Functions and Dysfunctions of GIT Motility

Question 1

List 3 types of contractions of the small intestine.

What are the functions of these contractions?

Answer 1

Contractions of the small intestine include:

1 - Segmental contractions.

• These are associated with non-propulsive movement of chyme.
• The result is the increased mixing that enhances the digestion and absorption of nutrients.

2 - Peristaltic contractions.

• These are associated with propulsive movement of chyme.
• The result is propagation of chyme in the caudal direction for elimination of undigested material.

3 - Sphincter contractions.

• These allow the stomach and large intestine to act as reservoirs for chyme by separating these spaces of the GIT from the small intestine.

Question 2

What is the average length of the small intestine?

Answer 2

The average length of the small intestine is 9ft.

Question 3

How do segmental contractions result in the mixing of chyme?

Answer 3

• Intervals of rings of circular muscle contract and relax.
• This is followed by the same movement by adjacent rings, causing the chyme to move rhythmically between adjacent rings.

Question 4

How do peristaltic contractions result in the propulsion of chyme in the caudal direction?

Answer 4

• Rings of circular muscle contract sequentially from the rostral to the caudal direction, starting initially behind the bolus.
• At the same time, the muscle in front of the bolus relaxes, giving passage to the bolus.

Question 5

How do small intestine contractions differ in the fed state compared with the fasting state?

Answer 5

• In the fed state, segmental and peristaltic contractions occur.
• In the fasting state, the migrating motor complex occurs.

Question 6

What is the migrating motor complex (MMC)?

How does it change along the length of the small intestine?

When does it occur?

Answer 6

• The migrating motor complex is a large sweep of contraction that occurs during the fasting state that begins at the stomach and progresses caudally to the large intestine.
• As the contraction advances in the small intestine, the contractions become weaker.
• It occurs every 1-2 hours.

Question 7

List the functions of the migrating motor complex (MMC).

Answer 7

1 - MMCs clear the small intestine of its residual content, including undigested food, desquamated cells and intestinal secretions.

2 - MMCs prevent migration of intestinal bacteria to inappropriate sites.

Question 8

Give an example of a peptide hormone that has a major influence on the MMC patern.

How many amino acids comprise this peptide?

Where is this peptide synthesised?

Answer 8

• Motilin is a peptide hormone that has a major influence on the MMC pattern.
• It comprises 22 amino acids.
• It is synthesised in the mucosa of the duodenum.

Question 9

List 2 reflexes of the ileum.

Briefly describe the reflex.

Answer 9

1 - The gastroileal reflex increases ileal motility in response to increased gastric activity, preparing the small intestine for digestion of chyme.

2 - The ileogastric reflex decreases gastric activity in response to ileal distension, preventing further chyme from entering the duodenum.

*Mediated by the ENS.

Question 10

How is the opening of the ileocaecal sphincter controlled?

Answer 10

• Stimuli proximal to the sphincter cause sphincteric relaxation.
• Stimuli distal to the sphincter cause sphincteric contraction.
• These stimuli affect the ileocaecal sphincter through the ENS.

Question 11

What is the average length of the large intestine?

Answer 11

The average length of the large intestine is 4 feet.

Question 12

How does the surface area of the large intestine compare to that of the small intestine?

Why?

Answer 12

• The surface area of the large intestine is 1/30 that of the small intestine.
• This is because, although the large intestine has crypts, it has no villi.

Question 13

What are taeniae coli?

Which tissue layers comprise the taeniae coli?

Answer 13

• Taeniae coli are muscular bands which span the length of the large intestine.
• They are formed of 3 thickened layers of longitudinal muscle.

Question 14

List the functions of the large intestine.

Answer 14

Functions of the large intestine include:

• Functions specific to the proximal large intestine:

1 - Absorption of water.

2 - Absorption of electrolytes.

3 - Absorption of short chain fatty acids - a product of carbohydrate fermentation.

• Functions specific to the distal large intestine:

4 - Reservoir function.

• General functions:

5 - Regulation of release of faeces.

6 - Producing a microenvironment that is favourable for commensal bacteria which synthesise vitamins.

7 - Secretion of mucus.

8 - Secretion of ions.

Question 15

List 2 types of contractions of the large intestine.

What are the functions of these contractions?

Answer 15

Contractions of the large intestine include:

1 - Rhythmic phasic contractions (RPCs).

• These are the large intestine equivalent of segmental contractions.
• These are subdivided into short RPCs (2-3 seconds) and long RPCs (15-20 seconds).
• Short-duration RPCs don’t cause propagation, and their amplitudes vary considerably. They function to mix the chyme.
• Long-duration RPCs causes propagation over short distances. They function to turn the semi-solid contents into solid contents.
• RPCs enhance fermentation and desiccation of the faeces.

2 - Giant migrating contractions (GMCs).

• These are the large intestine equivalent of peristaltic contractions and migrating motor complex.
• They are large-amplitude contractions that propagate rapidly in the caudal direction, leading to the defecation reflex.
• GMCs occur 2-10 times per day in the proximal, middle and descending colon.

Question 16

Which component of the ENS mediates the control of motility in the colon?

Answer 16

The myenteric plexus mediates the control of motility in the colon.

Question 17

List the types of laxatives.

How do they work?

Give one example of each.

Answer 17

1 - Stimulant laxatives (act by increasing motility through chemoreceptor activation).

• E.g. senna glycoside.

2 - Osmotic laxatives (act by drawing water into the bowel through osmosis).

• E.g. lactulose.

3 - Emollient laxatives (non-absorbable lubricants that soften the faeces).

• E.g. docusate.

4 - Bulk-forming laxatives (dietary fibre and fibre-based products that form bulk in the GIT, causing intestinal distension and therefore increasing motility).

• E.g. bran.

Question 18

What is the effect of the sympathetic nervous system on GIT motility?

What about the parasympathetic nervous system?

Answer 18

• The sympathetic nervous system inhibits GIT motility.

- The parasympathetic nervous system stimulates GIT motility.

Question 19

Describe the parasympathetic innervation of the large intestine.

Answer 19

• Parasympathetic innervation of the caecum, ascending and transverse colon is via branches of the vagus nerve.
• Parasympathetic innervation of the descending and sigmoid colon is via the pelvic nerves from the sacral spinal cord.
• Vagal stimulation causes generation of rhythmic phasic contractions (RPCs).
• Stimulation of the pelvic nerves causes generation of giant migrating contractions (GMCs).

Question 20

Describe the defecation reflex.

Via which nerves is this reflex carried out?

Answer 20

1 - Faeces enter the rectum and cause distension.

2 - This causes relaxation of the internal anal sphincter, which is composed of circular involuntary smooth muscle.

3 - This is reinforced by parasympathetic amplification of the peristaltic waves.

4 - This is accompanied by stimulation of somatic motor neurones innervating the external anal sphincter, which is composed of striated voluntary skeletal muscle. This inhibits defecation, but can be overridden as the external anal sphincter is under voluntary control.

• The myenteric plexus brings about the defecation reflex. More specifically, it is mediated via sacral spinal segmental nerves.