PHYS GIT Motility - Week 4

Question 1

What structures make up the small intestine?

Answer 1

Duodenum, jejunum, ileum.

Question 2

Motility processes in the small intestine.

Answer 2

1. Mixing contractions/segmental contractions – local contractions which mix chyme from stomach w alkaline secretions & digestive enzymes.
   (Post nutrient absorption in the small intestine)
2. Peristaltic contractions/migrating myoelectric complex – initiated by increasing chyme pH or motilin concentration -> remaining luminal contents are pushed into the large intestine.

Question 3

What nerves modulate mixing contractions in the small intestine?

Answer 3

Stretch receptors of stomach wall -> sensory information to CNS -> parasympathetic control.

Question 4

What nerves modulate peristaltic contraction of the small intestine?

Answer 4

ENS & ANS.

Question 5

What structures make up the large intestine?

Answer 5

Cecum, ascending, transverse, descending & sigmoid colons, rectum.

Question 6

Motility processes in the large intestine.

Answer 6

1. Mixing contractions/haustrations - local contractions which allow time for absorption of water & electrolytes
   (Post water & electrolyte absorption in the large intestine)
2. Peristaltic contraction/mass movement – haustra disappear -> faeces move into the rectum -> trigger defecation reflex.

Question 7

What nerves modulate haustrations in the large intestine?

Answer 7

Localised activation of myenteric nerves & circular smooth muscle cells.

Question 8

How many mass movements do most people have per day?

Answer 8

3-4, but can be up to 10.

Question 9

What nerves modulate peristaltic contractions in the large intestine?

Answer 9

Gastrocolic/duodeno-colic reflexes mediated by parasympathetic nerves.

Question 10

Explain consistency of faeces through large intestine.

Answer 10

Chyme (fluid) from SI -> fluid @ ileocecal valve -> semi-fluid @ ascending colon -> mush @ transverse colon -> semi-mush @ descending colon -> semi-solid @ sigmoid colon -> solid @ rectum.

Question 11

Excess motility results in ….. and what type of faeces?

Answer 11

Less absorption & diarrhoea/loose faeces.

Question 12

Poor motility results in ….. and what type of faeces?

Answer 12

Greater absorption & hard faeces/constipation.

Question 13

2 types of constipation

Answer 13

Slow transit or obstruction.

Question 14

2 types of diarrhoea

Answer 14

Organic or functional.

Question 15

What does a mass movement cause?

Answer 15

Movement of faeces from sigmoid colon -> rectum. Triggers defecation reflex.

Question 16

Explain the roles of autonomic, enteric & somatic motor nerves in the defecation reflex.

Answer 16

Autonomic Nerve Activation: stretch receptors activated -> stretch receptors synapse @ spinal cord neurons -> activation of parasympathetic nerves @ rectal wall & internal anal sphincters -> rectal contractions -> internal anal sphincter relaxation.
& Enteric Nerve Activation: rectal contractions -> activation of inhibitory myenteric neurons -> internal anal sphincter relaxation.
& Somatic Motor Nerve: inhibition of somatic motor nerves -> external anal sphincter relaxation -> defecation.
NB: Increased gut secretions also involved in the defecation reflex.

Question 17

What happens if you cannot or do not wish to defecate when you feel the urge to?

Answer 17

If we cannot or do not wish to defecate, then somatic motor nerve inhibition -> external anal sphincter contraction -> reverse peristalsis -> movement of faeces back into sigmoid colon (faeces will wait here until the next mass movement).

Question 18

What chemical substance triggers the migrating myoelectric complex?

Answer 18

Motilin (hormone).