*Mechanisms of GI Motility Flashcards
What are the two plexuses locally control the muscularis externa?
Myenteric/Auerbach’s plexus
Submucosal/Meissner’s plexus
What is the function of the myenteric plexus?
Mainly involved in coordination of muscle contraction
Influenced by ANS
What is the function of the submucosal plexus?
Mainly involved in sensory functions and local responses to stimulation of sensory nerve endings in mucosa
Where are the locally innervating plexuses located?
Myenteric - between muscle layers of muscularis externa
Submucosal - submucosa/closer to lumen than circular layer of muscle
What do the motor neurons of the enteric nervous system supply?
Smooth muscle (excitatory and inhibitory)
Vasomotor neurons for intrinsic arterioles to control blood flow in gut
Secretomotor neurons for cells regulating acid secretion
Epithelium to sense what is occurring in lumen (enteroendocrine cells)
Why does the enteric nervous system have interneurons?
Coordinate reflexes
What are the two types of sensory neurons of the enternic nervous system?
Chemosensitive
Mechanoreceptors
What is the function of sphincters/valves at certain points?
Ensure one-way traffic
Appropriate delaying
Describe the phases of swallowing.
Oral - tongue pushes bolus against palate and back of mouth (detected by back of palate) to trigger swallowing reflex
Pharyngeal - upper oesophageal sphincter relaxes while epiglottis closes (prevent aspiration)
Oesophageal - bolus moves downwards into oesophagus propelled by peristalsis and gravity into stomach (seconds)
What controls swallowing?
Oesophageal muscle and its innervation
What controls the striated muscle of the oesophagus and therefore the swallowing reflex?
Brainstem motor neurons
What controls the smooth muscle of the oesophagus?
Parasympathetic nervous system via the vagus nerve modulate activity of ENS
What is peristalsis?
Progressive contraction behind bolus to propel it forwards
Why is the lower oesophageal sphincter important?
Prevents reflux of acid into oesophagus/mouth
Why does the food enter the stomach at an angle (rather than at the top)?
Allows intragastric pressure to close the end of the oesophagus/lower oesophageal sphincter by contracting one wall
What helps shut the lower oesophageal sphincter?
Surrounding diaphragm
Tonic contraction of circular smooth muscle in abdominal oesophagus
When does the lower oesophageal sphincter shut?
Inspiration
When intra-abdominal pressure rises
Why do pregnant women tend to experience excess gastric reflux?
Greater pressures below diaphragm/in abdomen alter closing of lower oesophageal sphincter
How does the capacity of the stomach change when food arrives?
50ml - 1.5L
Internal rugae flatten
Muscle wall relaxes (reflex)
What initiates stomach contractions?
Pacemaker cells
Describe the contractions of the stomach wall.
Pacemaker cells cause waves of contractions (3/min) along the wall angles towards the pyloric region
Waves get stronger as they approach pyloric region to force chyme into duodenum
What happens to food which is too large to pass into the duodenum?
Returned to body of stomach for further breakdown
How is gastric emptying controlled?
Duodenum detects increased acidity, fat, amino acids, hypertonicity, distension
Increases secretion of enterogastrones
Stimulates:
- Short neural reflexes via enteric neurons
- Long neural reflexes to CNS (increase SNS and decrease PNS)
To decrease gastric emptying
What is the main form of motility in the small intestine?
Segmentation
What is the difference between segmentation and peristalsis?
Segmentation = no forward movement, for mixing
What is segmentation?
Simultaneous contraction of different segments
What is the transit rate in the small intestine?
2 hours
Why is peristalsis less frequent in the small intestine?
Allow time for digestion and absorption
What helps forward movement in the small and large intestines?
Small pressure gradient (higher pressure in proximal regions)
Which valve separates the small and large intestines?
Ileo-caecal valve
What causes the ileo-caecal valve to contract/relax?
Local control:
- relaxes in response to upstream pressure (mechanoreceptors)
- contracts in response to downstream pressure AND sympathetic stimulation
What are haustrations?
Short segments where circular and longitudinal muscle in wall constrict
How is the contents propelled forward in the large intestine?
Mass/bulk movements
Contraction of longer segments of circular muscle every 25-30mins
How does the contents of the large intestine change as it passes through?
Fluid –> mush –> solid
What type of muscle are the internal and external anal sphincters?
External - voluntary/skeletal
Internal - smooth (parasympathetic)
What aids in defecation other than sphincters?
Levator ani pull up
Increased abdominal pressure
What is the process of defecation?
- Filling of rectum detected by afferents results in the desire to defecate
- Rectal circular muscle contracts while internal anal sphincter relaxes
- External anal sphincter relaxes and levator ani contract (voluntary), rectal smooth muscle also contracts
Stimulation by what part of the ANS will usually increase gastric motility?
Parasympathetic
What is the interdigestive period?
Fasting period between meals
What does motilin do?
During interdigestive period, regulates ‘background’ motility (migrating contractile activity starting stomach and propagating along small intestine)
What is the post-prandial period?
When digesting a meal
What hormonal factors are involved during the post-prandial period?
Initially gastrin
Then cholecystokinin and GIP and GL1-P
What effect does gastrin have on gastric motility?
Increases
What effect does cholecystokinin have on gastric motility?
Decreases
What stimulates the release of cholecystokinin?
Lipids
What stimulates the release of GIP and GL1-P?
Glucose in duodenum
What effects do GIP and GL1-P have?
Decrease gastric motility
Increase insulin secretion
Where is cholecystokinin released?
Duodenum
How can diet composition affect digestion?
Influence water retention by gut contents = stool consistency
Influences microbiota in gut lumen which may affect local ENS neurons
Give an example of how microbiota in the gut lumen can affect digestion.
Some produce short chain fatty acids which aid water reabsorption
What physiological factors can influence motility?
ANS
Hormones
What extrinsic factors can influence motility?
Diet composition
Medication
Age
Why might the elderly experience constipation?
Degeneration of ENS neurons results in decreased reflexes
What can abnormal motility cause?
Regurgitation
Malabsorption
Bacterial overgrowth in intestine
Diarrhoea
Constipation
What can cause acid reflux/regurgitation?
Diaphragmatic hernia (open lower oesophageal sphincter)
Obesity
Bulimia
Stroke (affects central reflexes like swallowing)
Medications
Why is diarrhoea bad?
Dehydration
Acid-base issues
Why is constipation bad?
Discomfort
Dangerous straining
What is the danger of the Valsalva manouvre?
Large changes in blood pressure and heart rate due to straining with increased abdominal pressure
Death if you have a weak heart
