the physiology of defecation Flashcards
functions of the colon (3)
- absorption of water and electrolytes (osmosis)
- excretion of waste (motility)
- production of vitamins (microbiome)
layers of the colonic wall (inner to outermost)
- mucosa
- muscularis mucosae
- submucosa
- muscularis propia
- subserosa
- serosa
describe the muscular layers of the colon
- continuous circular muscle
- 3 ‘ribbons’ of longitudinal muscle - taeniae coli
main muscular layer
muscularis propia - generates high amplitude contractions
outer muscular layer
longitudinal muscle - formed as 3 ribbons/ strands
histology of the colon
- simple columnar epithelium
- goblet cells
nerve supply of colon and rectum
intrinsic and extrinsic
what is intrinsic nervous system made up of
- myenteric plexus
- submucosal plexus
these are dense networks of neurones
what is extrinsic nervous system made of
- parasympathetic
- sympathetic
what is the anal sphincter
a group of muscles at the end of the rectum that surrounds the anus and controls the release of stool, thereby maintaining continence
what are the 2 muscles of the anal sphincter
- internal anal sphincter
- external anal sphincter
4 phases of defecation
- basal
- pre - expulsive
- expulsive
- termination
function of rectum
temporary storage of faeces before defecation
what causes the desire to defecate
- as the rectum becomes filled the rectal walls expand and stretch receptors stimulate the desire to defecate.
- the urge to defecate arises from:
- contraction of rectal muscles,
- relaxation of the internal anal sphincter
-and an initial contraction of the external anal sphincter
what happens if the urge to defecate is not acted upon
further water is absorbed and the faeces is stored until the next mass movement occurs
what is the internal anal sphincter controlled by
parasympathetic fibres which relax involuntarily
what is the external anal sphincter controlled by
it is s skeletal muscle
is controlled by somatic nerve supply from the Inferior anal branch of the Pudendal nerve (S2,3,4)
which allows conscious control of defecation
what happens when the rectum is distended
the rectosphincteric reflex is initiated and relaxes the internal sphincter
what happens if defecation is not desired
voluntary contraction of the external sphincter can delay it
what happens if defecation is appropoiate
a series of reflexes take place that lead to:
- Relaxation of the external sphincter
- Contraction of abdominal wall muscles
- Relaxation of pelvic wall muscles
what do peristaltic waves do
facilitate the movement of faeces through the anal canal
what is the Valsalva maneuver
when defecation is assisted by taking a deep breath and attempting to expel the air against a closed glottis
what happens if a delay in defection is needed
voluntary contraction of the external sphincter is usually sufficient to override the reflexes that anal distension initiates
what occurs in the colon during the basal phase of defecation
segmental contractions (mixing)
what occurs in the rectum during the basal phase of defecation
motor complexes keep it empty
what occurs in the anal sphincter during the basal phase of defecation
tonic contraction
what happens to puborectalis muscle during basal phase of defecation
it is contracted
at a 90 degree anorectal angle
what occurs in the colon during the pre expulsive phase of defecation
there are high amplitude propagating contractions
mass movement of stool
gastro colic reflex
what occurs in the rectum during the pre expulsive phase of defecation
it fills causing distension
rectal compliance ( adaptive relaxation)
what occurs in the anal sphincter during the pre expulsive phase of defecation
the external anal sphincter maintains contraction
reflex relaxation of internal anal sphincter (RAIR) - for stool sampling
what happens to puborectalis muscle during the pre expulsive phase of defecation
it remains contracted
what happens in the explusive phase of defecation
- rectum contracts
- internal & external anal sphincter and puborectalis relaxes
- valsalva manoeuvre / posture aid emptying
what happens in sitting posture
the puborectalis muscle chokes the rectum to maintain continence
what happens in squatting posture
the puborectalis muscle relaxes and straightens the pathway to the anus
what happens in the termination phase of defecation
- traction loss causes sudden contraction of external anal sphincter (“closing reflex”)
- valsava closes
- change in posture (to standing)
2 disorders of defecation
- constipation
- diarrohea
what are the things that go wrong to cause constipation
- consistency of stool
- bowel motility
- physical blockage to the bowel
- pelvic floor disorders
what are the disorders that go wrong to cause diarrhoea
- consistency of stool
- frequency of movements
- diseased bowel mucosa
- reduced rectal capacity
- pelvic floor disorder
causes of faecal incontinence
- Physical injury
- Nerve injury
- Surgery
- Intense fright
- Inflammatory bowel disease
- Childbirth
