The Physiology Of Defecation

Question 1

Functions of the colon

Answer 1

Absorption of water and electrolytes (osmosis)

Excretion of waste (motility)

Production of vitamins (microbiome)

Question 2

Layers of the colonic wall

Answer 2

Mucosa- comprised of column, epithelial and goblet cells

Muscularis mucosae- thin layer of smooth muscle cells

Submucosa- dense layer of lymphatics, blood vessels and enteric nerves

Muscularis propria- main muscular layer comprised of inner circular layer and outer longitudinal layer

Subserosa- contain cells that are capable of creating surface fluid, which is important for the lubrication of the colon

Seroasa- contain cells that are capable of creating surface fluid, which is important for the lubrication of the colon

Question 3

Muscular layers

Answer 3

Continuous circular muscle: Role of mass movements (high amplitude contractions) needed for propelling contents along the colon

3 ‘ribbons’ of longitudinal muscle- teaniae coli: Localised
Segmental contractions important for mixing and allowing time for the absorption of that water and electrolytes

Question 4

Histology of the colon

Answer 4

Goblet cells- secrete mucus main role is to lubricate the colon

Simple columnar epithelium

Question 5

Nerve supply

Answer 5

Enteric Nervous System (Intrinsic) - made up of 2 dense networks of neurones called the:
-Myenteric Plexus
-Submucosal Plexus

Extrinsic -
-Predominantly from the Parasympathetic branch of the autonomic nervous system
-Sympathetic

Vagus nerve innervates the ascending and transverse colon
Pelvic nerve innervates descending and the anal rectum

Question 6

The anal sphincter

Answer 6

The internal anal sphincter is the thin ending of the rectum wall- involuntary muscle under the control of the parasympathetic pelvic nerve and localised enteric nerves
- Always contracted and generates around 85% of the resting tone of the anal canal

The external anal sphincter striated muscles- voluntary muscle, circulates outside if the anal canal
-Controlled by the somatic pudendal nerve

Question 7

4 phases of defecation

Answer 7

Basal

Pre-expulsive

Expulsive

Termination

Question 8

Basal phase

Answer 8

Colon – segmental contractions (mixing) -under control of the longitudinal muscles

Rectum - motor complexes (to keep rectum empty)
“braking mechanism”- random bursts of contractile activity this works to keep the rectum empty the vast majority of the time

Anal Sphincter - tonic contraction

Puborectalis - remains contracted to create 90o anorectal angle at the junction between the rectum and anus
-Rectal angle is one of the main mechanisms In place to preserve continence

Question 9

Pre-expulsive phase

Answer 9

Colon – circular muscles gets involved to produce high amplitude propagating contractions
-Mass movement of stool ~8 times day
These occur in response to us eating, when we eat the stomach wall stretches which sends signals to colon to start these movements
This is called the Gastro-colic reflex- when we eat

Rectum –
Fills causing distension
Rectal compliance (adaptive relaxation) Can accommodate increasing volumes of fetal matter without change in pressure

Anal Sphincter –
EAS maintains contraction (under voluntary control)
Reflex relaxation of IAS (RAIR) – for stool sampling
This allows for a tiny bit of faecal content to enter the canal
The sensory epithelium within the anal canal will detect that content as either gas, liquid or solid

Puborectalis – remains contracted to preserve the anal rectal angle

Question 10

Expulsive phase

Answer 10

Rectum contracts- propelling stool into anal canal

IAS, EAS and PR relaxes

Valsalva manoeuvre/posture aid emptying by increasing abdominal pressure

Posture further aids emptying by opening the anal rectal angle and straightening pathway for stool exit

Question 11

Termination phase

Answer 11

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

Valsalva ceases

Change in posture (to standing)

Question 12

Parasympathetic defection reflex

Answer 12

There’s a dense network of neural fibres within the
cone on the rectum, both extrinsic and intrinsic that have
to work together in a coordinated manner to allow for
defaecation

When stool comes down into the rectum, it stretches the rectal wall, sending signals via some afferent metres

It sends sensory signals to sacral part pf the spinal cord

This then sends messages via efferent nerves to the rectum and the internal sphincter

The rectum contracts, the internal sphincter relaxes

During this process, there’s continuous downward signals from the brain to the external sphincter via the pudendal nerve, which is allowing for the external sphincter to remain contracted unless the person can access a toilet at which the sphincter would relax

Question 13

Disorders of defecation- constipation

Answer 13

Less than 3 bowels movement a week, strain in patient and hard stool

Multitude of factors cause this

Consistency of stool- diet (not enough fibre), medication, fluid intake, drugs like codeine

Bowel motility- underactive thyroid, diabetes and Parkinson’s disease effect the nerves
The stool stays in the colon for longer, more water is absorbed from it and so the stool becomes dry

Physical blockage to the bowel- scar structures, tumours

Pelvic floor disorders- obstructive defecation. Due to presence of rectal prolapse, sphincter contracting rather than relaxing which prevents stool from leaving

Question 14

Diarrhoea

Answer 14

Loose stool, going toilet more than 3 times a day

Diet- too much fibre, too much diary, too much caffeine, lactose intolerance, fluid intake

Frequency y of bowel movements- colon inflamed, crohns disease, anxiety, colon cancer

Diseased bowel mucosa- crohns disease creates a diarrhoea pattern

Reduced rectal capacity- rectal wall can not descend due to inflammation which can reduce compliance of pelvic wall which will make rectal wall stiff

Pelvic floor disorder- sphincter becomes damaged