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Gastrointestinal motility

Anna Hogarth Metabolism English 43 cards

How does the pyloric sphincter perform a sieving function and what are the three phases of sieving?

Cards in this deck (43)

  1. 1What is the function of extrinsic regulation of the gut? Describe the extrinsic regulation of the gut.1) Adds an extra element of control to something which is self-regulatory
    2) Autonomic:
    PSN: Vagus nerve projects from the brainstem to the stomach and small intestine (less so to colon). The pelvic-splanchnic nerves project to the colon and the anus. Excitatory through ACh.
    SNS: Prevertebral projections throughout the GI (greater splanchnic, lumbar colonic and hypogastric nerve). Inhibitory through NA.
  2. 2What are the two components of the enteric nervous system/where are they located? What do they regulate?1) Myenteric plexus (located between the longitudinal and circular muscle layers of the muscularis externa) and the submucosal plexus (located in the submucosa).
    2) Gi motility, local blood flow and secretions. Contain sensory elements, interneurons and motor neurons. Interacts with gut endocrine and immune systems.
  3. 3Where do sensory enteric neurons project to?The villi in the mucosa.
  4. 4What are the interstitial cells of Cajal? Where are they located?1) Group of cells which spontaneously depolarise, create a rhythm of electrical slow waves, causing phasic muscle contractions (myogenic) of the smooth muscle - electrically coupled via gap junctions, form a syncytium.
    2) Located around the myenteric plexus (stomach and colon), located around the submucosal plexus (colon)
  5. 5How do interstitial cells of Cajal interact with extrinsic regulation?Influence ability of hormones and neurotransmitters to induce propulsive and other movements. Facilitate nerve transmission to muscle.
  6. 6What are the dominant control systems of the following: 1) Oesophagus 2) Stomach 3) Small intestine 4) Colon 5) Rectum1) CNS control (vagus) - starts off digestion with swallowing
    2) Myogenic (ICC), ENS, CNS
    3) ENS
    4) ENS
    5) ENS and CNS (spinal control)
  7. 7What is the duration of a migrating motor complex? Where do they originate from? What is their purpose?1) 3 phases every 90-120 minutes (1=quite, 2=rhythmic movement, 3=large surge in activity)
    2) Stomach (vagus-dependent) or small intestine (vagus-independent).
    3) Clear undigested material, prevent bacterial overgrowth and possibly to cause hunger sensation.
  8. 8What are the three stages of food intake?1) Cephalic - thought, sight, smell, and taste (learned responses). Abolishes MMC, prepares GI tract - saliva, gastric acid, pancreatic secretions, gastrin and ghrelin secretion
    2) Gastric phase - Satiation, early digestion, gastric emptying, triggered mechanically.
    3) Intestinal phase - feedback and satiation, triggered by chemoreceptor activation in the small bowel
  9. 9Describe the two parts of swallowing/peristalsis in the oesophagus.Initiated by the vagus, acting on striated muscle. As a bolus moves through the oesophagus it causes distention of stretch receptors which results in a local reflex (ENS) initiating a second peristaltic wave.
  10. 10What happens to the LES during (and just after) swallowing?1) LES relaxes through shortening of the longitudinal muscle aids food entry into the stomach.
    2) Reflux back into the oesophagus is prevented by a closure of the LES and tighething of crural fibres - LES is a physiological sphincter, acute angle and contraction of diaphragm.
  11. 11In the proximal stomach, how are receptive and adaptive relaxation mediated/what is the point of this? How is build up of gas alleviated?1) Receptive relaxation is a vago-vagal reflex mediated by CCK. Adaptive relaxation then takes over - enteric reflex releases NO (ENS). This accommodates food entering the stomach and prevents vomiting.
    2) Transient relaxation LES allows gas out (burping), relieves pressure, prevents early satiety and allows you to keep eating (vagus)
  12. 12What is the function of the distal stomach? (In the fed state)Contractions/movements different to those MMCs (as this the fed state). Stomach contracts against a closed pyloric sphincter - builds up mechanical pressure and generates propulsion and retropulsion.
  13. 13Describe the generation of slow wave activity in the stomach (where is the dominant pacemaker located). How does it vary within the stomach?1) ICCs generate the activity, propagate from the dominant pacemaker in the corpus, down and around the pylorus.
    2) Higher frequency near the greater curvature of the stomach, slow waves reaching mid-lower corpus form into incomplete wave-fronts.
  14. 14How does the pyloric sphincter perform a sieving function and what are the three phases of sieving?How does the pyloric sphincter perform a sieving function and what are the three phases of sieving?
  15. 15What does the contraction of the mid-corpus of the stomach facilitate? Movement into the antrum?1) Mixes and breaks down food together with gastric acid and peptidases.
    2) Movement into the antrum enables powerful contraction (against a closed pylorus) which further breaks food into particles small enough to enter the duodenum.
  16. 16How is the rate of expulsion from the stomach regulated? (3)1) Physical properties (liquids vs. solids/particle size)
    2) Neuronal and hormonal feedback
    3) Nutritional value
  17. 17Describe the rate of gastric emptying for liquids, solids and viscous content.1) Liquids is exponential
    2) Solids - lag period as large particles can only pass through after sufficient grinding.
    3) Viscous chyme (from break down of solids) emptied mainly linearly.
  18. 18When does fast emptying occur? How does volume affect this?1) When there are not solids to grind and liquify
    2) Large volumes empty faster than small volumes
  19. 19Define slow gastric emptying. How is it effect by nutritional value?1) 20-30 minute lag time in digestion
