Control of gut motility

• Describe the motility of the GI tract

  • secretory cells are located within the gut mucosa
  • Smooth muscle layers contract and relax to regulate movement of food alongside sphincters.
  • Plexi are responsible for controlling muscle contraction and hormone secretion
  • Circular muscle narrows and longitudinal muscle shortens to restrict food passage
• Describe the structure of the mucosa

  • Mucosa consists of an epithelial layer surrounded by lamina proparia and muscularis mucosa. These layers contain mucosal glands and ducts that deliver secretions to the gut.
  • Many blood vessels and lymphatic vessels to transport absorbed nutrients
  • Clusters of lymphocytes defend the mucosa from ingested bacteria
• Describe muscle contraction in the gut wall

  • PNS- increase motility and gastric secretions by integrating with the enteric nervous system
  • Long reflexes- impulses sent via sensory vagus nerve fibres from the intestinal wall to the brainstem. Then motor vagal fibres cause contraction of the smooth muscle of the gut.
  • Sympathetic nerves from the thoracic to lumbar regions of the spinal cord stimulate relaxation, which reduces motility and increases vasoconstriction
• Describe how autonomic control is mediated in the gut

  • Sympathetic innervation of myenteric plexus, submucosal plexus and epithelial cells through release of noradrenaline
  • Parasympathetic innervation of myenteric plexu and submucosal plexus through release of acetylcholine. The PNS generally increase GI functions.
  • ENS secretes NO, GLP and VIP
• Describe the slow waves in the stomach

  • Unstable resting potential in smooth muscle fibres: -40 to -60 mV. If membrane potential exceeds -40 mV, ion channels open, which allows influx of Na+ and Ca2+ and spike potentials occur.
  • These channels are slow to open and close, resulting in longer action potentials that trigger contraction.
  • Size and duration of contraction is related to the frequency of spike potentials.
• Describe how spike potentials lead to muscle contraction

  During spike potentials, calcium enters the cells and binds to calmodulin to activate myosin filaments, leading to contraction.
• Describe the structure of ICC cells

  • fuse form bodies with multiple processes from the central body
  • Found within the submucosa where they form an extensive network within the same region as the myenteric ganglion network.
• Describe how ICC cells can act as pacemakers

  The ICC cells have unique ion channels that periodically open, generating inward currents which may generate slow wave activity. They produce slow waves that initiate phasic contractions of the gastrointestinal smooth muscles.
• Describe the steps in slow wave formation in ICC cells

  1. Voltage gated Ca2+ channels open, which leads to a threshold for the upstroke potential.
  2. This leads to further calcium entry.
  3. Then the plateau phase occurs when the membrane is repolarised to the maximum negative membrane potential. This occurs due to inward current through non-selective cation channels.
• Describe how interstitial cells can initiate contraction in the gut

  If the slow wave is of great enough amplitude, L-type Ca2+ channels are activated in smooth muscle cells, which can result in a Ca2+ action potential superimposed upon the slow wave, which results in sufficient Ca2+ entry to elicits contraction.
• Describe the modulation of slow waves

  • Food stimulates nerve and hormone activity by stretching the muscle or stimulation by ACh or gastrin cause depolarisation of the membrane
  • SNS stimulation with noradrenaline leads to hyperpolarisation, which makes the muscle less excitable
• Describe the role of enterochromafiin cells in gut wall contraction

  1. Release serotonin across the basolateral membrane
  2. Serotonin binds to receptors to stimulate afferent neurones, which stimulates the ENS to increase peristaltic contractions
  3. Signal is terminated by 5HT transporter; SERT, which transport serotonin into enterocytes.
• What is PIEZO 2?

  This is a mechanosensor that is required to stimulate serotonin release. When it is mechanically stimulated, its channel opens allowing an influx of calcium, which leads to exocytosis of serotonin containing vesicles.
• Describe the oral phase in swallowing

  Oral stage: food is pushed towards the pharynx by the tongue
• Describe the pharyngeal stage in swallowing

  1. Soft palate elevates to close off nasopharynx
  2. Successive constriction of pharyngeal constrictors forces bolus through the pharynx and the upper oesophageal sphincter relaxes
  3. Bolus moves through the pharynx, which forces the epiglottis to bend and close of the larynx
• Describe the oesophageal stage in swallowing

  1. Peristaltic contractions of the oesophagus propel food towards the stomach, causing the lower oesophageal sphincter to relax and open
  2. Food enters stomach
• Describe how the stomach acts as an adaptive storage

  • Mechanoreceptors in the fundus detect small increases in pressure when food enters the stomach, resulting in NO release from the myenteric plexus, which relaxes circular muscle.
  • This enables the stomach to hold large volumes.
• Describe the function of peristalsis in the movement of food through the stomach

  • Muscles in the stomach exhibit basal electric rhythm, which generates weak peristaltic constrictor waves
  • This forces antral contents into the pylorus and contractions force contents back towards the antrum to create a churning action
• Describe stimulation of stomach emptying

  1. Increased distension of the stomach causes myenteric reflexes and gastrin release, which increases activity of the pyloric pump.
  2. Generates strong peristaltic contractions
• Describe inhibition of stomach emptying

  1. Peptides and acid enter duodenum and indicates reflexes that inhibit propulsive contractions and increase tone in the pyloric sphincter
  2. Fats enter duodenum cause CCK release, which blocks gastrin effects.
  3. Secretin is released in response to gastric acid and decreases contractions
• Describe segmentation in the small intestine

  • Contraction of circular muscle leads to segmentation of the small intestine, food mixing and slows the passage of food.
  • Mechanosensory receptors detect stretch as food enters and initiates reflex’s to induce smooth muscle contraction
• Describe peristalsis in the small intestine

  • Sensory neurones detect stretch in the gut wall and triggers reflex contraction
  • Food moves along towards the ileo-caecal valve
  • Contractions can be increased by parasympathetic input or decreased by sympathetic input
  • Gastrin and CCK enhance motility, while secretin inhibits it
• Describe small intestinal reflexes

  • Gastra-ileal reflex: initiated when stomach fills. Moves chyme towards ileo-caecal valve
  • Ileogastric reflex: initiated due to distension of the ileum and feeds back to the stomach to reduce gastric motility.
• Describe propulsion in the colon

  • Chyme moved along by haustral contractions and mass movements. This is a form of peristalsis where a ring contraction occurs in response to distension of the colon.
  • Initiated by gastrocolic and duodenocolic reflexes.
• Describe the short reflex in the rectum

  • Mass movements allow some faeces to pass into the rectum and increase intraluminal pressure, which imitates rectosphincteric reflex.
  • This causes relaxation of internal sphincter and contraction of external sphincter.
• Describe the long reflex in the rectum

  • Stimulated by the PNS
  • Strengthens peristaltic waves and sufficient to empty large intestine
  • Contraction of abdominal muscles increases intra-abdominal pressure, which relaxes puborectalis and widens anorectal angle, leading to voluntary relaxation of external anal sphincter.