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Digestion - Dr Ann Wechsler Room 1135 McIntyre(514)398-4341...

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Unformatted text preview: Dr. Ann Wechsler Room 1135 McIntyre (514)398-4341 [email protected] March 19 – April 11, 2008 PHYSIOLOGY OF THE GASTROINTESTINAL TRACT (GIT) AND ITS ACCESSORY STRUCTURES GIT Role in HOMEOSTASIS – provide nutrients External Environment DIGESTIVE SYSTEM Absorbable Molecules FOOD we will focus on what happens to the food when it enters the digestive system before it can be absorbed into the intestinal system INTERNAL ENVIRONMENT “Processed” Cells ENERGY & RAW MATERIALS Growth & Repair Function & Regulation GIT structure Ext.env. EARTHWORM Ext.env. - the digestive system is open at both ends - the central lumen is an extension of the external environment GROWTH two properties of the digestive tract: DIFFERENTIATION 1. Tubular Nature 2. Communication with External Environment GROWTH 1 - in a cadaver, the muscle elements of the digestive tract are relaxed and the length of the GIT increases by a factor of two - in a living human, the muscle tone shortens the overall length of the GIT Mouth 4.5m Stomach LENGTH of adult GIT Small Intestine Large Intestine Anus 1.5m GROWTH 2 Internal Surface area = 200-250 m2 LUMEN - the internal surface area is increased by 600x over the external surface area - this is achieved by villi, microvilli etc. - this increased surfaced area makes for increased absorption and the efficiency of transport DIFFERENTIATION - different portions of the digestive tract are specialized to perform specific functions - there is always communication with the external environment at both ends despite the differentiation - associated with the digestive tube are a number of accessory organs: salivary glands, pancreas, liver Pp 581 - 4 layers of identifiable tissue fibers are extended parallel to the long axis of the gut - when contracted, the length of the gut shortens Longitudinal fibres Circular fibres fibers are at right angles to the length of the tract - when contracted, the diameter of the lumen gets smaller striated LUMEN S M O O T H - most of the muscle in the GIT is smooth muscle - only the muscle near the mouth and pharynx are striated - the muscle from about 1/3 of the esophagus down is smooth striated SEROSA thin, tough layer of connective tissue MUSCULARIS EXTERNA double layer of muscle P 581 Longitudinal fibres Circular fibres Muscularis smooth mucosae muscle Lamina propria LUMEN layer of loose connective tissue Epithelial layer epithelial cells with secretory and absorbing properties MUCOSA made up of three layers SEROSA MUSCULARIS EXTERNA loose connective tissue with nerves lymphatics and blood vessels SUBMUCOSA GUT WALL STRUCTURE (mid-esophagus to anus) 1. Serosa – thin, tough layer of connective tissue (continuous in places with abdominal mesentery) 2. Muscularis Externa – outer layer longitudinal fibres (when it contracts, GIT shortens), inner layer circular fibres (when it contracts, lumen narrows); (musculature in oral cavity, pharynx, upper 1/3 esophagus and external anal sphincter is striated; the rest is smooth) 3. Submucosa – loose connective tissue, housing neuronal network, lymphatics, blood vessels 4. Mucosa – muscularis mucosae (sm. muscle) - lamina propria (loose connective tissue) - epithelial layer (secretory - exocrine and endocrine - and absorptive cells) GIT FUNCTIONS To convey food along GIT, allowing it to be disrupted into small molecules which can be absorbed into circulation. 3 Activities: 1. MOTILITY (muscular activity) Propulsion & physical breakdown Chemical Breakdown digestion in its truest sense 2. SECRETION (glandular activity) 3. ABSORPTION - Transfer to circulation DIGESTIVE / ABSORPTIVE EFFICIENCY CARBOHYDRATE FAT PROTEIN 99% 95% 92% - most of what we ingest, we digest and absorb - we can absorb much more than we are capable of eating in a normal day Pp 588-590 how are we able to be so efficient in digestion and absorption? PROPULSIVE – SECRETORY– ABSORPTIVE ACTIVITIES integrated for high functional efficiency by NEURAL and HORMONAL MECHANISMS ENTERIC INNERVATION (ENS) - the digestive system has its own neural system INDEPENDENT, INTEGRATIVE NERVOUS SYSTEM of the CNS, autonomic system - you can separate the gut from the brain and you will still have fundamental neural control INITIATES PROGRAMS REGULATES COORDINATES activities of muscular and secretory elements GUT WALL INNERVATION lumen - the enteric nervous system exists as a large number of neurons and associated fibers that are collected into ganglia - the ganglia are further collected into two plexuses - the submucosal plexus of meissner and the myenteric plexus of auerbach (located between the two muscle layers) - there are interconnections between the two plexuses - it contains everything necessary for reflex arcs - receptors (osmo, pH, mechano, chemo) - sensory fibers that carry information to other ganglia - large number of effector neurons - large number