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Major function of pharynx and esophagus is propulsion that starts with deglutition (swallowing) Deglutition involves coordination of 22 muscle groups and two phases Buccal phase: voluntary contraction of tongue and Pharyngeal-esophageal phase: involuntary phase that primarily involves vagus nerve Delgutition / swallowing steps: 1 Buccal Phase where the upper esophageal sphincter is contracted or closed. The tongue presses against hard palate forcing the food bolus into the oropharynx. 2 pharyngeal esophageal phase begins by tingue blocking the mouth soft palate uvula rise closes nasopharynx. Epiglottis blocks trachea. Upper esophageal sphincter relaxes food enters esophagus 3 pharyngeal esophageal phase continues pharyns contract forcing food into esphagus inferioly. Upper esophageal sphincter contracts after food enters. 4 peristalsis moves food through the esophagus to the stomach 5 gastroesophageal sphincter surrounding the cardial orifice opens. Food enters stomach sphincter closes preventing regurgitation • Stomach is a temporary storage tank that starts chemical breakdown of protein digestion – Converts bolus of food to paste-like chyme – Empty stomach has ~50 ml volume but can expand to 4 L – When empty, stomach mucosa forms many folds called rugae • Major regions of the stomach – Cardial part (cardia): surrounds cardial orifice – Fundus: dome-shaped region beneath diaphragm – Body: midportion – Pyloric part: wider and more superior portion of pyloric region, antrum, narrows into pyloric canal that terminates in pylorus • Pylorus is continuous with duodenum through pyloric valve (sphincter controlling stomach emptying) • Greater curvature: convex lateral surface of stomach • Lesser curvature: concave medial surface of stomach • Mesenteries extend from curvatures and tether stomach to other digestive organs – Lesser omentum Runs from lesser curvature to liver Types of gland cells Glands in fundus and body produce most gastric juice Glands include secretory cells • Mucous neck cells • Parietal cells • Chief cells • Enteroendocrine cells – Mucous neck cells • Secrete thin, acidic mucus of unknown function – Parietal cells • Secretions include: • Hydrochloric acid (HCl) • pH 1.5–3.5; denatures protein, activates pepsin, breaks down plant cell walls, and kills many bacteria • Intrinsic factor • Glycoprotein required for absorption of vitamin B12 in small intestine – Chief cells • Secretions include: • Pepsinogen: inactive enzyme that is activated to pepsin by HCl and by pepsin itself (a positive feedback mechanism) • Lipases • Digests ~15% of lipids – Enteroendocrine cells • Secrete chemical messengers into lamina propria • Act as paracrines • Serotonin and histamine • Hormones • Somatostatin (also acts as paracrine) and gastrin • Processes carried out by stomach – Carries out breakdown of food – Serves as holding area for food – Delivers chyme to small intestine – Denatures proteins by HCl – Pepsin carries out enzymatic digestion of proteins • Milk protein (casein) is broken down by rennin in infants • Results in curdy substance – Lipid-soluble alcohol and aspirin are absorbed into blood – 0nly stomach function essential to life is secretion of intrinsic factor for vitamin B12 absorption • B12 needed for red blood cells to mature • Lack of intrinsic factor causes pernicious anemia • Treated with B12 injections • Gastric mucosa secretes >3 L of gastric juice/day and are regulated by: – Neural mechanisms • Vagus nerve stimulation increases secretion • Sympathetic stimulation decreases secretion – Hormonal mechanisms • Gastrin stimulates enzyme and HCl secretion • Gastrin antagonists are secreted by small intestine Gastric secretions are broken down into three phases 1 Cephalic (reflex) phase Conditioned reflex triggered by aroma, taste, sight, thought 2 Gastric phase Lasts 3–4 hours and provides two-thirds of gastric juice released Stimulation of gastric phase Distension activates stretch receptors, initiating both long and short reflexes Chemical stimuli, such as partially digested proteins, caffeine, and low acidity, activate enteroendocrine G cells to secrete gastrin 3 Intestinal phase Begins with a brief stimulatory component followed by InhibitionStimulation of intestinal phase Partially digested food enters small intestine, causing a brief release of intestinal (enteric) gastrin Encourages gastric glands of stomach to continue secretory activities Stimulatory effect is brief and overridden by inhibitory stimuli as intestine fills Inhibition of intestinal phase • Four main factors in duodenum cause inhibition of gastric secretions: – Distension of duodenum due to entry of chyme – Presence of acidic chyme – Presence of fatty chyme – Presence of hypertonic chyme • Inhibitory effects protect intestine from being overwhelmed by too much chyme or acidity • Inhibition is achieved in two ways: enterogastric reflex and enterogastrones • Enterogastric reflex – Duodenum inhibits acid secretion in stomach by: – Enteric nervous system short reflexes – Sympathetic nervous system and vagus nerve long reflexes • Enterogastrones – Duodenal enteroendocrine cells release two important hormones that inhibit gastric secretion – Secretin – Cholecystokinin (CCK) CHOLECYSTOKININ CCK produced in duodenal mucosa target stomach liver pancreas gallbaladder Gastrin in stomach mucosa g cells increase HCI secretion HISTAMINE in stomach activates parietal to release hci Intestinal gastrin in duodenal stimulates gastric glands and mobility Motililin stimulates migrating motor complex Secretin in duodenal cck increase bile output gastric gland Parietal cells make HCI • Response of the stomach to filling – Stretches to accommodate incoming food – Two factors cause pressure to remain constant until 1.5 L of food is ingested • Receptive relaxation: reflex-mediated relaxation of smooth muscle coordinated by swallowing center of brain stem • Gastric accommodation: intrinsic ability of smooth muscle to exhibit stress-relaxation response, which enables hollow organs to stretch without increasing tension or contractions • Gastric contractile activity Basic electrical rhythm (BER)