Mechanisms of GI tract motility and its control Flashcards
What’s involved in gut motility
-Muscle in wall of GI tract (mainly smooth, but skeletal at a few critical points)
-Nervous system (enteric, plus some extrinsic control by the ANS)
-Endocrine influences (linking presence of food in the gut to increased motility)
-Sphincters/valves at some critical points to ensure correct timing and one-way traffic
Gut cross section
Submucosal plexus - nerves cells
Circular muscle - squeezes lumen
Myenteric plexus - nerve cells
Longitudinal muscle - flatten and elongate lumen
What is the myenteric plexus (Auerbach’s plexus)?
-Mainly involved in coordination of muscle contraction
-Influenced by autonomic supply
What is the submucosal plexus (Meissner’s plexus)?
-Mainly involved in sensory functions and local responses to stimulation of sensory nerve endings in mucosa
Features of enteric nervous system
Motor neurones:
-to smooth muscle (excitatory & inhibitory)
-vasomotor (to intrinsic arterioles)
-secretomotor (to cells regulating acid secretion by the stomach)
-to epithelium (enteroendocrine cells)
Interneurons:
-various kinds, for co-ordination of reflexes
Sensory neurons:
-chemosensitive (e.g. for H+)
-mechanoreceptors (e.g. for distension)
What is the enteric nervous system?
-Largely independent system
-Overall regulation by ANS
-And contributions from somatic motor neurones to specific regions (upper oesophagus)
-Controls GI system
Sphincters in the GI tract
-Upper oesophageal sphincter
-Lower oesophageal sphincter
-Pyloric sphincter
-Ileo-caecal sphincter
-Anal (internal and external) sphincter
Phases of mastication and swallowing
Oral
-Tongue pushes bolus against palate & back of mouth, triggering swallowing reflex
Pharyngeal
-Upper oesophageal sphincter relaxes while epiglottis closes to protect airway
Oesophageal
-Bolus moves downwards into oesophagus, propelled by peristalsis (and gravity)
Innervation of the oesophageal muscle
-Upper oesophagus is striated muscle
-Lower oesophagus is smooth muscle
-Striated muscle is innervated by axons of somatic motor neurons
-Smooth muscle is innervated by neurones of the ENS
3 ways lower oesophageal sphincter prevents reflux?
Functional sphincter
-Formed by tonic contraction of circular smooth muscle in wall of abdominal oesophagus
-Only relaxes during swallowing and vomiting
Angle at which the oesophagus enters the stomach
-Means intragastic pressure can close the end of the oesophagus by squashing one wall
Surrounding diaphragm restricts the orifice diameter
-Contracts during inspiration & when intra-abdominal pressure rises
How does stomach increase capacity?
-Capacity can increase from 50ml to 1.5l
-Internal rugae flatten
-Muscle wall relaxes (receptive relaxation) by reflex action (brainstem, PNS)
-Contractions of stomach wall mix contents and propel it to pyloric region
Gastric motility
-Pacemaker sets off contractile waves (3/min)
-Waves get stronger as they approach pyloric zone
-Raising pressure in pyloric antrum squirts liquid chyme through narrow sphincter
-Narrow sphincter and incomplete occlusion of pyloric antrum lumen by contraction, causes larger lumps to be regurgitated back into antrum for more mixing
Control of gastric emptying
-Duodenum senses increase in acidity, increase in fat/amino acids, hypertonicity and distension
-This stimulates neural receptors which inhibit gastric emptying via short and long neural reflexes
-This stimulates hormone secretion by duodenum
Motility of small intestine
-Simultaneous contractions of different segments results in mixing
-Peristalsis less frequent (allow time for digestion/absorption)
-Small pressure gradient (higher in proximal end) helps forward movement of chyme
-Overall transit rate is slow (2 hours)
Passage from small to large intestine
-Ileo-caecal valve/sphincter prevents reflux back into ileum
-Relaxes in response to upstream pressure
-Constricts in response to downstream pressure, and to sympathetic stimulation
What helps motility and water absorption in large intestine
-Haustrations for mixing (short segments where circular and longitudinal muscle in wall constricts)
-Mass movements (for bulk movement) due to contraction of longer segments of circular muscle once/30 mins
What controls defecation?
Co-ordination of contraction and relaxation of:
-skeletal muscle under voluntary control
-smooth muscle under parasympathetic control
How does defecation occur?
-Filling of rectum detected by afferents -> desire to defecate
-Rectal circular muscle contracts while internal anal sphincter relaxes (both under PS control) External anal sphincter remains contracted
-When pooping
-External anal sphincter relaxes and levator ani contract (both under voluntary control) Rectal smooth muscle also contracts
Physiological factors regulating gut motility
Neural regulation by ANS:
-Generally, motility increased by PS, decreased by S
-Few specific exceptions to this, mainly affecting sphincter muscle
Hormonal factors:
During ‘inter-digestive’ period:Motilin -Regulates the ‘background’ motility in the fasting period between meals
-migrating contractile activity which starts in the stomach and propagates along the SI in ‘post-prandial’ period
-Initially gastrin (increases gastric motility)
-As digestion proceeds:
-CCK (decreases gastric motility) -GIP & GL1-P (decrease gastric motility)
Factors that affect gastro-intestinal motility
Diet composition
-influences water retention by gut contents, and therefore ‘consistency’ of stools
-influences microbiota in the gut lumen (may have direct effects on local ENS neurones)
Medication
-prescribed, ‘over-the-counter’ and other
Age
-ENS neuron degeneration in the elderly
Problems caused by abnormal gut motility
Regurgitation of acidic contents of gut can result in erosion of teeth, and damage to oesophagus and larynx
Causes: diaphragmatic hernia, obesity, bulimia, stroke (affects swallowing and upper oesophageal motility), side effect of some medications
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
