogenic control of gastric motility:

Answer 1

Rhythmic contractions spread through gap junctions. Interstitial cells of Cajal are pacemaker

Question 2

Describe the enteric neural control of gastric motility:

Answer 2

Stimulation of post-ganglionic cholinergic enteric nerves increases force of contraction of gut, through local nerves and plexuses

Question 3

Where is Auerbach’s plexus located?

Answer 3

Between circular and longitudinal muscle layers

Question 4

Where is Meissner’s plexus located?

Answer 4

Submucosa

Question 5

Where is Henle’s plexus located?

Answer 5

Circular muscle adjacent to submucosa

Question 6

Where is Cajal’s plexus located?

Answer 6

Circular muscle adjacent to longitudinal muscle

Question 7

What is the intestino-intestinal inhibitory reflex?

Answer 7

Distension of one intestinal segment causes intestinal inhibtion

Question 8

What is the ano-intestinal inhibitory reflex?

Answer 8

Distension of anus causes intestinal inhibition

Question 9

What are the gastrocolic and duodenocolic reflexes?

Answer 9

Stimulates motility after material enters given organ

Question 10

What hormones are involved in the control of gastric motility?

Answer 10

Gastrin, secretin, CCK, motilin, paracrine transmitters

Question 11

Describe the pathway of emesis:

Answer 11

Triggers -> vomiting centre -> preliminary signs -> vomiting

Question 12

Which areas are involved in the control of emesis?

Answer 12

Vestibular apparatus
Postrema on floor of 4th ventricle
Medullary centre

Question 13

Give examples of anti-emesis drugs:

Answer 13

Domperidone
Ondansteron
Metoclopramide
Hyoscine
Cyclizine

Question 14

What is the mechanism of action of domperidone?

Answer 14

D2 antagonist, acts on psotrema on floor of 4th ventricle and increases gastric emptying

Question 15

What are some side effects of domperidone?

Answer 15

Stimulates prolactin release

Rare: dystonia

Question 16

What is the mechanism of action of ondansteron?

Answer 16

Serotonin antagonist - effective at postrema on floor of 4th ventricle and against vagal afferent nerves in GIT (stops initiation of vomiting)

Question 17

What are some side effects of ondansteron?

Answer 17

Headaches, constipation, flushing (IV)

Question 18

What is the mechanism of action of metoclopramide?

Answer 18

Dopamine antagonist at floor of 4th ventricle, serotonin antagonist. Causes increased gastric emptying and blocks vagal afferents

Question 19

What are some side effects of metoclopramide?

Answer 19

Extra-pyramidal reactions (dystonia), galactorrhoea

Question 20

What is the mechanism of action of hyoscine?

Answer 20

Direct antagonist of muscarinic cholinergic receptors

Question 21

What are some side effects of hyoscine?

Answer 21

Bradycardia, may get tolerance

Question 22

What is the mechanism of action of cyclizine?

Answer 22

Histamine antagonist and anti-muscarinic effects

Question 23

What are some side effects of cyclizine?

Answer 23

Prolonged QT interval, sedation

Question 24

What non-pharmacological treatment is there for constipation?

Answer 24

Increase fluid intake
Exercise
Fibre

Question 25

How do bulk laxatives work?

Answer 25

Insoluble and non-absorbable substances which distend gut and draw water in

Question 26

What are some contraindications to using bulk laxatives? What can be used instead in this case?

Answer 26

Adhesions/ulceration | Faecal softeners - lubricate and soften stool

Question 27

How do magnesium and sodium salts work in constipation treatment?

Answer 27

Cause water retention in small and large bowel to increase peristalsis

Question 28

How does lactulose work in constipation treatment?

Answer 28

Isn't hydrolysed by enzymes so fermented by colonic bacteria forming acetic and lactic acid which have osmotic effect

Question 29

What is the mechanism of action of irritant laxatives?

Answer 29

Excitation of sensory nerve endings leads to water and electrolyte retention in lumen and peristalsis

Question 30

When are irritant laxatives indicated?

Answer 30

Rapid treatment e.g. faecal impaction and surgical prep

Question 31

What are some side effects of irritant laxatives?

Answer 31

Abdominal cramps | Repeated use: colonic atony, hypokalemia

Question 32

In general what laxatives should be given if patient has soft faeces?

Answer 32

Stimulant laxataives e.g. senna, glycerol

Question 33

In general what laxatives should be given if patient has hard faeces?

Answer 33

Osmotic laxatives e.g. movicol or bulk laxatives

Question 34

What are the 3 types of anti-diarrhoeal drugs?

Answer 34

Anti-motility Bulk forming Fluid adsorbents

Question 35

What is the mechanism of action of anti-motility drugs?

Answer 35

Act via opioid receptors in the bowel to reduce motility and allow more time for fluid absorption

Question 36

When should bulk forming drugs be used?

Answer 36

IBS, ileostomy

Question 37

What drug can be used in irritable bowel syndrome and why?

Answer 37

Mebeverine - relieves spasm of intestinal muscles | Bulk forming agents