Gastrointestinal Physiology-I Lecture 27 PDF

Gastrointestinal (GI) System & Accessory Organs Components: – Tubular portion= series of hollow organs separated by sphincters* – Accessory glands and organs: ae Gastrointestinal Physiology-I Spring 2023 Pharynx i.e., salivary glands, pancreas, liver & gallbladder. Function: – Digestion & absorption...

Gastrointestinal (GI) System & Accessory Organs Components: – Tubular portion= series of hollow organs separated by sphincters* – Accessory glands and organs: ae Gastrointestinal Physiology-I Spring 2023 Pharynx i.e., salivary glands, pancreas, liver & gallbladder. Function: – Digestion & absorption of nutrients Major activities: – Motility: Dr. Salwa Abdullah FOM – Department of Physiology [email protected] Office hours by appointment only b a Propulsion & mixing of ingested food – Secretions (exocrine & endocrine): Rectum i.e., fluid, electrolytes, mucus, hormones, & enzymes – Digestion: Chemical breakdown of ingested food into absorbable molecules – Absorption: Of nutrients, electrolytes, and water from the lumen into the bloodstream small of intestine Sphincter = a ring of muscle surrounding and serving to guard or close an opening or tube. It’s a one-way valve that’s high-tone at rest. G of foodfrom onecompartment regulate passage 2 totheother Layers of Gastrointestinal Tract 1- Mucosa extrinsic1. - Innermost layer; direct contact with food - Directly involved in secretion & absorption - Contains goblet cells & other secretory cells - 3 Layers: – Constricts sphincters Parasympathetic innervationnestdigest – Stimulates GI motility & secretions – Relaxes sphincters - Highly vascular connective tissue below mucosa; supportive role - Consists of collagen, elastin, glands, blood and lymphatic vessels, and submucosal nerve plexus L - Responsible for the motility of the GIT - Has inner circular and outer longitudinal smooth muscle - Contains myenteric nerve plexus y modulate when it'sactivateddigestionisinhibited – Inhibits GI motility & secretions 3 3- Muscularis Externa is Autonomic nerve supply Sympathetic innervation - Epithelial cells - Lamina Propria - Muscularis mucosa folds shapingthe surface oftheepithelium 2- Submucosa Innervation of the GIT Mthegutbrain intrinsic2. Enteric nervous system (ENS) = intrinsic nervous system of the gut o Located in the wall of GIT. o Can direct all GI functions, even in the absence of extrinsic innervation. » Submucosal (Meissner’s) plexus: the controls motility Git of 4- Serosa - Outermost layer serves supportive and protective functions Qi.sn Located in the submucosa Katia Mainly controls GI secretion & local blood flow. man A cross-section through a generic piece of the stomach or intestine shows the characteristic layered structure of mucosa, submucosa, muscularis externa, & serosa. 3 bcontrolmuscularismucosa » Myenteric (Auerbach’s) plexus: Located in the muscularis externa g Mainly controls GI motility *The two systems communicate extensively. in 4 Chewing (Mastication) Interaction of ANS with ENS The ganglia of the ENS receive input from the parasympathetic and sympathetic nervous systems, which modulate their activity (Extrinsic Control) They also receive sensory information directly from mechanoreceptors and chemoreceptors in the mucosa and send motor information directly to smooth muscle, secretory, and endocrine cells (Local ENS Reflex) reflexcircuit NONEEDFORTHEBRAIN Secrete NE pphissial It is the mechanical process of breaking the food into smaller particles while simultaneously mixing it with the saliva. This process prepares food for swallowing. It may involve tearing, chopping, squashing, cutting & grinding the food between the upper and lower teeth. wi ti m IT Chewing has the following functions: 1) Reduction of the size of food particles to facilitate swallowing 2) Mixing and lubrication of food with the saliva to facilitate swallowing 3) Oral pre-digestion of carbohydrates prior to entry into the stomach digestioninthe Most of the chewing process is caused by an involuntary chewing reflex; however, Ischemical voluntary