12. Motility & Control

Question 1

Describe muscular contraction in muscularis of GI tract

Answer 1

Phasic: peristalsis/segmentation; oesophagus, small intestine, lower stomach

Tonic: minutes/hours; sphincters and fundus

Question 2

How does electrical conductivity spread in the GI tract?

Answer 2

Gap junctions - low electrical resistance

Question 3

What is the polarity of SMC of the GI tract?

Answer 3

Polarised - intracellularly negative

Question 4

Describe the level of polarisation and how it is set

Answer 4

Basic electrical rhythm set by pacemaker cells (non-contractile cells of Cajal)

Pacemaker cells may depolarise to reach the threshold for an action potential

Question 5

Why would an empty stomach contract?

Answer 5

Hunger pangs - vagus nerve activated due to a drop in glucose level

Question 6

Explain the filling of the stomach

Answer 6

50ml expands up to 1L

Rugae flatten, SMC plasticity and receptive/adaptive relaxation

Question 7

Where do peristaltic waves move towards?

Answer 7

Pylorus

Question 8

Where are the peristaltic waves most intense?

Answer 8

Near pylorus

Question 9

What is the purpose of the pyloric end in peristalsis?

Answer 9

Pours small amount of chyme into the duodenum whilst simultaneously emptying majority of contents back into stomach for further mixing

Question 10

Explain the stimulation of gastric emptying

Answer 10

Wall distension and food presence releases gastrin, activation of CNS and local nerves, motility increased and stomach emptied

Question 11

Explain the inhibition involved during the intestinal phase

Answer 11

Stomach emptied into duodenum causes release of inhibitory hormones such as secretin, CKK and GIP.

This causes inhibition of gastric motility and nerve reflexes.

Question 12

How long is the material retained?

Answer 12

18-24 hours but up to a week

Question 13

What is the main type of motility?

Answer 13

Segmentation

Question 14

How often does peristalsis occur and what is the fate of the material in that time?

Answer 14

1-3 times a day, propels food into empty descending and sigmoid colon and rectum

Question 15

What is a likely reflex as a result of peristalsis after a few times in a day?

Answer 15

Defaecation - gastrocoloic reflex

Question 16

Describe the defaecation reflex

Answer 16

Distension of rectal walls by faecal movement
Depolarises sensort fibres
Parasympathetic motor fibres stimulate rectal wall contraction and relaxation of internal anal sphincter

Question 17

What is diarrhoea?

Answer 17

Passage of watery faeces with increased volume/frequency

Question 18

Give 4 reasons why food accumulates in the intestinal lumen

Answer 18

Osmosis
Defective ion transport
Hyper-motility of intestine
Active secretion

Question 19

What is the difficulty in defaecation associated with?

Answer 19

Excessive dehydration of the faeces in large intestine

Question 20

What may constipation result in?

Answer 20

Haemorrhoids

Over exposure of colonic mucosa to carcinogens

Question 21

Give 4 factors that could contribute to constipation

Answer 21

Drugs
Lifestyle/exercise habits
IBS
Female hormones

Question 22

Main function of large intestine

Answer 22

Storage of faecal material
Further absorption of liquids and electrolytes
No villi, therefore no further absorption

Question 23

What drugs enhance motility of intestines?

Answer 23

Stimulant laxatives e.g senna, biscodyl

Question 24

What drugs reduce motility?

Answer 24

Opiates

Anticholinergics

Question 25

What drugs enhance motility of stomach and small intestines?

Answer 25

Dopamine antagonists (used in migraine treatment)

Question 26

Give 4 factors contributing to motility disorders

Answer 26

Damage to extrinsic nerves e.g spinal injury

Damage of intrinsic nerve plexus (myenteric)

Migraine

IBS