L7 - Defaecation

Question 1

Constipation

Answer 1

Infrequent passage of hard stools, straining, sensation of incomplete evacuation.

Question 2

Hirschsprung’s disease

Answer 2

Birth defect, characterised by absence of particular nerve cell (ganglion) in segment of bowel in an infant.

Absence of ganglion cells causes muscles in bowels to lose their ability to move through intestine via peristalsis

Question 3

Symptoms of IBS

Answer 3

Rectal bleeding
Pain
Wight loss
Excessive straining

Question 4

How does peristalsis occur?

Answer 4

Peristalsis induced by release of serotonin (5HT) from neuroendocrine cells in response to luminal distention

Serotonin activates the HT4 receptors.

Results in activation of sensory neurons
e.g. calcitonin gene related peptide CGRP

Question 5

How might we suppress urge to defeacate

Answer 5

Vigorous contraction of external sphincter and pubo rectalis

Question 6

Process of defaecation

Answer 6

• diaphragm contracts
• abdo muscle contracts
• pelvic floor muscles contract (puborectalis)
• anal sphincter muscle relax
• stool expelled

Question 7

What drugs might result in constipation

Answer 7

Opiates 
Anti-muscarinic's 
Calcium channel blockers 
Anti-depressants 
Iron

Question 8

Pelvic floor dyssynergia

Answer 8

External anal sphincter and puborectalis contract rather than relaxes during an attempted bowel movement.

Question 9

Anismus

Answer 9

Failure of normal relaxation of pelvic floor during attempted defecation.

Question 10

Anterior rectocele

Answer 10

Weakness of rectovaginal septum.

Anterior wall of rectum will impede into it.

Question 11

Osmotic laxative

Answer 11

Increase colonic inflow of fluid and electrolytes, softens stool and stimulates colon contractility.

Question 12

Ischaemic disease of the colon

Answer 12

Occlusion of branches of superior mesenteric artery or inferior mesentric artery.

Commonly present with sudden onset of abdominal pain and passage of red blood per rectum.

Question 13

Diverticulitis

Answer 13

Diverticula are inflamed.
Thickening of muscle layer.
Due to to high intraluminal pressure, pouches of mucosa extrude through muscular wall through weakened areas.
These form diverticula

Question 14

Internal anal sphincter is controlled by

Answer 14

Circular smooth muscle

Question 15

External anal sphincter controlled by

Answer 15

Striated voluntary muscle

Pudendeal nerve fibres

Question 16

Describe the intrinsic defaecation response?

Answer 16

• mediated by ENS in rectal wall
• food enters rectum - distension of rectal wall, initiated afferent signal that spreads through the myenteric plexus to initiate peristalsis. waves in descending colon, sigmoid and rectum
• peristalsis wave approaches anus, internal anal sphincter is relaxed by inhibitory signals from myenteric plexus.
• if external anal sphincter voluntarily relaxed then defecation will occur.

Question 17

What is a good way to test action of external anal sphincter?

Answer 17

Digital rectal exam.

Ask patient to squeeze on your finger.