Physiology of Defaecation

Question 1

Functions of the colon

Answer 1

Absorption of water and electrolytes
Production of vitamins
Excretion of waste

Question 2

Muscular layer of colon

Answer 2

Inner continuous circular muscle

3 “stripes” of longitudinal muscle - taeniae coli (localised segmental contractions

Question 3

What cells lubricate the lining of the colon

Answer 3

Goblet cells

Question 4

Nerve supply of colon

Answer 4

Enteric nervous system (intrinsic) - myenteric plexus and submucosal plexus

Extrinsic - parasympathetic and sympathetic

Question 5

Internal and external anal sphincter

Answer 5

Internal under parasympathetic pelvic nerve and myenteric nerve?

External sphincter is striated, voluntary muscle, somatic con-_ nerve?

Question 6

Basal phase

Answer 6

Colon - segmental contractions (mixing)

Rectum - motor complexes (to keep rectum empty)
Anal sphincter - tonic contraction

Puborectalis - contracts

Question 7

Pre-expulsive phase

Answer 7

Colon - high amplitude propagating contractions (mass movement of stool, 8 times a day, gastric-colic reflex)

Rectum - fills causing distension, rectal compliance (adaptive relaxation)

Anal sphincter – EAS maintains contraction, reflex relaxation of IAS (RAIR) for stool sampling

Puborectal remains contracted to preserve anorectal angle.

Question 8

Expulsive phase

Answer 8

Rectum contracts

IAS, EAS and PR relaxes

Valsalva manoeuvre/posture aid emptying

Question 9

Termination phase

Answer 9

Traction loss causes sudden contractions of EAS (“closing reflex”)

Valsava ceases

Change in posture