Lecture 9: colonic motility and defaecation

Question 1

What is the function of the colon?

Answer 1

• retain contents for long periods
• storage
• **mix contents **over a long period
• facilitate reabsorption of water & electrolytes
• allow** voluntary elimination of faeces**

Question 2

Describe the structure of the large intestine

Answer 2

• ascending colon - half of chyme entering caecum is cleared within 90 mins
  transverse colon **-material is retained for 24 hours - shaping and compaction of matter happens here
  **Descending colon**- storage of material after leaving transverse
  ** recto-sigmoid region - reservoirs for faeces

Question 3

Describe the histology of the large intestine

Answer 3

• mucosa of the LI contains** colonic glands & crypts** that are packed together, they contain coloncytes (absorptive cells) & goblet cells (mucous secretion to lubricate LI)
• in the muscularis externa- they contain inner circular and outer longitudinal muscle -also** tenia coli - 3 seperate longitudinal ribbons** surrounded by haustra (pouches) which give the folds a concertina look

Question 4

What is the ileo-caecal junction?

Answer 4

• the junction/boundary** between the ileum and the large intestine**
• contains a region of high pressure - which drives contents from ileum into the LI & controls flow of content
• the high pressure zone** responds to both ileal and colonic distension**
• also contains an** ileocaecal valve** - this prevents contents from moving backwards from the caecum into the terminal ileum

Question 5

What are the tenia coli & haustra?

Answer 5

• muscular structure of the colon is different to other GIT regions
• longitudinal muscle is aggregated (joined together) **into 3 bands **called the tenia coli
• the longitudinal tenia are** shorter than the length of the colon**, this results in the formation of haustra (pouches)

Question 6

what is** diverticulosis**?

Answer 6

a disease that causes the development of small sacs of the colon

Question 7

Describe the main methods of colonic motility

Answer 7

• Haustral shuttling- most common, movement along short distances & in both directions, contraction of circular muscle layer (back and forth)
• segmental propulsion (peristalsis)- wave like contraction, contraction behind bolus and relaxation ahead of bolus
  *** mass contraction/mass movement ** -infrequent, can be 2/3 times a day, produced by the gastrocolic reflex and this moves fecal mass over long distances, normally followed by a bowel mvt

Question 8

Describe the 2 reflexes that trigger mass movement motility

Answer 8

• gastroileal reflex - stimulated by the presence of food in the stomach & gastric peristalsis. Initiation of the reflex causes peristalisis of the ileum & opening of the ileocecal valve (which allows content to enter caecum from ileum)
• gastrocolic reflex - involves an increase in the motility of the colon in resonse to stretch in the stomach and byproducts of digestion in the SI

Question 9

what 4 elements **control **colonic motility?

Answer 9

1. basal electrical rhythm
2. intrinsic nerves
3. extrinsic nerves
4. endocrine/paracrine control

Question 10

How does the basal electrical rhythm control colon motility?

Answer 10

• remember - basal electrical rhythm is the spontaneous depolarisation of the pacemaker cells of the SI and LI - the interstitial cells of cajal
• BER can be increased by stretch, Ach, histamine, serotonin
• BER can be** decreased by noradrenaline and adrenaline**

Question 11

How do instrinsic nerves control colonic motility?

Answer 11

• the myenteric plexus (ENS) is required for normal colonic motility
• if there is an absence of nerve ganglia in the myenteric plexus, there will be a delayed passage of faeces & abdominal distension

Question 12

what is Hirschprung’s disease?

Answer 12

• a disease seen at birth (congenital), where** ganglia from the ENS** of the intestine are absent, this causes a delayed passage of the first stool in a newborn
• can be extremely life threatening & cause megacolon or abdominal distension

Question 13

what is chaga’s disease?

Answer 13

• a disease caused by Trypanosoma which causes damage to the myenteric plexus of colon and oesophagus
• can cause megacolon

Question 14

what is megacolon?

Answer 14

• abnormal dilation of the colon - hypertrophy of the colon
• can lead to paralysis of the peristalic movements of the colon

Question 15

How do extrinsic nerves control motility of the colon?

Answer 15

• parasympathetic nerves release Ach which has an excitatory effect on the smooth muscle in the colon - increases movements
• sympathetic nerves release NA and A which have an inhibitory effect on the smooth muscle in the colon - decreases movements

Question 16

Describe & compare the 2 sphincters in the anal canal

Answer 16

1. internal anal smooth muscle sphincter -** not under voluntary control, contraction is initaited by the sympathetic NS** and** relaxation is initiated** by the** pelvic parasympathetic nerves** via nitric oxide
2. external anal straited sphincter- composed of skeletal muscle, supplied by the pudenal nerve and is under learned voluntayr control

Question 17

What nerve supplies the external anal sphincter?

Answer 17

• pudenal nerve

Question 18

What is the puborectalis?

Answer 18

• muscle fibers that form a sling around the rectum and anal canal
• relaxation increases the angle between the rectum and the anus
  *

Question 19

briefly describe the process of defacation

Answer 19

• complex process involving both reflex and voluntary actions
• when faeces enter the rectum,** causes distension and an urge to have a bowel movement **
• under influence of parasympatheic NS, walls of sigmoid colon and rectum contract to move faeces towards anus
• the anal sphincters relax to allow faeces to move into the anal canal
• expulsion of faeces through anus is aided by voluntary contractions of the abdominal wall muscles, this increases the pressure and helps move faeces against relaxed sphincters