GASTROINTESTINAL PHYSIOLOGY

• The gastrointestinal tract is a continuous tube that stretches from the mouth to the anus

  
• Primary function of the gastrointestinal tract


  To serve as a portal whereby nutrients and water can be absorbed into the body
• Digestion and absorption in the gastrointestinal tract


  1. Meal is mixed with secretions from the gastrointestinal tract and other organs

  2. Intestine displays motility patterns to mix the meal and move it along the tract

  3. Residues that cannot be absorbed are expelled from the body
• The gastrointestinal system has evolved a large number of regulatory mechanisms that act both locally and to coordinate the function of the gut and the organs that drain into it over long distances
• The lumen of the gastrointestinal tract is functionally contiguous with the outside of the body

  
• The intestine has a very substantial surface area, which is important for its absorptive function

  
• The gut is an unusual organ in that it becomes colonized, almost from birth, with a large number of commensal bacteria (particularly in the colon, or large intestine)
• Parts of the gastrointestinal tract

  • Mouth
  • Esophagus
  • Stomach
  • Duodenum
  • Jejunum
  • Ileum
  • Cecum
  • Colon
  • Rectum
  • Anus

  
• GIT Sphincters


  • Upper esophageal sphincter
  • Lower esophageal sphincter
  • Pylorus
  • Ileocecal valve
  • Inner anal sphincter
  • Outer anal sphincter
• Layers of the GIT wall


  • Columnar epithelial cell layer
  • Lamina propria
  • Muscularis mucosa
  • Submucus layer
  • Muscular layer
  • Serous or fibrous layer
• Slow waves


  Slow, undulating changes in the resting membrane potential of gastrointestinal smooth muscle that determine the rhythm of contractions
• Spike potentials


  True action potentials that occur when the resting membrane potential becomes more positive than -40 millivolts, causing muscle contraction
• Differences between the myenteric and submucosal plexuses


  • Myenteric plexus controls gastrointestinal movements
  • Submucosal plexus controls gastrointestinal secretion and local blood flow

  
• Types of neurotransmitters secreted by enteric neurons

  • Acetylcholine
  • Norepinephrine
  • Adenosine triphosphate
  • Serotonin
  • Dopamine
  • Cholecystokinin
  • Substance P
  • Vasoactive intestinal polypeptide
  • Somatostatin
• Parasympathetic control of the gastrointestinal tract

  Increases activity of the enteric nervous system, providing extensive innervation to the esophagus, stomach, pancreas, and intestines

  
• Sympathetic control of the gastrointestinal tract


  Usually inhibits gastrointestinal tract activity, with effects opposite to the parasympathetic system
• Mastication (chewing)


  • Breakdown of foodstuffs into smaller particles
  • Mixing of saliva with food substances
  • Lubrication and moistening of dry food
  • Appreciation of taste of the food
• Muscles of mastication


  • Masseter
  • Temporalis
  • Medial and lateral Pterygoid
  • Buccinator
• Mastication


  The first mechanical process in the gastrointestinal (GI) tract, by which the food substances are torn or cut into small particles and crushed or ground into a soft bolus
• Teeth


  • The anterior teeth (incisors) provide a strong cutting action, and the posterior teeth (molars) provide a grinding action
  • All the jaw muscles working together can close the teeth with a force as great as 55 pounds on the incisors and 200 pounds on the molars

  
• Significances of mastication


  • Breakdown of foodstuffs into smaller particles
  • Mixing of saliva with food substances thoroughly
  • Lubrication and moistening of dry food by saliva, so that the bolus can be easily swallowed
  • Appreciation of taste of the food
• Muscles of Mastication


  • Masseter muscle
  • Temporalis muscle
  • Medial and lateral Pterygoid muscles
  • Buccinator muscle
• Most of the muscles of chewing are innervated by the motor branch of the fifth cranial nerve (mandibular branch of trigeminal nerve), and the chewing process is controlled by nuclei in the brain stem
• Stimulation of specific reticular areas in the brain stem taste centers will cause rhythmical chewing movements
• Stimulation of areas in the hypothalamus, amygdala, and even the cerebral cortex near the sensory areas for taste and smell can cause chewing
• Chewing reflex


  1. Presence of a bolus of food in the mouth initiates reflex inhibition of the muscles of mastication, allowing the lower jaw to drop

  2. Drop in jaw initiates a stretch reflex of the jaw muscles that leads to rebound contraction, causing closure of the teeth

  3. Compression of the bolus against the linings of the mouth inhibits the jaw muscles once again, allowing the jaw to drop and rebound another time

  4. This process is repeated again and again
• Swallowing (Deglutition)

  A complicated mechanism, principally because the pharynx subserves respiration and swallowing
• Stages of Swallowing

  • Oral or voluntary stage
  • Pharyngeal stage
  • Esophageal stage
• Oral or Voluntary Stage of Swallowing


  1. Bolus is placed over postero-dorsal surface of the tongue

  2. Anterior part of tongue is retracted and depressed

  3. Posterior part of tongue is elevated and retracted against the hard palate, pushing the bolus backwards into the pharynx

  4. Forceful contraction of tongue against the palate produces a positive pressure in the posterior part of oral cavity, pushing the food into pharynx
• Pharyngeal Stage of Swallowing

  1. Soft palate is pulled upward to close the posterior nares

  2. Palatopharyngeal folds are pulled medially to form a sagittal slit for food passage

  3. Vocal cords are strongly approximated, and the larynx is pulled upward and anteriorly, causing the epiglottis to swing backward over the opening of the larynx

  4. Upward movement of the larynx pulls up and enlarges the opening to the esophagus, while the upper esophageal sphincter relaxes

  5. Muscular wall of the pharynx contracts, beginning in the superior part and spreading downward, propelling the food by peristalsis into the esophagus
• The most sensitive tactile areas for initiating the pharyngeal stage of swallowing lie in a ring around the pharyngeal opening, with greatest sensitivity on the tonsillar pillars
• Impulses from these areas are transmitted through the sensory portions of the trigeminal and glossopharyngeal nerves into the medulla oblongata

  
• The successive stages of the swallowing process are then automatically initiated in orderly sequence by neuronal areas of the reticular substance of the medulla and lower portion of the pons
• The motor impulses from the swallowing center to the pharynx and upper esophagus that cause swallowing are transmitted successively by the fifth, ninth, tenth, and twelfth cranial nerves and even a few of the superior cervical nerves

  
• Esophageal Stage of Swallowing

  1. Primary peristalsis is continuation of the peristaltic wave from the pharynx into the esophagus

  2. Secondary peristaltic waves result from distention of the esophagus by retained food, initiated by intrinsic neural circuits and reflexes from the pharynx

  
• The musculature of the pharyngeal wall and upper third of the esophagus is striated muscle, controlled by skeletal nerve impulses from the glossopharyngeal and vagus nerves

  
• In the lower two thirds of the esophagus, the musculature is smooth muscle, but this portion is also strongly controlled by the vagus nerves acting through the esophageal myenteric nervous system

  
• When the esophageal peristaltic wave approaches the stomach


  A wave of relaxation, transmitted through myenteric inhibitory neurons, precedes the peristalsis, and the entire stomach and duodenum become relaxed to receive the food

  
• Lower Esophageal Sphincter (Gastroesophageal Sphincter)


  Circular muscle at the lower end of the esophagus that normally remains tonically constricted, but relaxes ahead of the peristaltic wave to allow easy propulsion of swallowed food into the stomach
• Dysphagia


  Difficulty in swallowing