lecture 5- the large intestine

Question 1

Large intestine primary function

Answer 1

is to store faeces before defecation

Question 2

L.I structure

Answer 2

consists of the caecum, appendix, colon, and rectum.

Question 3

Colon has three relatively straight parts

Answer 3

Ascending, Transverse and Descending colon

Question 4

Haustra

Answer 4

divided pouches of the colon seen from caecum to rectum

Question 5

Taeniae coli

Answer 5

3 separate bands of longitudinal muscle layers gathered into ‘pouches’ called haustra. Myenteric plexus concentrated beneath

Question 6

Haustra contractions

Answer 6

colons main motility, initiated by autonomous rhythmicity of colonic pacemakers signalling to smooth muscle cells. shuffles chyme back and forth for absorption of salt and water

Question 7

Haustral contractions

Answer 7

controlled by locally mediated reflexes in the intrinsic plexus (response to distension)

Question 8

Mass movements

Answer 8

controlled by neural and hormonal stimuli- predominantly in response to a meal

Question 9

Gastrocolic reflex

Answer 9

Food enters stomach> release of gastrin & activation of extrinsic autonomic nerves> stimulation colon motility> mass movement. pushes contents into rectum which is followed by urge to defecate

Question 10

Defecation

Answer 10

Distension caused by mass movement triggers defecation reflex. Internal anal sphincter (smooth muscle) relaxes to allow anal canal to open. Distension elicits sensation that signals urge to defecate. If external anal sphincter is relaxed voluntarily then defecation occurs

Question 11

Delaying defecation

Answer 11

If environment conditions are not conducive to defecation: voluntary contraction of the external anal sphincter can overcome the reflex. Internal sphincter regains tone. Defecation reflex is initiated again after subsequent mass movement

Question 12

Constipation

Answer 12

delayed transit> increased H2O absorbed> dry faeces, commonly dietary related, direct correlation with increased fibre, increased intraluminal bulk & enhanced transit through colon.

Question 13

Hirschsprungs disease (congenital megacolon)

Answer 13

Absence of enteric nervous system in distal colon involving internal anal sphincter and often rectum (area has increased tone, narrow lumen, no propulsive activity)

Question 14

Secretion

Answer 14

secretes a protective alkaline mucus solution. secretion increases in response to mechanical and chemical stimulation

Question 15

Absorption

Answer 15

water and salts but not nutrients