GI Tract Overview

GI Tract Overview

1. Structure and Function

   • Basic primary function is the transfer of nutrient molecules, water and electrolytes from the food into the body internal environment Achieved by 4 processes: Motility Secretion Digestion Absorption Secondary Functions: Fluid Balance Protection
   • GI Tract Wall
     • Motility provides both mixing and propulsion. It is also predomnately smooth muscle. Exceptions are upper third of oesophagus. upper oesophageal sphincter, external anal sphincter.

2. Physiological Basis of GI Tract Motility

   • Electrical Activity
     • Smooth Muscle are electrically active. It has continual oscillations in membrane potentials due to calcium ion mediated Spike potentials are periodically slow waves to give rise to smooth muscle contractions.
   • ICC
     • Interstitial Cells of Cajal (ICC) The ICC muscle layers act as pacemakers and transfer excitability to the smooth muscle cells. 2 types of neural networks. Myenteric Plexus located between circular and longitudinal muscle layers. Pacemaker Generators. Intramuscular ICC: Scattered throughout circular and logitudinal muscle, transfer excitability from ENS to the smooth muscle cells.
   • Neural Innervation
     • Intrinsic to GI tract: ENS (enteric nervous system) 1. Sensory enteric neurons 2. Interneurons 3. Motor Enteric Neurons Mostly independent of the CNS. Extrinsic to GI tract: Efferent, Parasympathetic and sympathetic. Afferent: Vagal and dorsal root ganglia. (may interface thru ENS)
     • ENS
       • Complex network of neurone Located in the gut wall from the oesophagus to the anus (myenteric plexus and submucosal plexus) Controls moment to moment motility and secretions in the GI tract. (hormonal etc)
       • Intrinsic Sensory Neurons
         • Intrinsic within the GI tract IPAN (intrinsic primary afferent neurons) Sensory neurons receive info from the sensory receptors in the mucosa and muscle. Sensory receptors in muscle respond to mechanical, thermal, osmotic and chemical stimuli
       • Enteric Motor Neurons
         • Motor neurons control motility and secretion by acting directly on effectors cell. Excitatroy motor neurons: ACh and substance P. Inhibitory Motor Neurons: relaxation/vasodilation ATP, NO and VIP It works through separate and distinct group of neurotransmitters.
       • Interneurons
         • Interneurons are responsible integrating information from the sensory neurone and providing it to the ENS. Ascending (oral) Descending (anal) Mainly in the myenteric plexus
   • Reflexes
     • Short enteric Relfex are integrated within the ENS: secretions peristalsis, MMC, segmentation. Initiated by changes in pH, distension, osmolarity (sensory neurons) Medium Reflexes: Via ANS, gastro colonic (make room reflex) enter-gastric, colono ileal. Long Reflexes: Via CNS, gastric accommodation(brain stem) pain mediated inhibition of GIT, defecation via spines

3. Mouth Digestive Proccesses

   • mastication
     • Chewing Teeth, insciors cut... Chewing reflex, rebound. Neural control by the motor branch of the 5th cranial nerves.
   • Salivary Secretions
     • Parotid Submandibular Sublingual Serous acinar cells and it contains alpha-amylase but no mucins. Mucus acinar cells, contains mucin glycoproteins.
     • Saliva
       • Secrete about 1.5L per day Responsible for lubrication, initiation of carbohydrate digestion, oral hygiene, neutralization of pH Composed of 99.5% water and the other are electrolytes and organic compounds. Organic Composition are alpha amylase and mucin. There are also lingual lipase, lactoferrin lysozyme and murimidase for antibacterial actions, immunoglobulins (IgA) or epidermal growth factors.
       • Stimulus
         • Neural Salivatory nuclei is in the brain stem, Positive stimuli includes, taste and tactile stimuli from the tongue. Control is by parasympathetic using ACh acting on muscarininc M1 receptors, Sympathetic acting from NA acting on a and b receptor subtypes
   • Swallowing
     • 3 Phases: Oral Phase: Voluntary Pharyngeal Phase: Involuntary Initiated by the primary peristaltic wave. Esophageal Phase Involuntary: Assisted by gravity and secondary peristaltic waves.

4. Motility patterns

   • Propulsion of food and liquid. Peristalsis Segmentation MMC
   • Peristalsis
     • Wave-like propulsive motion down the GI tract. Alternate contraction and relaxation. Mediated ny short reflexes, peristalsis and local and intrinsic ENS. Occurs the the pharynx, esophagus, gastric anthrum, small and large intestines.
   • Segmentation
     • Non propulsive mixing motility Produces narrow areas of contracted segments between relaxed segments Enhances digestion and absorption. Occurs predominantly in small and large intestines.
   • MMC
     • Migrating Motility Complex Housekeeping. Only seen during fasting.

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