chewing can override involuntary chewing at any time. abt Secrete Ach a amylase savan carbohydrate insaliva asalivaryamylasewhichstartscomplex mouth involuntary Food in mouth Reflex inhibition of muscles of mastication Lower jaw drops the sense receptors n Compression of jawopen sothisisanyyouforget aboutityou doitbut just stop can atanyminuteyou control relaxdrop soitwinstretchhe muscles Stretch reflex of jaw muscles ayyy cheminger and T stopit both whenmouthisclosedyou Voluntary Involuntary (Swallowing Reflex) Closure of nasopharynx by soft palate. b) Closure of larynx by epiglottis. c) d) the tonge tipofthe bolus necks toward back the the Opening of upper esophageal sphincteris (UES). 1 oropharynx 2a gym 3) Esophageal Phase : the bolus is pushed down thenetax.sk Main apart it's ofswallowing othat f startedinphases esophagus by is primary peristalsis (3-5 cm /s). If this fails Emotion 2(b-d) to clear the bolus, a secondary peristalsis is initiated**. dear theesophagus a wig **This peristaltic wave is triggeredto by the distension of the esophagus doesn't cord involve brainorspinal behindrelaxes infront contracts o Contraction creates an area of high pressure just behind the bolus, pushing it down the esophagus Lower Esophageal Sphincter The path of food bolus through the esophagus: 2) Primary peristaltic contractions. 3) LES opens. This is mediated by peptidergic fibers in the vagus nerve that release VIP (Vasoactive intestinal polypeptide). rasympethutiomy.y.my xp by the retained food, and is mediated by a local ENS reflex 6 To 1) UES opens, allowing the bolus to move to the esophagus; then closes to prevent reflux into pharynx. É Initiation of a peristaltic wave of contraction in the pharynx (propels bolus through open UES). Note: Respiration is inhibited in phase 2. o Moves bolus along GIT due to contraction and relaxation of circular smooth muscle behind and in front of the bolus, respectively. butthelongitudinalwillactoppositely soft palate 0b pushes from esophagus separate pharynx i 0 1) Oral Phase: after chewing, a bolus of food is separated using the tip of the tongue then forced into the oropharynx to initiate next phase. Peristalsis: o Wave-like muscular contractions. g saliva pballmixedw a) andsoon Esophageal Motility (Peristalsis) Swallowing (deglutition) is initiated voluntarily but continues under reflex control. 2) Pharyngeal Phase: stimulation of touch receptors in the oropharynx initiates this phase then, the following events occur: I contractionrelaxation relaxation p Swallowing Phases of swallowing voluntary MMM against food willagainfedthe Rebound contraction sense roofofyourmouthyou it (lower jaw is raised causing the repeated then p rocessin and the closure of mouth) 5 Lumen so up 4) Food bolus enters the stomach followed by ayyy 3 immediate closure of LES. 5) Secondary peristalsis upon failure of bolus clearance. 7 0ft I me highest resting pressure away efffqfmoptf.jp Recordings of Intraluminal Pressures at Different Sites along the GIT: before, at, & after swallowing. The UES & LES are tonically constricted at rest. The act of swallowing induces relaxation of the UES, while the LES remains contracted. Only when the UES returns to its resting pressure does the LES begin to relax 8 LES Dysfunction Mechanisms of Control of LES 1. Achalasia: o Uncommon disorder characterized by: Sympathetic o o NE + _ Caffeine LES reflex feeling Emma we Fix.az o Causes: AChcholinergic Parasympathetic + _ peptidergics Vagus VIP dependingon fifshementionedragesneme fibers Impaired LES relaxation in response to swallowing. Thus, The food builds up in the esophagus behind the closed LES, causing esophageal distension. Aperistalsis (impaired peristalsis) in the distal 2/3 of the esophagus. o swituinthewale Selective loss of intramural inhibitory neurons that regulate the LES (peptidergic fibers which release VIP) o Symptoms: woutspecifyingandaskedwhat willitdo both o Most common: Dysphagia & regurgitation of bland undigested food or saliva o Others: chest pain, heartburn, weight loss, and esophagitis. s o Treatment: NE = Norepinephrine ACh = Acetylcholine VIP = Vasoactive Intestinal Polypeptide (+) = Excitatory signals, causing contraction (-) = Inhibitory signals, causing relaxation 9 a) b) Antispasmodic drugs. relaxmuscles Botulinum Toxin (Botox) injection. c) Pneumatic dilatation. d) Heller Myotomy. LES Dysfunction o Causes by reduced resting LES pressure, so the LES fails to withstand gastric pressure (gastric pressure > LES pressure). The result will be leakage of some of the gastric contents back into the esophagus. Repeated incidents of reflux may lead to esophagitis. o – A middle circular layer. – An outer longitudinal layer. Innervation: – Extrinsic innervation by the autonomic nervous system. throat stuckinyour feel somethingis you sounding roughharsh – Intrinsic innervation from the myenteric and submucosal plexuses. Treatment: a) Reducing gastric acid production. b) Neutralizing gastric acid (antacid). 3 Muscle layers: stomach). Classic symptoms of GERD are heartburn & acid regurgitation. change oftasteinyourmouth paininswallowing Other symptoms: dysphagia, is odynophagia, chest pain, globus sensation, chronic cough, hoarseness, wheezing, and nausea. cutLES – An inner oblique layer (unique to the o Symptoms: o 8.4 ofesophagus at 8 10 Structure of the Stomach 2. Gastroesophageal Reflux Disease (GERD): o y mechanicalexpansion 2 Regions (based on differences in motility) – Orad region: Proximal, thin-walled, exhibiting infrequent weak contractions Receives food (reservoir) – Caudad region: Bitcanstorethefood Distal, thick-walled, exhibiting strong peristaltic waves of contraction that become stronger as they propagate towards the pyloric sphincter Mixes food and propels it to small intestine. 3 Anatomic divisions: – Fundus. – Body. – Antrum. contraction strongest therearene realperistalsis 1J toempty pomp 11 12 Stomach Filling: Receptive Relaxation of the Stomach Functions of the Stomach 1- Food moves down the esophagus by peristalsis. Vagus 1) Food storage (reservoir): mainly in the orad region. Has up to 1.5 L capacity without increase in pressure. Thestomachisrelaxedbefore 2) Mixing of food with gastric secretions (HCl and pepsin); mostly the antrum. theentryoffoodthisprepares the closes relaxes firstonaUes itwill open 2- p LES opens before arrival of the food. 3) Controlled emptying into duodenum.perfectlytimed 4) Secretion of HCl to kill microorganisms and convert pepsinogen to its active form, pepsin. VIP stomach forstoringandreeieringthat volume food large VIP 3- The orad stomach relaxes before arrival of food.oncethefoodisthere one outanincreaseinpress it canexpandwe press onesuddenlyincreases regurgitation if of 4- Filling the stomach leads to further relaxation of smooth muscle of the stomach.toaccomedatefoodwout Pp 5) Secretion of intrinsic factor to absorb vitamin B12.byileum 6) Secretion of mucus and HCO3− to protect the gastric mucosa. 7) Secretion of water for lubrication and to provide aqueous suspension of nutrients. 13 Relaxation of both LES and stomach are through postganglionic peptidergic vagal fibers (releasing VIP). Receptive Relaxation is a vagovagal reflex. bothsensory3motorarefibersinthevagalnerve Receptive Relaxation allows intragastric volume to increase without an increase in intragastric pressure (can hold up to 1.5L of food). 14 Mixing of Food in the Stomach A. of Gastric Emptying Propulsion: B. Gastric contractions propel the bolus toward the Pyloric sphincter, which is closed. Grinding: C. Once the bolus is trapped near the antrum, it is churned to help reduce the size of the particles. mixed Emptying of the gastric contents to the duodenum takes ~ 3 hours. The rate of gastric emptying must be closely regulated Pyloric to provide adequate time for neutralization of gastric Sphincter H+ in the duodenum and for digestion and absorption of nutrients. thechyme the To control the rate of gastric emptying, contraction of willpass the antrum and pyloric sphincter must be controlled antrum Pyloric sphincter is closed. Retropulsion: Pyloric sphincter is partially opened. Only gastric material containing particles