Control of the alimentary tract

• Automatic Innervation of the Gut

  ANS enables food storage in the stomach
• Food movement through the alimentary tract

  1. Food in throat

  2. Moves past nasopharynx

  3. Reaches laryngopharynx and oropharynx

  4. Stomach relaxes to receive food

  5. Food travels to vagus centre

  6. Stomach undergoes adaptive relaxation
• Acetylcholine (ACh)


  Acts as an inhibitory vagal fibre, released presynaptically and postsynaptically, releases nitric oxide (NO)
• Movement of food into the duodenum

  1. Ripples of contraction move food towards antrum

  2. Pyloric sphincter relaxed but closes on peristaltic wave

  3. Repulsion of chyme opens pyloric sphincter

  4. Small partially digested material squirted into duodenum

  5. Repulsion of antral contents allows mixing/grinding

  6. Sieving effect retains viscous and solid matter in stomach
• Types of control in the alimentary tract

  • Endocrine
  • Paracrine
  • Neural
  • Vago-vagal reflex
  • Enteric or local reflex
  • Metabolic
  • Myogenic
• Endocrine control


  Hormones released, carried in blood to target site, bind to receptors on target cells to produce response
• Gut hormones


  All are peptides (sequence of amino acids)
• Gastric acid secretion and control

  1. Food in stomach stimulates more acid secretion

  2. Dietary peptides stimulate G cells to release gastrin

  3. Gastrin binds to receptors on ECL or parietal cells, causing acid secretion

  4. ACh can also bind to parietal cell receptors, causing acid secretion

  5. ACh or gastrin cause ECL cells to release histamine, which binds to H2 receptors, causing hypersecretion

  6. High luminal acid stimulates D cells to secrete somatostatin, inhibiting gastrin-secreting cells
• Paracrine control


  Paracrine agents go via interstitial fluid, not bloodstream
• Vago-vagal reflex


  • Reflex circuit within GI tract, with afferent and efferent axons in vagus nerve
  • Pathway via brain stem (medulla)
  • Reflex control of responses to gut stimuli via NTS and DMVN
  • Causes receptive relaxation of stomach in response to swallowing, and promotes motility and acid secretion

  
• Enteric Nervous System/Local Reflex

  • Two nerve fibre types: intrinsic (within gut wall) and extrinsic (act on gut from outside)
  • Intrinsic nerves in myenteric (Auerbach's) plexus (motor function) and submucosal (Meissner's) plexus (secretions)
  • Regulates GI functions entirely within gut wall, can function autonomously
  • Has similar number of neurons (100 million) as spinal cord
  • Neurotransmitters: ACh, NO, NA, 5HT, GABA, ATP
• Intrinsic Neuronal Plexuses of the Gut

  • Submucosal Meissner plexus regulates digestive glands
  • Myenteric Auerbach plexus primarily connected with gut motility
• Nervous Control of the GI Tract

  • Intrinsic controls: short reflexes mediated by local enteric plexuses
  • Extrinsic controls: long reflexes involving CNS centres and extrinsic autonomic nerves, including parasympathetic reflexes
• Metabolic control


  • Rate of gastric emptying depends on food type: liquids empty quickly, large chunks slowly (need grinding), carbohydrates empty quickly, proteins slowly, fats slowest (need emulsification)
  • Fatty acids in duodenum increase pyloric sphincter contractility, slowing emptying
• Myogenic control of gastric motility

  • Intrinsic basic electric rhythm (BER) - stomach muscle cells produce rhythmic depolarisation-repolarisation cycles
  • Slow waves from ICC create regular migrating ripples (3/min)
  • BER allows smooth muscle to contract rhythmically when exposed to hormonal signals