26) Drugs Affecting Gut Motility Flashcards
Describe the myogenic control of gastric motility:
Rhythmic contractions spread through gap junctions. Interstitial cells of Cajal are pacemaker
Describe the enteric neural control of gastric motility:
Stimulation of post-ganglionic cholinergic enteric nerves increases force of contraction of gut, through local nerves and plexuses
Where is Auerbach’s plexus located?
Between circular and longitudinal muscle layers
Where is Meissner’s plexus located?
Submucosa
Where is Henle’s plexus located?
Circular muscle adjacent to submucosa
Where is Cajal’s plexus located?
Circular muscle adjacent to longitudinal muscle
What is the intestino-intestinal inhibitory reflex?
Distension of one intestinal segment causes intestinal inhibtion
What is the ano-intestinal inhibitory reflex?
Distension of anus causes intestinal inhibition
What are the gastrocolic and duodenocolic reflexes?
Stimulates motility after material enters given organ
What hormones are involved in the control of gastric motility?
Gastrin, secretin, CCK, motilin, paracrine transmitters
Describe the pathway of emesis:
Triggers -> vomiting centre -> preliminary signs -> vomiting
Which areas are involved in the control of emesis?
Vestibular apparatus
Postrema on floor of 4th ventricle
Medullary centre
Give examples of anti-emesis drugs:
Domperidone Ondansteron Metoclopramide Hyoscine Cyclizine
What is the mechanism of action of domperidone?
D2 antagonist, acts on psotrema on floor of 4th ventricle and increases gastric emptying
What are some side effects of domperidone?
Stimulates prolactin release
Rare: dystonia
What is the mechanism of action of ondansteron?
Serotonin antagonist - effective at postrema on floor of 4th ventricle and against vagal afferent nerves in GIT (stops initiation of vomiting)
What are some side effects of ondansteron?
Headaches, constipation, flushing (IV)
What is the mechanism of action of metoclopramide?
Dopamine antagonist at floor of 4th ventricle, serotonin antagonist. Causes increased gastric emptying and blocks vagal afferents
What are some side effects of metoclopramide?
Extra-pyramidal reactions (dystonia), galactorrhoea
What is the mechanism of action of hyoscine?
Direct antagonist of muscarinic cholinergic receptors
What are some side effects of hyoscine?
Bradycardia, may get tolerance
What is the mechanism of action of cyclizine?
Histamine antagonist and anti-muscarinic effects
What are some side effects of cyclizine?
Prolonged QT interval, sedation
What non-pharmacological treatment is there for constipation?
Increase fluid intake
Exercise
Fibre
How do bulk laxatives work?
Insoluble and non-absorbable substances which distend gut and draw water in
What are some contraindications to using bulk laxatives? What can be used instead in this case?
Adhesions/ulceration
Faecal softeners - lubricate and soften stool
How do magnesium and sodium salts work in constipation treatment?
Cause water retention in small and large bowel to increase peristalsis
How does lactulose work in constipation treatment?
Isn’t hydrolysed by enzymes so fermented by colonic bacteria forming acetic and lactic acid which have osmotic effect
What is the mechanism of action of irritant laxatives?
Excitation of sensory nerve endings leads to water and electrolyte retention in lumen and peristalsis
When are irritant laxatives indicated?
Rapid treatment e.g. faecal impaction and surgical prep
What are some side effects of irritant laxatives?
Abdominal cramps
Repeated use: colonic atony, hypokalemia
In general what laxatives should be given if patient has soft faeces?
Stimulant laxataives e.g. senna, glycerol
In general what laxatives should be given if patient has hard faeces?
Osmotic laxatives e.g. movicol or bulk laxatives
What are the 3 types of anti-diarrhoeal drugs?
Anti-motility
Bulk forming
Fluid adsorbents
What is the mechanism of action of anti-motility drugs?
Act via opioid receptors in the bowel to reduce motility and allow more time for fluid absorption
When should bulk forming drugs be used?
IBS, ileostomy
What drug can be used in irritable bowel syndrome and why?
Mebeverine - relieves spasm of intestinal muscles
Bulk forming agents
