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Digestion Summary.doc

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PHGY 210
Ann Wechsler

SUMMARY: DIGESTION General information about the GIT: - Main role in homeostasis: provide appropriate amount and type of nutrients to body. - Main properties: o Tubular nature o Communication with external environment: The lumen is considered as being part of the external environment - 2 types of evolutionary development: o Growth: In length (the human GIT is 3x the height of an average man) Growth of the internal surface (inner surface is 600x larger than the outer surface because of convolutions). Role is to increase efficiency of diffusion. o Differentiation: The GIT is not a simple structure; it is a sequence of interconnected organs However, if you take a cross section anywhere, you will get the same 4 typical layers making the wall of the GIT. - 4 layers of the gut wall (from outside to lumen): o Serosa: thin but though layer of tissue o Muscularis externa: Longitudinal layer: muscles fibers parallel to the tube responsible for the shortening of the GIT. Circular layer: muscle fibers at right angle with the long axis of the gut. They cause constriction of the GIT when they contract. Note: the GIT is composed of smooth muscle everywhere except in mouth, pharynx, upper 1/3 of the esophagus and anus, which have striated muscle. o Submucosa: loose connective tissue. Characterized by having a lot of nerves, lymphatics and blood vessels. o Mucosa: Muscularis mucosa: smooth muscle Lamina propria: connective tissue with immune properties Epithelial cells: secretory and absorptive cells - GIT functions: o Motility: Propulsion Physical breakdown Contractile activity of the muscle of the GIT o Secretion: Chemical breakdown o Absorption: Transfer to circulation Raison dtre of the GIT Digestive/absorptive efficiency: Carbohydrates: 99% Fat: 95% 1 Protein: 92% The enteric nervous system: - Unique to the GIT - Independent, integrative nervous system - Initiates, programs regulates and coordinates activities of muscular and secretory elements of the GIT Gut wall innervation: Purple: sensory neurons Red: excitatory neurons (release ACh on muscarinic receptors of the muscle and secretory cells) Orange: inhibitory neurons (release NANC) You also have interneurons in the plexuses We will consider the submucosal plexus and the myenteric plexus as one functional unit. All elements of a reflex arc are present Note 1: predominant innervation to the circular muscle layer is inhibitory Note 2: muscarinic receptors are blocked by atropine Sphincters : Name Anatomy Properties Roles Hormonal Neural regulation regulation UES Corresponds Normally Prevents Closure: impulses (upper to the closed from from the CNS esophageal cricopharyng Relaxed entering mediated by the sphincter) eus muscle during esophagus, vagus, releasing deglutition where ACh on nicotinic pressure is receptors causing lower than muscle contraction in the Relaxation: pharynx cessation of impulses during deglutition LES Anatomicall Closure is Prevent Small Mall influence on (lower y myogenic: it reflux of influence on resting tone esophageal insignificant, is an gastric juice resting tone sphincter) but intrinsic in the Note that Relaxation: functionally physiologic esophagus gastrin does neurogenic. very sphincter NOT play a Caused by local important Assisted by role in release of NANC by Consists of presence of regulating enteric inhibitory 2 lower 4 cm intraabdomi the LES neurons of esophagus nal segment under Inferior 2 cm physiologica are in the l conditions. abdominal Estrogen cavity may lower pressure within the sphincter Pyloric Anatomicall Very narrow Functionally sphincter y well lumen that insignificant developed behaves as Open at rest - - (massive sphincter Closed by muscle) antral peristalsis Ileocecal Between SI Prevents valve and colon bacteria of colon from - - entering in SI Spincter of At the When Control flow Oddi intersection closed, bile of bile in of common is stored in duodenal bile duct and gallbladder lumen. - - duodenal lumen, near pancreatic duct Autonomic innervation: Parasympathetic Sympathetic Synapses Preganglionic: synapses only Postganglionic: synapses first on the enteric neuron outside of the wall, and the postganglionic fiber than synapses on the enteric neuron Neurotransmitter ACh First synapse: ACh In GIT wall: NA (NE) Receptor Nicotinic cholinergic First synapse: nicotinic receptors cholinergic receptors In the GIT wall: adrenoreceptors Origin Medulla Spinal cord Overall effect Excitatory Inhibitory Also innervates smooth muscle in blood vessel in the wall, causing vasoconstriction 3 Additional facts about the autonomic innervation of the gut wall: - The vagus nerve innervates almost all the GIT - Only the distal regions of the large intestine are the ANS innervations taken over by the pelvic nerve. - Note that the ANS does not act directly on the GIT; it only modulates the activity of the ENS. - Both parasympathetic and sympathetic divisions can act both on excitatory and inhibitory neurons. - Sensory neurons also exist, allowing for long reflexes, which can integrate activity over long distances alon
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