WINDSOR UNIVERSITY SCHOOL OF MEDICINE

1. WINDSOR UNIVERSITYSCHOOL OF MEDICINE The Digestive System-1 Dr.VishalSurender .MD.

2. LUNCH MENU • Function and processes of the digestive system • Anatomy of the digestive system • Motility • Secretion • Regulation of GI function • Digestion and absorption

3. The gastrointestinal system includes the gastrointestinal tract (mouth, esophagus, stomach, small intestine, large intestine, rectum and anus) and the associated organs and accessory glands (salivary glands, liver, gallbladder and pancreas) that add secretions to the gastrointestinal tract. • The overall function of the G.I. system is to absorb nutrients, electrolytes and water and Eliminate waste products. • To achieve this it requires (1) movement of food through the alimentary tract-motility (2) secretion of digestive juices and digestion of the food (3) absorption of water, various electrolytes, and digestive products (4) circulation of blood through the gastrointestinal organs to carry away the absorbed substances and (5) control /regulation of all these functions by local, nervous, and hormonal systems.

4. Most food is taken in as solids and as macromolecules that cannot be readily transported across the intestinal epithelial cells. Digestion which involves physical and chemical alterations of the food has to first take place and this is facilitated by secretions from the G.I. tract and associated organs. • The absorbed nutrients are then metabolised through a series of chemical processes making it possible for cells to continue living. • Waste substances from the ingested food are stored and excreted out of the G.I. tract. Basic Processes of the Digestive System

5. Digestive System Anatomy Oral cavity  esophagus  stomach  small intestine  large intestine  rectum • Each part is adapted to its specific • functions: • some to simple passage of food- e.g., esophagus, • others to temporary storage of food such as the stomach and • others to digestion and • absorption, such as the small intestine

6. The Wall of the Digestive Tract • A typical section of the digestive tract reveals four main layers. From inside (the lumen) to outside they are: o Mucosa o Submucosa o Muscularis (externa) o Serosa (a.k.a. visceral peritoneum)

7. Example: Four-layered Wall of the Small Intestine absorption secretion endocrine Epithelium Lamina propria Muscularis mucosae Submucosal plexus Circular muscle Myenteric plexus Longitudinal muscle

8. The Upper Part of the GI Tract- Mouth,esophagus,stomach.

9. The Upper Part of the GI Tract- Stomach.

10. The Lower Part of the GI Tract Small Intestine. lymphoid nodules

11. The Lower Part of the GI Tract Large Intestine

12. Sphincters Sphincters regulate the passage of food from one region of the digestive tract to the next, and finally, out of the body as feces The sphincters of the digestive tract, from mouth to anus, are the: o Upper esophageal sphincter or UES (circular skeletal muscle – an anatomical sphincter) o Lower esophageal sphincter or LES (a physiological sphincter) o Pyloric sphincter (circular smooth muscle) o Ileocecal sphincter or valve (circular smooth muscle) o Internal anal sphincter or IAS(circular smooth muscle) o External anal sphincter or EAS (circular skeletal muscle)

13. Accessory Glands The accessory glands that produce secretions to aid in digestion are the salivary glands (3 pair), liver, and pancreas. Salivary glands moisten food, cleanse and protect the mouth, and produce amylase to begin enzymatic digestion of starch The liver produces bile, which emulsifies fats to increase their surface area for subsequent chemical digestion by lipases; bile is stored in and released from the gall bladder into the duodenum The pancreas is the main digestive enzyme-producing exocrine organ in the body. It releases a host of digestive enzymes into the duodenum via the pancreatic duct; it also produces bicarbonate to neutralize the chyme from the stomach

14. Clinical correlaton- -Peptic Ulcer. -Gastroesophageal reflux disease(GERD) -Achalasia- Chagas disease Barium swallow showing bird's beak“ or "rat's tail" appearance in achalasia.

15. THE DIGESTIVE SYSTEM Motility Chewing occurs in the mouth Swallowing initiates primary peristalsis in the esophagus Stretch of the esophageal wall initiates secondary peristalsis • Motility in esophagus : • Primary and secondary peristalsis. • Function of peristalsis is to • Propel a bolus of food to stomach.

16. Control of the Digestive System The autonomic nervous system, hormones, and other chemicals control motility and secretion in the digestive system to maximize digestion and absorption.

17. Control of the GI tract depends on the location of food • Control of the digestive system progresses from head to stomach to intestine as food enters and moves through the GI tract. • We can distinguish three phases of control:- • Cephalic • Gastric • Intestinal. The sight, smell, taste, and mental images of food trigger the cephalic phase of digestion via the vagus nerve (N X) which includes: o salivation o gastric juice production o gastric contractions

18. Increased volume of food in the stomach and subsequent stimulation of stomach stretch receptors triggers the gastric phaseof digestion which includes: o gastric juice production o increased gastric motility As food moves into the small intestine (duodenum), the chemical composition and volume of that food triggers specific reflexes during the intestinal phase of digestion which may include: o pancreatic secretion of bicarbonate into the duodenum o pancreatic secretion of digestive enzymes into the duodenum o gall bladder release of bile into the duodenum o segmentation contractions of the small intestine • The small intestine reflexively slows gastric emptying to allow for neutralizing, enzymatic digestion, and absorption of its contents The phases of control occur in sequence only at the beginning of a meal. Once a meal is underway the phases are simultaneous.

