Large Intestine Flashcards
What is the function of the L.I.?
DRYING AND STORAGE!
Bacteria: digestion of CHO
Absorption of water (and Na+ & Cl-)
Stores faeces before defecation
What is the transit time for substances in the L.I.?
12-24 hrs
What are the main forms of motility in the L.I.?
HAUSTRAL CONTRACTIONS (main form)! -Initiated by autonomous rhythmicity of colonic pacemakers (cajal) signalling to smooth muscle cells
SEGMENTAL
-one secretion that is relaxed gradually contracts while a previously contracted area relaxes to form a new sac
Shuffles chime back and forth for absorption of salt and water → drying leaves faeces
What are the characteristics of motility in the L.I.?
Non propulsive - (storage and absorption)
Slow - (1 per 30mins)
Mass movement 2-3 times day (after meal)
Large segments of the ascending and transverse colon contract simultaneously-‐drives faeces to distal colon, where it is stored before defecation.
–Drives faeces 1/3 to 3/4 way through colon in few seconds
What controls motility in the form of Haustral contractions?
LOCALLY MEDIATED REFLEXES!
(*changes the likelihood of slow wave reaching threshold)
– Intrinsic innervation:
ENS, response to distension
–Extrinsic innervation:
ANS, in proximal colon = vagus nerve, distal colon = pelvic nerve (parasympathetic); in proximal colon = superior mesenteric ganglion, distal colon = inferior mesenteric ganglion (sympathetic)
What controls motility in the form of mass movements?
GASTROCOLIC REFLEX
Food enters stomach → release of gastrin & activation of extrinsic autonomic nerves → stimulate colon motility → mass movement
-Pushes contents into rectum which is followed by urge to defecate
When/how does defecation occur?
- Rectum usually empty
- Anal canal closed due to contraction of internal anal sphincter
- When rectum fills following a mass movement, causes distension
- This triggers the defecation reflex!
- -Internal sphincter relaxes
- -Rectal distension also elicits a sensation that signals the urge for defecation
-If the defecation reflex occurs when evacuation is convenient, then defecation will occur following voluntary relaxation of the external anal sphincter.
How is defecation regulated by neural control?
DISTENSION OF RECTUM INITIATES:
- SHORT REFLEX (via myenteric plexus)
- contraction of muscle in rectum and sigmoid colon,
- relaxation of internal sphincter - LONG REFLEX (via spinal cord)
- action reinforced by parasympathetic neurons (pelvic nerve)
What are the voluntary components that assist defecation?
External anal sphincter relaxes
Abdominal contractions (increase intra-abdominal P)
- expiration against closed glottis
- pelvic floor muscle relax to allow increased P to force pelvic floor downward
What is constipation?
Delayed transit → ↑ H2O absorbed→ hard dry faeces
What are the causes of constipation?
~Most commonly dietary in origin!
~Direct correlation w. increased intraluminal bulk, & enhanced transit thru colon
~Other causes:
- Ignoring need to defecaye
- Decreased colonic motility due to aging, emotion, or a low bulk diet
- Obstruction (tumour or colonic spasm)
- Impairment of defecation reflex (injury of nerve pathway)
What is Congenital megacolon (hirschsrpungs disease)?
Characterised by an absence of the ENS in the distal colon!
-Involves internal anal sphincter and often the rectum
The involved segment of intestine exhibits:
- increased tone
- very narrow lumen
- no propulsive activity
THUS, the area proximal to the narrow section becomes dilated, giving the enlarged mega colon appearance
What is secreted in the L.I.?
[NO DIGESTIVE ENZYMES -(nutrient digestion and absorption complete!)]
Secretes a PROTECTIVE ALKALINE (sodium bicarb.) MUCUS solution!
- Neutralise acids from bacterial fermentation, lubricates passage of faeces
- Secretion increases in response to mechanical and chemical stimulation (mediated by short reflexes & parasympathetic innervation)
Discuss absorption in the L.I.?
Relatively smooth surface (no villi etc.)
∴↓ absorption than S.I.
Actively absorbs Na+
(Cl-‐ passively follows down electrical gradi.)
Absorbs H20 (osmotically) to form firm faecal mass
Also absorbs electrolytes, vitamin K (synthesised in the colon by bacteria).
TRUE OR FALSE:
There are 10x more bacteria in the colon than there are cells in the body
TRUE
