The Digestive System Part I (3) PDF

The Digestive System Chapter 23 p. 874 – 924 Section 23.1 – 23.11 1 Dr. Savory Université d’Ottawa | University of Ottawa Disclosure...

The Digestive System Chapter 23 p. 874 – 924 Section 23.1 – 23.11 1 Dr. Savory Université d’Ottawa | University of Ottawa Disclosure You may only access and use this PowerPoint presentation for educational purposes. You may not post this presentation or the associated videos online or distribute it without the permission of the author. uottawa.ca 2 Objectives 1.1 Summarize the general anatomical features of the digestive system 1.1.1. Differentiate between the components of the alimentary canal and the accessory digestive organ 1.1.2. Describe the relationship between the various digestive organs and the peritoneum 1.1.3. Outline the blood supply serving the main components of the GI tract 1.1.4. Describe the histology of the GI tract wall 1.2 Describe the functional anatomy of the GI tract 1.2.1. Describe the functional anatomy of the mouth, pharynx & esophagus 1.2.2. Describe the functional anatomy of the stomach 1.2.3. Describe the functional anatomy of the liver & gallbladder 1.2.4. Describe the functional anatomy of the pancreas 1.2.5. Describe the macroscopic and microscopic anatomy of the small intestine 1.2.6. Describe the anatomy of the large intestine 3 Objectives cont. 1.3 Describe the region-specific digestive activities of the GI tract and their regulation 1.3.1. List, define and provide examples of the 6 digestive processes 1.3.2. Summarize the basic concepts pertaining to the control of digestive function 1.3.3. Describe the digestive activities associated with the oral cavity and their regulation 1.3.4. Describe the voluntary & involuntary regulation of deglutition; define & indicate some potential causes of heartburn 1.3.5. Describe the digestive functions of the stomach and their regulation 1.3.6. Summarize the digestive roles of the liver and gallbladder & their regulation 1.3.7. Describe the exocrine function of the pancreas and its regulation 1.3.8. Summarize the digestive functions of the small intestine and their regulation 1.3.9. Summarize the chemical digestion of carbohydrates, proteins, lipids & nucleic acids 1.3.10. Outline the processes by which the various nutrient breakdown products are absorbed by the small intestine 1.3.11. Describe the digestive functions (& regulation) of the large intestine 1.3.12. Describe the neural regulation of defecation 4  Functions of the Digestive System o Take in food o Break it down into nutrient molecules o Absorb molecules into the bloodstream o Rid body of any indigestible remains o Nutrient production http://image.slidesharecdn.com/15-140427232211- phpapp02/95/digestive-system-3-638.jpg?cb=1398640996 Synthesis of vitamins by bacteria that live in the intestine o Examples: Vitamin K, biotin (one of vitamins B) and other B vitamins o Production of neurotransmitters, hormones and hormone-like compounds o hormones: gastrin, ghrelin, cholecystokinin, secretin, VIP, motilin, GIP (gastric inhibitory peptide) o neurotransmitters: acetylcholine, serotonin, histamine, nitric oxide 5 Alimentary Canal and Accessory Digestive Organs 6 Gastrointestinal Tract Activities 7 Figure 23.2 GI Tract Activities Peristalsis vs Segmentation 8  Relationship of the Digestive Organs to the Peritoneum Peritoneum Serous membranes of the abdominal cavity that consists of o Visceral peritoneum: membrane on external surface of most digestive organs o Parietal peritoneum: membrane that lines body wall Peritoneal cavity o Fluid-filled space between two peritoneums o Fluid lubricates mobile organs What is peritonitis? 9 Relationship of the Digestive System Organs to the Peritoneum Mesentery: double layer of peritoneum fused together that extends to the organs from the body wall mostly posterior. o Provides support for the organs; holds them in place o Provides support for vessels & nerves supplying the organs o Stores fat Intraperitoneal o organs located within the peritoneum Retroperitoneal o located outside of/posterior to the peritoneum o Includes most of pancreas, duodenum, and parts of large intestine 10  Histology of the Alimentary Canal All digestive organs have the same four basic layers or tunics Mucosa / Submucosa / Muscularis externa / Serosa The alimentary