of interneurons - important for the coordination of the muscle - stimulating one sensory neuron can coordinate both muscular and secretory activity - an interneuron can activate effectors at a distance from the original stimulus - some interneurons are inhibitory - as a result of some reflex arcs, inhibitory neurons can be activated - the two plexuses are anatomically distinct - they are too many interconnections between them that it is difficult to distinguish different functions serosa The ENS consists of the myenteric plexus (between the longitudinal and circular muscle layers) and the submucosal plexus. Though anatomically distinct, the two plexuses behave as a functional unit, which includes all the elements required for reflex arcs: sensory neurons, effector neurons, and interneurons. ENS consists of ganglion cells and their processes which synapse with sm.muscle cells, endocrine and exocrine cells, and other ganglion cells. Some of the enteric neurons are excitatory (release mostly ACh – acting on muscarinic receptors -); others are inhibitory (release NANC (Non Adrenergic, Non Cholinergic transmitters). Also present are enteric sensory fibres with cell bodies in plexuses. - an enteric neuron is excitatory (orange) releases Ach and acts on muscarinic receptors (can be blocked by atropine) - some enteric neurons are inhibitory (yellow) do not release Ach or NE, but a neurotransmitter NANC - non-adrenergic-non-cholinergic, e.g. NO - there can be a variety of levels of activity which depends on the sum of all the excitatory and inhibitory impulses Secretory Cell intraneural SHORT REFLEXES G U T W A L L Chemoreceptors, osmoreceptors, mechanoreceptors sensory input bringing information from the gut, enteric layers integration of information modification of activity of smooth muscle Nerve plexus Smooth Muscle or Gland Gut Wall algebraic sum of all excitatory and inhibitory impulses Stimulus Response CNS *pp.199-202 - sympathetic connection to the CNS is through a sympathetic interneuron - release of NE on badrenergic receptors CNS GIT ACh Parasympathetic (preganglionic) - normally there is input from the CNS and autonomic system - there is no synapse between the CNS along the parasympathetic preganglionic fiber - the first synapse is with an enteric interneuron (can be excitatory or inhibitory) - they release Ach acting on nicotinic receptors NA ACh ENS Sympathetic (postganglionic) Autonomic Innervation of GIT AUTONOMIC INNERVATION OF GUT WALL - parasympathetic innervation begins in the medulla and pelvis and information travels along the vagus nerve - there is no synaptic interruption of the parasympathetic system - the sympathetic innervation begins in the spinal cord - lumbar and thoracic regions - there is synaptic interruption before forming synapses with the ENS - the gut provides information to the brain that is integrated centrally --> regulation of the level of activity in the enteric neurons - there is sensory input to the medulla and spinal cord - this results in parasympathetic and/or sympathetic activity to enteric interneurons (excitatory or inhibitory) - when you stimulate an excitatory enteric neuron there is increased activity - when you stimulate an inhibitory enteric neuron there is increased inhibition - inhibiting an inhibitory enteric neuron results in increased activity Secretory cell The ANS modulates the ENS The parasympathetic reaches the wall of the GIT as preganglionic fibres, synapsing (via nicotinic ACh receptors) with enteric neurons (both excitatory and inhibitory) exerting an excitatory effect. The sympathetic reaches the wall of the GIT as postganglionic fibres, synapsing (via NA receptors) with enetric neurons (both excitatory and inhibitory) exerting an inhibitory effect. The sympathetic also innervates smooth muscle in blood vessels in the wall, causing vasoconstriction. Sensory neurons also exist allowing for “long reflexes”. Emotional States integration of sensory input - if you stimulate at one point, because of the integration and wide distribution of the autonomic nervous system the distal ends of the digestive tract can be regulated - amplification of the sensory input Sight, smell, taste of food sensory fibers going from the gut to the CNS CENTRAL NERVOUS SYSTEM ps LONG REFLEXES s Efferent Autonomic modify, modulate the neurons the enteric innervationlevel of activity of - DO NOT regulate smooth muscle directly Afferent neurons G U T W A L L Chemoreceptors, osmoreceptors, mechanoreceptors Stimulus Nerve plexus SHORT REFLEXES Smooth Muscle or Gland Response ANS MODULATES ACTIVITY OF ENS ALLOWS FOR INTEGRATED ACTIVITY OVER LONGER DISTANCES ALONG GUT LONG, EXTRINSIC REFLEXES In general, PS – EXCITATORY (may also excite inhibitory neurons) S - INHIBITORY (may also inhibit inhibitory neurons) pp.589-590 HORMONAL REGULATION OF GUT ACTIVITY DES = DIFFUSE ENDOCRINE SYSTEM (scattered in mucosa) - individual cells, they do not collect together into an organ (such as in the thyroid) - cells are scattered out throughout the