7.7 Physiology of defecation and mechanisms of constipation Flashcards
what kind of motility occurs in the colon?
Segmentation
what are the four phases of defecation?
continence
initiation of reflex
defecation reflex
closure reflex
What is the top down hierarchy in establishing continence?
higher brain centres - ANS - prevertebral ganglia - ENS - smooth muscle/mucosa/vasculature
What are the anatomical elements of continence?
1st line of defence: controlled by ENS
- Kohlrausch’s fold, circular smooth muscle
2nd: controlled by ANS
- Internal anal sphincter
- Venous spongy body
3rd: controlled by CNS
- External anal sphincter (striated muscle)
- Puborectal muscles
what is the main control of defecation?
Parasympathetic innervation - S2-4 (onufs nucleus)
What causes the initiation of defecation?
Mass movements, small movements in sigmoid, emptying in sigmoid
what perceives the signal of filling of the ampulla recti and what is the outcome?
Pressure receptors send signals via the pelvic splanchnic nerves to the pelvic plexus (S2-4)
This will either initiate the recto-anal inhibitory or contractile reflex
What occurs during emptying?
circular contraction of sigmoid (CNS) and relaxation of internal (ENS and ANS) and external (CNS) sphincters
what occurs during the closure reflex?
removal of inhibitory drive to internal sphincter and contraction of the internal sphincter
Voluntary contraction of external sphincter closes off anus
smooth muscles in sigmoid relax
When does onufs nucleus establish?
2 years
What are the causes of constipation?
Insufficient water and/or dietary fibre
Drugs
Metabolic and endocrine disorders: diabetes, hypoparathyroidism
primary causes: myogenic, neurogenic, idiopathic
what is the major element for continence and what kind of control is it under?
Internal anal sphincter - unconscious control