canal extends from mouth to anus Most of the length is made up by the small intestine The walls consist of 4 tissue layers Figure 23.5 11  Histology of the Alimentary Canal 1. Mucosa (lines the lumen) Functions: Secretion: mucus, digestive enzymes, hormones Absorption: end products of digestion Protection: against infectious disease Made up of 3 sublayers: 1. Epithelium Simple columnar epithelium rich in mucus-secreting (Goblet) cells mucus (protects digestive organs from enzymes; eases food passage) may secrete enzymes and hormones (e.g., in stomach and small intestine) 2. Lamina propria loose areolar CT; capillaries for nourishment & absorption lymphoid follicles (part of MALT)- protection 3. Muscularis mucosae smooth muscle that produces local movements of mucosa 12  Histology of the GI tract Wall 2. Submucosa o areolar CT o Blood and lymphatic vessels, lymphoid follicles o submucosal nerve plexus o Abundant in elastic fibers (allows stomach to regain shape after large meal) 3. Muscularis externa o segmentation & peristalsis o inner circular & outer longitudinal layers o sphincters in organ-to organ junctions o myenteric nerve plexus 4. Serosa Visceral peritoneum - outermost protective layer o areolar connective tissue covered with mesothelium o replaced by the fibrous adventitia in the esophagus (adventitia: fibrous connective tissue that “binds” the esophagus to surrounding tissues) Retroperitoneal organs have both an adventitia & serosa o serosa on the side facing the peritoneal cavity & an adventitia on the side against the dorsal body wall 13  Blood Supply: Splanchnic Circulation Arteries that branch off the abdominal aorta to serve the digestive organs o Hepatic, splenic, and left gastric of the celiac trunk (serve the liver, spleen and stomach) o Inferior and superior mesenteric (serve small and large intestine) o Venous return from much of the abdominopelvic region is via inferior vena cava. Venous return from the digestive viscera is indirect via the hepatic portal 20 – 25% of CO circulation. Why?  after a meal  during exercise 14  Enteric (Gut) Nervous System GI tract has its own NS, referred to as Enteric Nervous System (ENS) ▪ Also called the gut brain; made up of enteric neurons that communicate extensively with each other ▪ major nerve supply to GI tract wall that controls motility Linked to the CNS via: o afferent visceral fibers o motor fibers of the ANS (synapses with neurons in the enteric plexuses) ▪ sympathetic impulses inhibit secretion and motility ▪ parasympathetic impulses stimulate secretion and motility (RESTING & DIGESTING!!) Relationship between the ENS and components of the PNS Nature Reviews Gastroenterology & Hepatology volume 13, pages517–528 (2016) 15  Enteric Nervous System Enteric neurons form two major intrinsic nerve plexuses that control GI tract motility Submucosal nerve plexus contains sensory & motor neurons regulates glands & smooth muscle in mucosa Myenteric nerve plexus located between the circular and longitudinal muscles provides the major nerve supply to GI controls GI tract motility (pacemaker cells and local reflex arcs between enteric neurons) Nature Reviews Gastroenterology & Hepatology volume 13, pages517–528 (2016) 16  Enteric Nervous System ▪ Short reflexes in response to stimuli ▪ Long reflexes in response to stimuli inside inside the GI tract (internal) (internal) or outside (external) the GI tract ▪ Control patterns of segmentation involve CNS centers and autonomic and peristalsis nerves 17  Basic Concepts of Regulating Digestive Activity Three key concepts regulate digestive activity: Neurons (intrinsic and extrinsic) and hormones control digestive activity ▪ Nervous system controls – Intrinsic controls: involve short reflexes (ENS only) – Extrinsic controls: involve long reflexes ▪ Hormonal controls – Hormones from stomach and small intestine stimulate targets (muscles, glands) in same or different organs to affect secretion or contraction Digestive activity is provoked by mechanical & chemical stimuli ▪ Receptors in walls of GI tract organs respond to stretch, changes in osmolarity and pH, the presence of substrate and end products of digestion Effectors of digestive activity are SM and glands ▪ Receptors initiate reflexes that stimulate SM to mix and move lumen contents ▪ Reflexes can also activate or