Introduction to the GI tract and Motility

Question 1

What are the four major functions of the alimentary canal?

Answer 1

1. Motility (movement)
2. Secretion
3. Digestion
4. Absorption

Question 2

How do electrical currents flow through smooth muscle cells?

Answer 2

Adjacent smooth muscle cells are coupled by gap junctions

Question 3

What drives slow wave electrical activity?

Answer 3

Interstitial cells of Cajal (ICCs) - pacemaker cells located largely between the circular and longitudinal muscle layers

Question 4

How do ICCs drive smooth muscle cells?

Answer 4

ICCs form gap junctions with each other and smooth muscle layers, electrically coupling them. Slow waves in ICCs drive slow waves in the smooth muscle cells coupled to them.

Question 5

What has to happen for contraction in the intestines to occur?

Answer 5

The slow wave amplitude must be sufficient to reach a threshold to trigger smooth muscle cell calcium action potentials (spikes)

Question 6

Whether slow wave amplitude reaches a threshold depends on:

Answer 6

Neuronal stimuli
Hormonal stimuli
Mechanical stimuli
(These generally act to depolarize smooth muscle cells - depolarization shifts slow wave peak to a threshold)

Question 7

Give an example of a local reflex

Answer 7

Peristalsis

Question 8

Give an example of a short reflex

Answer 8

Intestino-intestinal inhibitory reflex

Question 9

Give an example of a long reflex

Answer 9

Gastroileal reflex (a vago-vagal reflex)

Question 10

What is peristalsis?

Answer 10

A wave of relaxation, followed by contraction, that normally proceeds a short distance along the gut in an aboral direction - triggered by distension of the gut wall

Question 11

What is segmentation (mixing, churning movements)?

Answer 11

Rhythmic contractions of the circular muscle layer that mix and divide luminal contents

Question 12

What is colonic mass movement?

Answer 12

Powerful sweeping contraction that forces faeces into the rectum - occurs a few times a day

Question 13

What is migrating motor complex (MMC)?

Answer 13

Powerful sweeping contraction from stomach to terminal ileum

Question 14

What is the pressure of tonic contractions?

Answer 14

Low pressure - organs with a major storage function (e.g. stomach)
High pressure - sphincters

Question 15

What are sphincters?

Answer 15

Act as one way valves by maintaining a positive resting pressure relative to two adjacent structures (e.g. oesophagus and stomach). In general, stimuli (pressure) proximal and distal to a sphincter cause opening and closing, respectfully

Question 16

What are the 6 sphincters?

Answer 16

1. Upper oesophageal sphincter (UOS)
2. Lower oesophageal sphincter (LOS)
3. Pyloric sphincter
4. Ileocaecal valve
5. Internal anal sphincter
6. External anal sphincter

Question 17

What is motility (movement)?

Answer 17

Mechanical activity mostly involving smooth muscle (skeletal at mouth, pharynx, upper oesophagus and external anal sphincter)

Question 18

What does secretion involve?

Answer 18

Into the lumen of the digestive tract occurs from itself and accessory structures in response to the presence of food, hormonal and neural signals. Required for: (i) digestion, (ii) protection and (iii) lubrication

Question 19

What is digestion?

Answer 19

Chemical breakdown by enzymatic hydrolysis of complex foodstuffs to smaller, absorbable, units

Question 20

What is absorption?

Answer 20

Transfer of the absorbable products of digestion (with water, electrolytes and vitamins) from the digestive tract to the blood, or lymph – largely mediated by numerous transport mechanisms