intestine and GI tract largest, most diversified endocrine system in the body it can operate in a variety of ways - over 20-30 substances are released in many different ways Different modes of regulation AUTOCRINE PARACRINE diffusion of hormone through the ISF ENDOCRINE diffusion of hormone through the vascular system capillary ENDOCRINE CELL GUT REGULATORY PEPTIDES - there are many ways of increasing or decreasing the activity of the intestinal tract 1. Released from mucosa into portal blood - all substances released are peptides, no steroid hormones liver systemic circulation are released 2. Have multiple targets intestine, gall bladder, pancreas - the substances are released from the mucosa into the portal blood, the blood flow from the gut to the liver - once they have gone through the liver, they enter the systemic circulation (they can be modified in the liver) - they are then transported to muscular and secretory elements the rest of the body to multiple targets excitatory inhibitory 3. Interact with one another and with neurotransmitters that are released locally at the level of the cell a) synergistically b) antagonistically GUT PEPTIDES A number of peptide agents are released from endocrine cells in the mucosa of the stomach and the small intestine by nervous, chemical, and mechanical stimulation, coincident with the intake of food. Released into the portal circulation, the gut peptides pass through the liver to the heart, and back to the digestive system to regulate its movements and secretion SUMMARY OF GIT REGULATION 1. Short enteric (intramural) reflexes the main control of the GI tract, merely modified by long reflexes and hormonal activity 2. Long extrinsic (ANS) reflexes modify short enteric reflexes, sympathetic and parasympathetic components 3. Hormonal Factors interact with neural regulation (#2 and #3 modulate effects of #1) GIT STRUCTURE, FUNCTIONS, REGULATION 3 Activities: 1. MOTILITY (muscular activity) Propulsion & physical breakdown Chemical Breakdown 2. SECRETION (glandular activity) 3. ABSORPTION - Transfer to circulation Pp 581 Longitudinal fibres Circular fibres narrow the lumen shorten the length of the tract striated LUMEN S M O O T H striated SEROSA MUSCULARIS EXTERNA basic muscular layer - 2 fibrous layers most of the muscular tube is smooth muscle, excluding the pharynx, larynx, uppwe 1/3 of the esophagus and the anal sphincter PROPULSION (FLOW) IN THE GIT Gradients of pressure Variations in resistance Coordinated contractions Normally, of muscular elements in segmentation - standing rings ofat little/no resistance contraction that are established - normally sphincters open in the different points along the tube arrival of a meal wall of GIT followed by relaxation and perastalsis - propagated wave of contraction that moves over the wall of the organ causing the narrowing of the lumen - causes the contents to move in the aboral direction, mainly responsible for propulsion contraction of the adjacent region of the tube - important in essential mixing activity, also responsible for a certain amount of propulsion - they do not offer any resistance - in the absence of a meal moving in the GI tract they are closed Normally, flow is slow, aboral and meets little/no resistance increased resistance of sphincters and the GI tract are manifestations of disease PHASES OF DEGLUTITION (swallowing) Pp.590-593 ORAL - three phases to swallowing correspond to the transfer of food through the oral cavity, pharyx and esophogus and into the stomach - the first propulsive activity of the GI tract PHARYNGEAL ESOPHAGEAL DEGLUTITION (Swallowing) is accomplished through a complex series of highly coordinated muscular movements aimed at building up pressure, temporarily sealing off of compartments to prevent dissipation of pressure, and decreasing resistance. ORAL PHASE – transport from anterior mouth to pharynx VOLUNTARY CONTROL - food is ingested into the mouth and is moved to the tip of the tongue - the food begins to move to the back of the mouth by elevation of the tip of the togue and depression of the back of the tongue - the food is thrusted into the pharynx by elevation of the middle part of the tongue - the ability to initiate swallowing in under oral control ORAL PHASE – transport of bolus (masticated, insalivated mass of food) from anterior to posterior portion of mouth. This involves a series of reflexes coordinated in DEGLUTITION CENTRE in medulla oblongata CORTICAL vs MEDULLARY CENTRES “Voluntary” facilitates the involuntary reflexes - makes them more smoothly integrated swallowing centers Deglutition Centre “Involuntary” ORAL PHASE CORTEX 1. ABILITY TO INITIATE: VOLUNTARY 2. COORDINATED MOVEMENTS: REFLEX (INVOLUNTARY) the actual movements that allow for the transport of the food from the anterior to the posterior regions of the oral cavity are involuntary MEDULLA PHARYNGEAL PHASE - the pharyx is the site where the respiratory and digestive tracts cross - the first thing that must happen during swallowing, the two tracts must be separated - the bolus enters the pharyx it initiates a series of protective movements that closes the openings back to the nose and to the trachea - only then can it enter the esophagus PHARYNGEAL PHASE INVOLUNTARY 1. - the bolus begins to pass downward and at the same time the vocal cords come together to seal the glottis and the whole larynx moves upward and forward to fit under the tongue --> main way to protect the respiratory tract - the epiglottis flips down to cover the lower respiratory tract --> secondary to the upward movement of the vocal cords to fit beneath the larynx - the upper esophageal tract relaxes (to decrease the resistance to flow) and the pharyx muscles contract (to generate a pressure difference)as the bolus enters into the esophagus - these involuntary contractions and relaxations are accompanied by deglutition apnea - inhibition of respiration Passages into nose, mouth, and trachea are blocked Apnea relaxes 2. 3. UES 4. Pharynx muscles contract PHARYNGEAL PHASE Under involuntary control, consists of a) a series of protective reflexes, initiated by stimulation of afferent fibres in the pharynx, organized in Deglutition Centre, closing off nasal, oral, and laryngeal cavities, preventing misdirection of the bolus. Simultaneously, respiration is briefly inhibited. b) transfer to esophagus, as pharyngeal muscles contract and Upper Esophageal Sphincter relaxes. DEGLUTITION REFLEXES afferent Pharyngeal Receptors DEGLUTITION CENTRE efferent “protective reactions” relax UES contract pharyngeal constrictor muscles “deglutition apnea” Vagus UES - striated muscle - in order to contract it needs to receive innervation - at rest in the absence of swallowing, the UES receives constant innervation from the CNS mediated by the vagus - releases Ach that acts on nicotinic receptors - on swallowing there is an inhibition of the vagally sent impulses - the muscle no longer receives regulation --> they relax ACh(N) CLOSURE – impulses originate in CNS, mediated by vagus, releasing ACh , causing muscle contraction RELAXATION – cessation of impulses, results in muscle relaxation PHARYNGEAL PHASE 1. Involuntary 2. Rapid 3. “Stereotyped” 4. Temporospatial Coordination - there are about 25 different muscles that must contract with the appropriate amount of strength at the correct times - it is one of the most vulnerable areas in neuromuscular disease - aspiration pneumonia - elderly people aspirate bacterial particles ESOPHAGEAL PHASE - main function is propulsion - transfer of the bolus from the UES to the LES - there is no absorption in the esophagus - the sole function is transport UES Body of esophagus lies within thoracic cavity - the pressure in the thoracic cavity is negative - the body of the esophagus lies in a subatmospheric region - the pressure in the pharynx is atmospheric - if there was no esophageal sphincters we would constantly be aspirating air each time you take a breath --> the UES keeps the top of the esophagus tightly closed - in the absence of swallowing, no air moves into the esophagus - if there was no LES we would constantly be refluxing the stomach contents into the lower esophagus - the esophagus does not have a mechanism to protect itself from stomach acid LES p h a r y n x UPPER GIT PRESSURES Intragastric (+ve) atmospheric pressure UES Body of Esophagus LES Intrathoracic (-ve) the vagus directly innervates the striated muscle - typical neuromuscular junction vagus UES Striated muscle vagus - in the smooth muscle region the vagus does not innervate the muscle, but it synapses with enteric interneurons which innervate the smooth muscle - autonomic innervation Smooth muscle LES ESOPHAGEAL FORCES 1. Gravity - you can swallow in the absence of gravity, if you stand on your head - gravity in the esophagus helps, but it is not the main mechanism 2. Peristalsis p.592 PERISTALSIS - a wave of contraction moving over the wall of the organ, narrowing the lumen and setting up a gradient of pressure favouring aboral movement Each time we swallow, a single PRIMARY PERISTALTIC WAVE is generated It takes 8-10 seconds to be propagated the length of the esophagus Primary peristalsis is part of the deglutition reflexes stimulation of pharyngeal receptors give rise to primary peristalsis cannot distinguish between the innervation or regions --> coordinated contraction - activation of the striated portions of the esophagus through vagal-somatic fibers - muscle contracts sequentially, propelling food toward the aboral end of the esophagus - vagal autonomic fibers activate almost at the same time as vagal somatic signals - but there is progressive delay in the enteric neurons - causes a propagated contraction - if the vagi are cut high up in the neck there cannot be the generation of a primary peristaltic wave - if the cut is lower down in the thorax, a wave can be generated because the striated muscle will still be intiated - a few of the enteric neurons will be initiated - it is sufficient to relay betwe...
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