inhibit digestive glands that secrete digestive juices or hormones 18  Functional Anatomy of the Digestive System 19 20  Mouth and Associated Structures Oral (buccal) cavity ▪ Bounded by lips, cheeks, palate & tongue ▪ Oral orifice is the anterior opening ▪ Walls lined with SSE ▪ Beginning of digestion & initiation of swallowing food is chewed & mixed with enzyme- containing saliva Associated organs include: Tongue Salivary glands Teeth 21  Mouth and Associated Structures Lips and cheeks ▪ Lips (labia): composed of fleshy orbicularis oris muscle ▪ Oral vestibule: recess internal to lips and cheeks, external to teeth and gums ▪ Cheeks: composed of buccinator muscles ▪ Oral cavity proper: lies within teeth and gums ▪ Lingual frenulum: attachment to the floor of the mouth ▪ Labial frenulum: median attachment of each lip to gum 22  Mouth and Associated Structures Hard palate: palatine bones and palatine processes of the maxillae – Slightly corrugated to help create friction against the tongue Soft palate: fold formed mostly of skeletal muscle – Uvula projects downward from its free edge – Closes off the nasopharynx during swallowing 23 Uvula  Mouth and Associated Structures Tongue Functions: o Repositioning and mixing food during chewing o Formation of the bolus o Initiation of swallowing, speech, and taste o Intrinsic muscles change the shape of the tongue o Extrinsic muscles alter the tongue’s position Ankyloglossia: congenital condition in which children are born with an extremely short lingual frenulum referred to as “tongue- tied” or “fused tongue” 24  Mouth and Associated Structures Tongue Palatine Tonsils Lingual Tonsils Terminal sulcus marks the division between ▪ Body: anterior 2/3 residing in the oral cavity ▪ Root: posterior third residing in the oropharynx Surface papillae (projections of lamina propria covered with epithelium): 1. Foliate—on the lateral aspects of the posterior tongue 2. Filiform—whitish, give the tongue roughness and provide friction 3. Fungiform—reddish, scattered over the tongue 4. Circumvallate (vallate)—V-shaped row in back of tongue ▪ Circumvallate, foliate and fungiform papilla contain taste buds involved in detecting the elements of taste perception ▪ taste qualities are found in all areas of the tongue, some regions are more sensitive than others 25  Mouth and Associated Structures Salivary Glands Secrete saliva, a fluid that: – Cleanses mouth; dissolves food chemicals for taste and moistens food to compact it into a bolus; begins breakdown of starch via amylase Glands: composed of two types of secretory cells ▪ Serous cells: produce watery secretion, enzymes, ions, bit of mucin ▪ Mucous cells: produce mucus Minor or intrinsic salivary glands (buccal glands and others) located throughout oral mucosa help keep it Sublingual moist (minor source of saliva) (mostly mucous cells) Parotid (mostly serous cells) Most saliva secreted (when we eat or anticipate it) by major or extrinsic salivary glands (paired) Submandibular (mostly serous cells) What is mumps? Figure 23.10a The salivary glands. 26  Mouth and Associated Structures Salivary Glands Composition of saliva – Mostly water (97–99.5%), therefore hypo-osmotic; slightly acidic (pH 6.75 to 7) – Electrolytes: Na+, K+, Cl-, PO43-, HCO3- – Salivary amylase and lingual lipase – Proteins: mucin, lysozyme, and I g A – Metabolic wastes: urea and uric acid Saliva protects against microorganisms because it contains: IgA, lysozyme, defensins Xerostomia: uncomfortably dry mouth caused by too little saliva being made – May lead to difficulty with chewing and swallowing, as well as oral infections – Can be caused by medications, diabetes, H I V/AIDS, and Sjögren’s syndrome (autoimmune disease affecting moisture-producing glands throughout body) 27  Mouth and Associated Structures Salivary Glands Control of salivation ▪ 1500 ml/day can be produced ▪ Minor glands continuously keep mouth moist ▪ Major salivary glands are activated by PNS when: Ingested food stimulates chemo- & mechanoreceptors in mouth, sending signals to: Salivatory nuclei in brain stem that stimulate parasympathetic impulses along fibers in cranial nerves VII (facial nerve) and IX (glossopharyngeal nerve) to glands DO NOT MEMORIZE https://doctorlib.info/physiology/illustrated/illustrated.files/image200.jpg Other stimuli: o Swallowing