Mechanisms of GI tract motility and its control

Question 1

What anatomical features are involved in gut motility?

Answer 1

1. Muscle in wall of GI tract (mainly smooth, some skeletal)
2. Nervous system (enteric but some control by ANS)
3. Endocrine influences (linking presence of food recognised by the epithelial cells in the lumen to increased motility)
4. Sphincters/valves at some critical points to ensure coming and one way movement

Question 2

Give the layers of the GI tract (inside outwards)

Answer 2

Lumen, mucosa, submucosal plexus, circular muscle, myenteric plexus, longitudinal muscle

Question 3

What are these two layers of the GI tract involved with?

1. Myenteric plexus
2. Submucosal plexus

Answer 3

1. Mainly involved in coordination of muscle contracted influenced by autonomic supply
2. Mainly involved in sensory functions and local responses to stimulation of sensory nerve endings in mucosa

Question 4

Give the details on the neurone types destinations of the enteric (intestine)

Where is overall regulation from?

Answer 4

Motor neurons:

• To smooth muscle
• Vasomotor to intrinsic arteriole (blood flow to get to the gut)
• Secretomotors (to cells regulating acid secretion by the stomach)
• To epithelium (enteroendocrine cells)

Interneurons:
- Various kinds (for co-ordination of reflexes)

Sensory Neurons:

• Chemosensitive
• Mechanoreceptors

Overall: the ANS

Question 5

How do we keep food going in one direction?

Give the 5 of the GI tract

Answer 5

This requires valves or sphincters between adjacent segments of the GI tract

Where are these … 
Upper oesophageal sphincter
Lower oesophageal sphincter
Pyloric sphincter  (controls movement from stomach to duodenum)
Lleo-caecal sphincter (to the colon)
Anal (internal and external) sphincters

Question 6

Give and explain the 3 stages of mastication

Answer 6

1. Oral: tongue pushes bolus against palate & back of mouth, triggering swallowing reflex
2. Pharyngeal: upper oesophageal sphincter relaxes while epiglottis closes to protect airways
3. Oesophageal: bolus moves downwards into oesophagus, propelled by peristalsis (and gravity)

Question 7

Explain the nervous innovation of the oesophagus

Answer 7

Upper: somatic motoneurons

Lower: autonomic neurones of the ENS

Question 8

In relation to the bolus, what part of the intestine contracts to get it to move forward?

Answer 8

Behind the bolus

Question 9

Give the 3 anatomical details of the lower oesophageal sphincter

Answer 9

1. Functional sphincter formed by tonic contraction of circular smooth muscle in wall of abdominal oesophagus - relaxes only when swallowing and vomiting
2. Angle at which the oesophagus enters the stomach is on the side so that intragastic pressure closes the oesophagus by squashing one wall
3. Surrounding diaphragm restricts the orifice diameter:
   contracts during inspiration and when intra-abdominal pressure rises

Question 10

What two things happen to the stomach when food arrives?

What region is the food moved towards?

Answer 10

1. Internal rughae flatten
2. Muscle wall relaxes by reflex action

The pyloric region (bottom of stomach).

Question 11

Explain the waves of gastric motility

Answer 11

Pacemaker sets off contractile waves.
Waves get stronger as you reach the pyloric zone.
Raising pressure in the pyloric antrum, squirting liquid chyme through narrow sphincter.

Narrow sphincter and incomplete occlusion of pyloric antrum lumen by the contraction, causes larger lumps to be regurgitated back into the wider part of the antrum for more mixing.

Question 12

Explain the motility of the small intestine

Answer 12

Main motility is segmentation.
Simultaneous contractions of different segments results in the mixing.
Overall transit rate is slow.

Peristalsis less frequent here (to allow time for digestion and absorption).

Small pressure gradient (higher in proximal end than distal end). Also helps forward movement of chyme.

Question 13

Explain the passage from small to large intestine

Answer 13

• Via another valve and sphincter

Valve prevents reflux back into ileum.
Smooth muscle sphincter.
Relaxes in response to upstream pressure.
Constricts in response to downstream pressure and to sympathetic stimulation.

Question 14

What are the two mechanisms for motility in the large intestine?

Answer 14

1. Haustrations: for mixing (short segments where the muscle in wall constricts)
2. Mass movements (for bulk movements) due to contraction of larger segments of muscle.

Question 15

What 2 sphincters controls the exit point?

Answer 15

Internal and external anal sphincter.

Question 16

What does defecation involve?

What nervous control are the internal and external sphincter under?

Answer 16

Involves co-ordination of contraction and relaxation of both skeletal muscle under voluntary control and smooth muscle under parasympathetic control.

Internal = autonomic 
External = somatic

Question 17

Give the 3 things that happen before defecation

Answer 17

1. Filling of rectum detected by afferents
2. Rectal circular muscle contracts while internal anal sphincter relaxed - both under PS control (external anal sphincter remains contracted)
3. When socially acceptable, external anal sphincter relaxes and levator ani contract both under voluntary control. Rectal smooth muscle also contracts.

Question 18

What are the two physiological factors regulating gut motility?

Answer 18

1. Neural regulation by ANS
   generally, motility increased by PS, decreased by S
2. Hormonal factors
   Inter-digestive phase: motilin - regulates the ‘background’ motility in the fasting period between meals – this is a migrating contractile activity which starts in the stomach and propagates along the SI

in the ‘post-prandial’ period (when digesting a meal):
initially gastrin (increases gastric motility) then, as digestion proceeds, later:
CCK (decreases gastric motility - helps with digestion)
GIP & GL1-P (decrease gastric motility - signals pancreas to increase insulin and means that glucose will be taken up)

Question 19

What 3 factors affect gastro-intestinal motility?

Answer 19

1. Diet composition: influences water retention. by gut contents, influences microbiota in the gut lumen
2. Medication
3. Age (ENS neuron degeneration in the elderly)

Question 20

What are problems caused by abnormal gut motility?

Answer 20

1. Regurgitation of acidic contents of gut can result in erosion of teeth and damage to oesophagus and larynx.
2. Malabsoprtion of foodstuffs including vitamins and minerals
3. Bacterial overgrowth within the intestine and a change in microbiota balance in gut lumen
4. Diarrhoea, with resulting dehydration and acid-base problems
5. Constipation