Continence mechanisms

Question 1

How can the rectum be distinguished from the sigmoid colon?

Answer 1

All common features of the sigmoid colon stop at the rectum.

Question 2

What are the sequence of events during defaecation?

Answer 2

1. Mass of movement of faeces into the rectum via gastocolic reflex induces feeling of fullness (via visceral afferents in pelvic splanchnic).
2. Sitting and relaxing pelvic floor (mainly puborectalis) and external anal sphincter causes loss of anorectal angle.
3. This is enhanced by the Valsalva manoeuvre.
4. Increased rectal pressure causes reflex distension of the rectum.
5. Rectum is pulled upwards and shortened by the longitudinal muscles.
6. Voluntary relaxation of the external anal sphincter.
7. Anal canal opens and the mucosa everts temporarily to ensure total emptying.
8. Puborectalis, anal sphincters contract, closing the anal canal in “closing reflex”.

Question 3

What are the mechanisms involved in maintaining faecal continence?

Answer 3

1. Internal anal sphincter (involuntary)
2. External anal sphincter (voluntary)
3. Pelvic floor (voluntary)
4. Anal cushions
5. Apposition of mucosal folds
6. Intra-abdominal pressure

Question 4

What are the relative positions of the anal cushion?

Answer 4

3, 7, 11 o’clock positions relative to anterior

Question 5

Where is faeces stored?

Answer 5

• Usually in the middle/distal colon.
• Only stored in rectum pre-defaecation.

Question 6

What is spurious diarrhoea?

Answer 6

Constipation causes reflex distension of rectum. Fluid leaks out, but no faeces.

Question 7

What is the type of elasticity is displayed by the bladder?

Answer 7

Vesico-elasticity: Distending force of urine on bladder walls causes reflex relaxation of bladder.

Question 8

What are the main factors contributing to urinary continence?

Answer 8

1. Internal/external urethral sphincter
2. Elastic tension in urethra
3. Water-tight seal formed by soft tissue
4. Pelvic floor muscles
5. Intra-abdominal lateral pressure on urethra
6. Vesico-elasticity of bladder
7. Nervous control

Question 9

What is the sequence of events during voiding of urine?

Answer 9

1. Voluntary relaxation of pelvic floor.
2. Reflex contraction of bladder and opening of intrinsic urethral mechanisms cause bladder neck to open.

Question 10

How can voiding be stopped mid-stream?

Answer 10

1. Contraction of externa urethral sphincter (voluntary) closes urethra and stops stream.
2. Intrinsic urethral mechanism milks urine in urethra back into bladder.
3. Smooth muscles of the bladder neck chronically contract to close it and stop urine.

Question 11

What are the CNS centres in control of voiding?

Answer 11

1. Cortical micturition centre: Decides when we void.
2. Pontine micturition centre: Decides how we void
3. Spinal micturition centre