irritating foods; nausea; smell/sight of food or upset GI can act as 28 stimuli  Mouth and Associated Structures Teeth ▪ lie in sockets (alveoli) in gum-covered margins of mandible & maxilla ▪ Mastication: process of chewing that tears and grinds food into smaller fragments ▪ Deciduous dentition: 20 primary/milk/baby teeth (lost between 6-12 years) ▪ Permanent dentition: 32 deep-lying permanent teeth that develop while the milk teeth roots are resorbed Tooth structure – Each tooth has two major regions: Crown: exposed part above gingiva (gum) Root: portion embedded in jawbone – Connected to crown by neck Teeth are classified according to shape: ▪ Incisors: chisel shaped for cutting ▪ Canines: fanglike teeth that tear or pierce ▪ Premolars (bicuspids): broad crowns with rounded cusps used to grind or crush ▪ Molars: broad crowns, rounded cusps: best grinders 29  Digestive Processes of the Mouth Mouth and its accessory organs are involved in (four of six) digestive processes 1. Ingests 2. Begins mechanical breakdown (via mastication) 3. Initiates propulsion (swallowing) 4. Starts digestion of polysaccharides (via salivary amylase) and lipids (via lingual lipase). Note: breakdown in the stomach but with the enzyme produced in the mouth Mastication (Chewing) – Cheeks and lips hold food between the teeth, tongue mixes food with saliva to soften it, and teeth cut and grind solid foods into smaller morsels  production of a bolus (lump) – Partly voluntary and partly reflexive ▪ Pattern and rhythm of continued jaw movements controlled mainly by stretch reflexes and in response to pressure via receptors in cheeks, gums, tongue 30  Digestive Processes of the Mouth Mastication (chewing) Production of bolus (lump) easy to swallow ▪ Mechanical – Closed lips and cheeks – Teeth – Tongue ▪ Chemical  enzymatic breakdown of starch by salivary amylase breaking of fats by lingual lipase (in the stomach but with the enzyme produced in the mouth) 31  Pharynx and Esophagus The Pharynx o Allow passage of food, fluids, and air o Food passes from mouth into oropharynx and then into laryngopharynx o Stratified squamous epithelium lining with mucus-producing glands External muscle layers consists of two skeletal muscle layers Inner layer of muscles runs longitudinally Outer pharyngeal constrictors encircle wall of pharynx Fig. 22.5 32  Pharynx and Esophagus Esophagus ▪ Flat muscular tube (~25 cm) that runs from laryngopharynx to stomach ▪ collapsed when not involved in food propulsion ▪ pierces diaphragm at esophageal hiatus to join stomach at cardial orifice which is surrounded by lower esophageal sphincter (also called gastroesophageal, or cardiac, sphincter) Keeps orifice closed when food is not being swallowed Mucus cells on both sides of sphincter help protect esophagus from acid reflux What is heartburn? https://assets.lybrate.com/imgs/tic/enadp/image-of-the-esophagus.jpg 33  Pharynx and Esophagus Esophagus has all four alimentary canal layers o Esophageal mucosa contains SSE that changes to simple columnar epithelium at the stomach o Esophageal glands in submucosa secrete mucus to aid in bolus movement o Muscularis externa: skeletal muscle superiorly; mixed in middle; smooth muscle inferiorly Figure 23.13 o Has adventitia instead of serosa 34  Digestive Processes of the Esophagus Swallowing (deglutition) ▪ Pharynx & esophagus: passage of food from mouth to stomach ▪ Major function of both organs is propulsion that starts with deglutition (swallowing) ▪ Involves the tongue, soft palate, pharynx, esophagus, 22 muscle groups ▪ Occurs in 2 phases: Buccal phase – Voluntary , occurs in the mouth (via the tongue) Pharyngeal - esophageal phase (primarily via the vagus nerve) https://www.christart.com/IMAG – Involuntary ES-art9ab/clipart/1696/jonah- whale.png – Control swallowing center in the medulla & lower pons The passage of food is regulated by: ▪ Two sphincters: upper and lower esophageal sphincters ▪ Peristalsis: (involuntary muscle movements controlled by medulla oblongata) & facilitated by mucus produced by the submucosal glands 35 Swallowing (Deglutition) Fig. 23.14 36  Swallowing (Deglutition) 37 Fig. 23.14 38  Gross Anatomy of the Stomach Stomach is a temporary storage tank that continues breaking down food both physically and chemically o Converts bolus of food to paste- like chyme (bolus + gastric juice) o Empty stomach has ~50 ml volume but can expand to 4 L when distended o When empty, stomach mucosa forms many folds called rugae https://ufhealth.org/sites/default/files /graphics/images/en/19223.jpg 39  Gross Anatomy of the Stomach Major regions of the stomach ▪ Cardial part (cardia): surrounds cardial orifice ▪ Fundus: dome-shaped region beneath diaphragm ▪ Body: midportion ▪ Pyloric part: funnel-shaped region, continuation of body ▪ 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 40  Gross Anatomy of the Stomach Mesenteries extend from curvatures & tether stomach to other digestive organs – Lesser omentum: from lesser curvature to liver – Greater omentum: inferiorly from greater curvature over intestine, spleen, and transverse colon; blends with mesocolon, mesentery that anchors large intestine to abdominal wall Contains fat deposits and lymph nodes http://158.132.198.175/lexicon/web/tmp_img/e658fc085647892b03bff66ae90ba292.jpg ANS innervation: Sympathetic fibers via the celiac plexus and parasympathetic via vagus nerve Blood supply: celiac trunk (gastric and splenic branches), veins of hepatic portal system 41  Microscopic Anatomy of the Stomach Four tunics but the muscularis & mucosa are modified ▪ Muscularis externa Three layers of smooth muscle Circular, longitudinal & inner oblique layer allows stomach to churn, mix, move & physically break down food ▪ Mucosa layer is also modified Simple columnar epithelium entirely composed From Visible Body of mucous cells Secrete 2-layer coat of alkaline mucus – Surface layer (viscous, insoluble mucus) traps bicarbonate-rich fluid layer that is beneath it – Dotted with gastric pits, which lead into gastric glands that produce gastric juice 42 Microscopic Anatomy of the Stomach 43  Microscopic Anatomy of the Stomach Types of gland cells Mucous neck cells Secrete thin, acidic mucus of unknown function different from mucus of the surface epithelium Parietal cells ▪ Hydrochloric acid (HCl) 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 44  Microscopic Anatomy of the Stomach Types of gland cells Chief cells ▪ Pepsinogen (activated to pepsin by HCl & by pepsin itself (+ve feedback mechanism) ▪ Lipases (digests ~15% of lipids) Enteroendocrine cells Secrete variety of chemical messengers into IF of lamina propria, including ▪ Hormones: – gastrin (needed for HCL secretion; opening of pyloric sphincter, stomach’s motility) – ghrelin (stimulates appetite, gastric motility and opening) – Somatostatin acts both locally and as a hormone ▪ Locally acting paracrine: histamine and serotonin 45  Mechanism of HCl Formation Parietal cells pump H+ (from carbonic acid breakdown) into stomach lumen via H+/K+ ATPase (proton pumps) – As H+ is pumped into stomach lumen, HCO3− is exported back to blood (alkaline tide) in exchange for Cl- (via the Cl− /HCO3− antiporter) Resulting increase of HCO3− in blood leaving stomach is referred to as alkaline tide – Cl− is pumped out to lumen to join with H+, forming HCl 1. H2CO3 → HCO3- + H+ (carbonic acid) 2. H+-K+ ATPase ▪ H+ → the lumen ▪ K+ → the cell (K+ returns to the lumen through membrane channels) − 3. Cl in the interstitial fluid is exchanged for intracellular HCO3−. − 4. Cl diffuses through membrane channels into the lumen 46  Microscopic Anatomy of the Stomach Mucosal barrier protects stomach from its own acid and proteolytic enzymes and is created by 3 factors 1. Thick coating of bicarbonate-rich mucus 2. Tight junctions between epithelial cells Prevent juice seeping underneath tissue 3. Damaged epithelial cells are quickly replaced by division of intestinal stem cells (ISC) Surface cells replaced every 3–6 days 47  Digestive Processes in the Stomach – Holding area for food – Mechanical breakdown of food via the churning action of peristalsis – Digestion of protein HCl denatures proteins, enhancing pepsin mediated enzymatic digestion Milk protein (casein) is broken down by rennin in infants – Results in curdy substance – Absorption of lipid-soluble alcohol and aspirin (but not nutrients) – Only 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 – Delivers chyme to small intestine 48  Regulation of Gastric Secretion Gastric mucosa secretes >3 L of gastric juice/day and are regulated by: – Neural mechanisms Vagus nerve stimulation  secretion Sympathetic stimulation  