Mechanisms of Gastrointestinal Motility and its control

• What can problems with gut motility result in?

  - malabsorption of foodstuffs, including vitamins and minerals

  - bacterial overgrowth within the intestine, and a change in the

  - balance of microbiota in the gut lumen

  - diarrhoea, with resulting dehydration and acid-base problems

  - constipation, causing discomfort & dangerous straining

  (it can lead to holding breath, heart attack and then death
• What is a problem caused by abnormal gut motility?

  regurgitation of acidic contents of gut can result in

  erosion of teeth, and damage to oesophagus and larynx

  
  

  -> this can be caused by diaphragmatic hernia (diaphragm doesnt close properly and acid goes up) , obesity (increases intragastric pressures), bulimia, stroke (affects swallowing and upper oesophageal motility), side effect

  of some medication, etc
• How can diet composition affect GI motility?

  - influences water retention by gut contents, and therefore 'consistency' of stools (hard/soft, etc)

  - influences microbiota in the gut lumen (may have some direct effects on local ENS neurons)
• what are additional extrinsic factors that affect GI motility?

  • diet composition

  • medication prescribed, 'over-the-counter' and other

  • age

  (especially ENS neuron degeneration in the elderly)
• how is the gut motility regulated by Hormonal factors?

  -> in the 'inter-digestive' period:

  motilin - regulates the 'background' motility in the fasting period between meals - this is a migrating contractile activity that 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)

  GIP & GL1-P (decrease gastric motility)
• how is the gut motility regulated by Neural regulation by ANS?

  generally, motility increased by PS and decreased by S

  -> there are a few specific exceptions to this, mainly affecting sphincter muscle
• What are normal physiological factors regulating gut motility?

  1. Neural regulation by ANS

  2. Hormonal factors
• rectum

  
  

  Describe the process of defecation.

  1. Filling of rectum with faeces detected by afferents, leads to a desire to defecate

  2. Rectal circular muscle contracts while internal anal sphincter relaxes- both under PS control

  for the moment, 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

  4. Faeces expelled

  
• rectum

  
  

  what is the external anal sphincter under control by?

  voluntary control
• rectum

  
  

  what is the internal anal sphincter under control by?

  the PNS
• rectum

  
  

  what two sphincters control the exit point?

  the internal and external anal sphincter
• rectum

  
  

  What is defecation and where does it occur?

  rids the body of indigestible waste; large intestine (defecation reflex in rectum)

  
  

  -> it involves co-ordination of contraction and relaxation of both skeletal muscle under voluntary control and smooth muscle under parasympathetic control
• large intestine

  
  

  What is the function of the colon?

  The progressive reabsorption of water, which changes the consistency of the gut contents
• large intestine

  
  

  What is the motility of the large intestine?

  • haustrations for mixing (short segments where circ. & longit.

  muscle in wall constricts)

  • mass movements (for bulk movement) due to contraction of longer segments of circ. muscle
• small intestine

  
  

  What does chyme move from the small to the large intestine via?

  Ileo-caecal 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.
• small intestine

  
  

  What helps the forward movement of chyme in the small intestine?

  There is a small pressure gradient (higher in the proximal than distal end of SI).

  
  

  Also, once absorption is complete, you get these stronger contractions that force chyme down the small intestine called migrating myoelectric complexes
• small intestine

  
  

  what is chyme?


  a thick semifluid mass of partially digested food and digestive secretions that is formed in the stomach and intestine during digestion.
• small intestine

  
  

  why is peristalsis less frequent in the small intestine?

  to allow time for digestion & absorption
• small intestine

  
  

  What is the main motility of the small intestine?

  Segmentation, and it's for mixing:

  Simultaneous contractions of different segments result in the mixing, but the overall transit rate is slow (2h)
• stomach

  
  

  How is gastric emptying controlled?

  the duodenum senses the presence of food/chyme and tells the stomach to stop delivering food into the duodenum until the food already there is successfully digested and is beginning its passage further down the small intestine

  
• stomach

  
  

  what is gastric emptying?

  the process by which the contents of the stomach are moved into the duodenum
• stomach

  
  

  How is gastric motility controlled by a 'pacemaker'?

  
• stomach

  
  

  how is bolus mixed and propelled towards the pyloric region?

  via Contractions of stomach wall
• stomach

  
  

  how does the capacity of the stomach increase?

  1. internal rughae flatten (increasing volume)

  
  

  2. muscle wall relaxes (= 'receptive relaxation') by reflex action

  (this allows stretch to occur)
• stomach

  
  

  When food arrives to the stomach, what does the capacity of the stomach increase to?

  It increases from 50ml to (up to) 1.5L
• what is peristalsis?

  contraction of the gut behind the bolus (mass of chewed food) causes the arithmic wave to move the bolus towards the stomach

  
• oesophagus

  
  

  Why does the oesophagus enter the stomach at an angle?

  Because it means that the intragastric pressure can close the end of the oesophagus by squashing one wall
• oesophagus

  
  

  What surrounds the lower oesophageal sphincter?

  The diaphragm which restricts the orifice diameter and contracts during inspiration & when the intra-abdominal pressure rises

  
  

  -> the diaphragm controls the sphincter and prevents reflux during breathing
• oesophagus

  
  

  When does the lower oesophageal sphincter relax?

  Only when swallowing, or during vomiting
• oesophagus

  
  

  How is the lower oesophageal sphincter formed?

  By tonic contraction of circular smooth muscle in wall of abdominal oesophagus
• oesophagus

  
  

  Describe the innervation of the oesophageal muscle wall.

  - The upper third consists of striated muscle innervated by axons of somatic motor neurones which deliver a brainstem-generated activity pattern to the striated muscle of the upper oesophagus.

  
  

  - The bottom two thirds are smooth muscle, innervated by vagus nerve of ANS and neurons of the ENS

  
  

  ----> the bit in between the upper and bottom areas is the overlap zone

  
• swallowing

  
  

  What are the 3 phases of swallowing?

  oral, pharyngeal, oesophageal

  
• swallowing

  
  

  What is swallowing?

  moving food from the mouth to the stomach
• swallowing

  
  

  what is mastication?


  breaking food up into smaller chunks (increases surface area of the food)
• What different sphincters are there in the GI tract?

  • Upper oesophageal sphincter

  • Lower oesophageal sphincter

  • Pyloric sphincter

  • Ileo-caecal sphincter

  • Anal (internal and external) sphincters
• what is the difference between a sphincter and a valve?

  - valve is a flap-like structure of a hollow organ that allows one-way fluid flow through it

  - sphincter is a ring-like muscle which is able to contract or close a bodily passage or opening.
• what is a sphincter?

  Sphincters are circular muscles that open and close passages in the body to regulate the flow of substances
• What controls the one-way system of the GI Tract and appropriate delay points during food consumption?

  Valves or sphincters between adjacent segments of the GI tract
• enteric nervous system

  
  

  what is the enteric nervous system regulated by?

  Overall regulation by the ANS & contributions from somatic motor neurons to a few specific regions

  (e.g. upper oesophagus)
• enteric nervous system

  
  

  what are the types of sensory neurons in the enteric nervous system?

  • chemosensitive (e.g. for H+)

  • mechanoreceptors (e.g. for distension)