Mechanisms of GI tract motility and its control Flashcards

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1
Q

What anatomical features are involved in gut motility?

A
  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
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2
Q

Give the layers of the GI tract (inside outwards)

A

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

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3
Q

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

  1. Myenteric plexus
  2. Submucosal plexus
A
  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
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4
Q

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

Where is overall regulation from?

A

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

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5
Q

How do we keep food going in one direction?

Give the 5 of the GI tract

A

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
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6
Q

Give and explain the 3 stages of mastication

A
  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)
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7
Q

Explain the nervous innovation of the oesophagus

A

Upper: somatic motoneurons

Lower: autonomic neurones of the ENS

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8
Q

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

A

Behind the bolus

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9
Q

Give the 3 anatomical details of the lower oesophageal sphincter

A
  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
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10
Q

What two things happen to the stomach when food arrives?

What region is the food moved towards?

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

The pyloric region (bottom of stomach).

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11
Q

Explain the waves of gastric motility

A

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.

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12
Q

Explain the motility of the small intestine

A

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.

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13
Q

Explain the passage from small to large intestine

A
  • 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.

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14
Q

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

A
  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.
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15
Q

What 2 sphincters controls the exit point?

A

Internal and external anal sphincter.

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16
Q

What does defecation involve?

What nervous control are the internal and external sphincter under?

A

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

Internal = autonomic 
External = somatic
17
Q

Give the 3 things that happen before defecation

A
  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.
18
Q

What are the two physiological factors regulating gut motility?

A
  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)

19
Q

What 3 factors affect gastro-intestinal motility?

A
  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)
20
Q

What are problems caused by abnormal gut motility?

A
  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