secretion – Hormonal mechanisms Gastrin stimulates enzyme and HCl secretion Gastrin antagonists are secreted by small intestine – Synergistic effect when ACh, gastrin, histamine all stimulate parietal cell receptors – Drugs that block H2 histamine receptors reduce acid secretion 49  Regulation of Gastric Secretion Gastric secretions are broken down into three phases 1. Cephalic (reflex) phase 2. Gastric phase 3. Intestinal phase 50 https://img.brainkart.com/imagebk20/Wb4qhBU.jpg  Regulation of Gastric Secretion 1. Cephalic phase ▪ Before food reached the stomach (minutes) ▪ reflexes initiated by sensory receptors in the head (sight, smell, taste), or by thought 2. Gastric phase ▪ Lasts 3–4 hours & provides 2/3 of gastric juice Stimulation ▪ stretch receptors – detect distention of stomach initiating neural (both long & short) reflexes ▪ chemical stimuli, partial digested proteins, amino acids, stimulate enteroendocrine (G) cells to secrete gastrin ▪ Gastrin directly (and indirectly via triggering histamine release) stimulates parietal cells to secrete HCl 51  Regulation of Gastric Secretion Gastric phase cont. Stimulation ▪ Buffering action of ingested proteins causes pH to rise, which activates more gastrin secretion ▪ initiates HCl release from parietal cells & activates enzyme secretion ▪ Prods parietal cells to secrete HCl by: 1. Binding to receptors on parietal cells 2. Stimulating enteroendocrine cells to release histamine Inhibition ▪ Low pH (< 2) inhibits gastrin secretion; common between meals ▪ Inhibitory action of sympathetic division overrides vagal (parasympathetic) stimulation during times of fight-or-flight (stress, fear, anxiety) 52  Regulation of Gastric Secretion 3. Intestinal phase ▪ Begins with a brief stimulatory component followed by inhibition Stimulation ▪ 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 Four main factors in duodenum cause inhibition of gastric secretions: ▪ Distension of duodenum due to entry of chyme ▪ Presence of acidic / fatty / hypertonic chyme 53  Regulation of Gastric Secretion 3. Intestinal phase cont. Inhibition: protects intestine from being overwhelmed by too much chyme or acidity and is achieved in two ways: ▪ Enterogastric reflex (Neural) Duodenum inhibits acid secretion in stomach by: ENS short reflexes & SNS and vagus nerve long reflexes ▪ Enterogastrones (Hormonal) Duodenal enteroendocrine cells release two important hormones that inhibit gastric secretion: secretin & cholecystokinin (CCK) 54 Neural and Hormonal Mechanisms that Regulate Release of Gastric Juice 55 Regulation of Gastric Motility and Emptying 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 https://d45jl3w9libvn.cloudfront.net/jaypee/static/books hollow organs to stretch without increasing /9789386056979/Chapters/images/407-1.jpg tension or contractions 56  Regulation of Gastric Motility and Emptying Gastric Contractile Activity ▪ Peristaltic waves move toward the pylorus at the rate of 3/minute ▪ Contractions: most vigorous & powerful near pylorus region (holds ~ 30 ml of chyme) ▪ Each wave spurts ~3 ml duodenum & rest forced back into stomach ▪ only liquids & small particles are allowed to pass through small pyloric valve Basic electrical rhythm (BER) also called the cyclic slow waves of the stomach ▪ initiated by enteric pacemaker cells (formerly interstitial cells of Cajal) which generate waves of depolarization ( ~3/ min) ▪ pacemaker cells (located between SM layer) connected to the rest of the smooth muscle by gap junctions/ entire much contracts when threshold is met – Same factors that increase gastric secretion also increase contractile activity ▪ Distension and gastrin strengthen the waves of depolarization 57 Regulation of Gastric Motility and Emptying Figure 23.21 Peristaltic waves in the stomach. 58 Regulation of Gastric Motility and Emptying Regulation of gastric emptying – Stomach empties in ~4 hours, but increase in fatty chyme entering duodenum can increase time to 6 hours or more Carbohydrate-rich chyme moves quickly through duodenum – Duodenum can prevent overfilling by controlling how much chyme enters Duodenal receptors respond to stretch and chemical signals Enterogastric reflex & enterogastrones inhibit gastric secretion & duodenal filling Figure 23.22 59 Table 23.1 Hormones and paracrines that act in Digestion 60 61