    2) The greater the nutrient density the slower the gastric emptying - don't want to to overload the intestine.
  20. 20How is the rate of gastric emptying regulated? What else is regulated by these features?1) Feedback from intestine - both neuronal and hormonal
    2) Regulates satiety and insulin secretion
  21. 21How does signalling from the GI change before and during food intake?1) Nutrient intake is initially regulated by the vagus in response to gastric distention and accomodation.
    2) Nutrient intake is then influenced by signalling from the intestine after exposure to nutrients.
  22. 22What do duodenal and jejunal 'brakes' respond to? How do they mediate their effect?1) Nutrients, osmolality and hydrochloric acid.
    2) Release hormones (cholecystokinin - CCK) from mucosa. This acts on vagal afferents to mediate effects of vagal efferents (ACh) - reduces opening of pyloric sphincter, reduces gastric contractions, increases release of NO in fundus to enhance relaxation and storage.
  23. 23Which cell type is responsible for the feed-back physiological regulation (in response to nutrients)?Enteroendocrine cells - act as nutrient receptors (e.g. have free fatty acid receptors, tastant receptors etc.)
  24. 24What is the ileal brake? What does it promote? What are the three candidate mediators?1) Fats are usually absorbed in the duodenum - if they reach the ileum this suggests that we are eating too fast/not digesting properly. Feedback slows gastric emptying and induces satiety
    2) Peptide YY (PYY), Glucagon-like Peptide-1 (GLP-1) and oxyntomodulin.
  25. 25In summary what are the five types of movement in the upper GI tract?1) Primary and secondary peristalsis in the oesophagus (vagus and then ENS)
    2) Lower Oesophageal Sphincter - prevents reflux and TLESRs (burping)
    3) Gastric fundus relaxation (initially vagal, then ENS)
    4) Gastric 'mixing', 'grinding' and emptying (against pyloric sphincter, dependent on particle size)
    5) Feedback regulation - nutrient, pH, osmolarity, hormonal and neuronal.
  26. 26—XX
  27. 27What is the basic movement in peristalsis?Contraction at the oral end, relaxation at the anal end.
  28. 28What do enteric sensory neurons detect? What can they initiate?1) INTRALUMINAL SIGNALS -
    Chemical signals - from endocrine cells (e.g. 5-HT). Nutrients and low pH. Mechanical signals (mucosal deformation, stretch)
    2) Initiate peristalsis, secretion and vascular flow
  29. 29How do enteric sensory neurons mediate both of the necessary movement of peristalsis?Release ACh (onto myenteric motor neurons) at the oral end to induce contraction, NO (and ATP, purinergic) at the anal end to induce relaxation.
    Also modulates the immune, vascular, muscular and epithelial transport systems.
  30. 30What are the functions of the 1) Ascending 2) Transverse 3) Descending colon and the transverse time of chyme through these regions?1) Mixing, absorption and fermentation - slow transit
    2) Absorption - rapid transit
    3) Storage - slow, partly voluntary transit
  31. 31Describe the structure of smooth muscle in the colon. What movements occur in the ascending colon? Transverse?1) Circular muscle - no functional longitudinal, instead three bands called taenia within the circular muscle - forms haustra (when relaxed). When the circular muscle contract semilunar (triangular) folds form.
    2) Propulsion, retropulsion and segmentation
    3) Mass movements
  32. 32What is the ascending colon rich in? What is it a site for? What is the point of haustra? How does movement differ to the small intestine?1) Bacteria
    2) Fermentation & absorption of water/ions/nutrients
    3) Increases the available surface area
    4) Peristalsis is unidirectional - movement in the ascending colon is more similar to the churning in the stomach.
  33. 33What is the point of segmentation? How is movement in the colon regulated?1) Mixes content and increases exposure to mucosa
    2) ICC - waves generate propagate for short distances in oral or anal directions. Local mediators also regulate (e.g. 5-HT; 5-HT antagonists can cause constipation).
    When a mass in a colon reaches a sufficient size this triggers the activity of endocrine cells to promote defecation. Also responds to local luminal environment.
  34. 34How is the ENS modulated?Functions independently but can be modulated by local and circulating hormones as well as by the central/spinal NS.
  35. 35Describe the mass movement through the ascending-descending colon.Chyme enters, haustra visible. As more chyme enters a large constrictive ring appears and the distal haustra disappear (20 cm distal) - contract as unit propelling faecal matter into the transverse colon. When a mass of faeces reaches the rectum this triggers the sensation of a need to defecate.
  36. 36How long do a series of mass movements usually last for?10-30 minutes - will then cease but usually re-appear within half a day.
  37. 37Describe the activity/pressure in rectal to anal movements.As contents enter the rectum this causes distention and a passive build up in pressure - this triggers relaxation of the internal anal sphincter and contraction of the external sphincter. As contents continue to enter the rectum this eventually triggers a conscious urge to defecate.
  38. 38Where is the defecation centre?Located in the lower spinal cord and regulated by cortical impulses (i.e. voluntary control). Feeds into pelvic ganglia and enteric reflex pathways.
  39. 39How is defecation usually prevented?1) Tone of internal anal sphincter and the puborectalis
    2) Mechanical effects of acute anorectal angle
  40. 40How is defecation initiated?1) Puborectalis and external anal sphincter relax
    2) Intra-abdominal pressure can be increased by squatting
  41. 41How is actual defecation mediated?Sphincter relaxation and rectal propulsive contractions.
  42. 42How do humans distinguish between faeces and gas?Both through mechanical and chemical (hydrogen sulphide and methane) signals
  43. 43—

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Dr. Sanger
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