0320 - Defaecation and Constipation Flashcards
What is the role of the colon?
Compact and portion chyme into faeces. Fluid absorption Electrolyte absorption Bacterial flora (Vit K, B12 prodn) Not necessary for life.
What are the anatomical elements of anal continence and defaecation?
(3 lines of defence)
1 - Controlled by ENS, transverse rectal fold, together with upright position, and circular smooth muscles of the rectum.
2 - Controlled by autonomic NS - internal anal sphincter and venous spongy body.
3 - Controlled by CNS (spinal reflexes) - External anal sphincter, and puborectal muscles.
Explain the sequence of events that initiate, maintain, and end defaecation
1 - Continence in between defaecation sessions
2 - Defaecation reflex initiates with the urge to void.
3 - Defaecation reflex proper, the opening of the anus
4 - Closure reflex, the closure of the anus.
Requires all 3 nervous systems to participate, with higher control systems amplifying and synchronising the ENS response.
What is the physiology of anal continence?
ENS constricts internal sphincter and relaxes sigmoid colon (as a reservoir).
Internal and external sphincters are both tonically active.
What is the physiology of initiation of defaecation?
Faeces moves from sigmoid colon to rectal ampulla, where pressure receptors signal a feeling of urge. This can then elicit either the
Recto-anal inhibitory reflex (of internal sphincter, leading to defecation) or
Recto-anal contractile reflex (or external sphincter, supressing defecation)
What is the physiology of the maintenance of emptying in defecation?
Contraction of sigmoid circular muscles (ENS) and relaxation of internal (ENS and ANS) and external (CNS) sphincters, with amplification and simplification of the response.
Behavioural response is relaxation of external sphincter, and straining - an increase in intraabdominal pressure, closure of glottis, and change in position to crouching.
What is the physiology of the closure reflex to finalise defecation?
After voiding, pressure receptors in rectal ampulla remove inhibition of internal sphincter, leading to its contraction. The external sphincter is voluntarily contracted, closing the anus, and smooth muscles in the sigmoid relax, re-establishing the reservoir function.
Outline the heirarchy of control systems for defaecation
Mostly out of voluntary control. Only abdominal muscles, external sphincter, and pubo-rectal muscles are under direct voluntary control. The levels serve to amplify and synchronise the efferent response.
1 - Higher brain centres is the only conscious control mechanism, which feeds into
2 - parasympathetic and sympathetic systems.
3 - Sympathetic activation occurs via prevertebral ganglia.
4 - SY and Para activate the ENS
5 - which controls the smooth muscle, mucosa and vasculature - the effector level.
Discuss what constipation is
Passage of small amounts of hard, dry bowel movements, usually fewer than 3 times a week. May be associated with pain and bloating.
List some elements that might lead to constipation.
Opioids Chronic laxative abuse Poor diet (fibre) Dehydration Ignoring urge to defecate.