19. Innervation of The Gastrointestinal Tract A Comparison In most body systems: In GI tract: SNS SNS PSNS PSNS Muscle, glands, blood vessels Muscle, glands, blood vessels Enteric nervous system (ENS)

20. Innervation of the GI Tract Extrinsic nervous system - Parasympathetic (vagus nerve, pelvic nerve) - Sympathetic (T5 – L2) - Neurotransmitters - Efferent fibers and afferent fibers Intrinsic/Enteric nervous system - Submucosal plexus - Myenteric plexus

21. excitatory Parasympathetic Nervous System (PSNS) • divided into cranial and sacral divisions. • usually ____________ for the functions of the GI tract. • is carried via the vagus and pelvic splanchnic nerves. • Preganglionic fibers synapse in the myenteric and submucosal plexuses. • Cell bodies in ganglia of the plexuses then send information to the smooth muscle, secretory cells, and endocrine cells of the GI tract. • The vagus nerve carries PSNS information to the esophagus, stomach, pancreas, and upper large intestine. • The pelvic splanchnic nerves carry information to the lower large intestine, rectum, and anus.

22. inhibitory Sympathetic Nervous System (SNS) • is usually _________ on the functions of the GI tract. • Fibers originate in the spinal cord between T5 and L2. • Preganglionic fibers travel in the Greater, Lesser and least Splanchnic nerves. • Preganglionic cholinergic fibers synapse in prevertebral ganglia. • Postganglionic fibers leave the prevertebral ganglia, travel with arteries and synapse in the myenteric and submucosal plexuses. • Direct postganglionic adrenergic innervation of blood vessels and some smooth muscle cells also occurs.

23. Intrinsic / Enteric Nervous System The submucosal and myenteric plexuses comprise the enteric nervous system (ENS). • Submucosal plexus: • controlling inner wall functions • - secretion • - local blood flow • - mucosal movement Intrinsic and independent - lies entirely in the wall of the gut. - can function on its own in the absence of extrinsic innervation. - controls most functions of the GI tract, especially motility and secretion. Integrated - stimulation by the parasympathetic and sympathetic systems can greatly enhance or inhibit its functions. • Myenteric plexus: • controlling muscle activity • along the gut

24. Reflexes coordinate and modulate digestive activity • Reflexes (stimulus, integration, and response) that are totally controlled by the enteric nervous system are termed short reflexes. • Reflexes that involve the CNS as an integration center are called long reflexes. • Short and long reflexes can occur simultaneously • Larger volumes of food in the stomach produce stronger gastric contractions than smaller volumes of food. • Chemical composition of food influences the rate of digestion: lipid rich meals take longer to digest than carbohydrate rich meals • Emotional stressors cause the brain to override the intrinsic controls of digestion and subsequently cause problems such as constipation, diarrhea, and stomachaches.

25. There are many neurotransmitters in the GI tract • All preganglionic fibers of the ANS release acetylcholine (ACh). • Parasympathetic postganglionic fibers also release ACh • Sympathetic postganglionic fibers release norepinephrine • Other enteric nervous system neurotransmitters include serotonin, vasoactive intestinal peptide (VIP), nitric oxide (NO), and somatostatin (SST). • ACh and substance P stimulate smooth muscle of the digestive tract. • Smooth muscle of the digestive tract is inhibited by norepinephrine, VIP, and NO • Enkephalins released in the submucosal and myenteric plexuses slow intestinal motility, inhibit intestinal secretion, and contract the LES, pyloric and ileocecal sphincters.

26. Hormones modulate digestive activity • GI hormones are peptides (amino acid based). • GI hormones include -gastrin, -cholecystokinin (CCK), - secretin, -glucose dependent insulinotropic peptide (GIP) and -motilin.

27. Gastrin- • Enteroendocrine cells of the GI mucosa secrete the peptide hormones. • Pyloric antrum G cells secrete gastrin, which stimulates HCl production and growth of the gastric mucosa Cholecystokinin (CCK)- • CCK is secreted by the “I” cells in the duodenum and jejunum. • CCK cause gall bladder contraction and subsequent release of bile into the duodenum • CCK causes the pancreas to release digestive enzymes into the duodenum • CCK inhibits gastric emptying • CCK stimulates growth of the pancreas and gall bladder mucosa

28. Secritin- • Duodenal “S” cells produce secretin • Secretin causes the pancreas, and to a lesser extent the liver, to release bicarbonate into the duodenum • Secretin inhibits HCl secretion by the stomach Glucose dependent insulinotropic peptide (GIP) • Duodenal and jejunum cells secrete GIP • When glucose is present, GIP stimulates the pancreas to secrete insulin, thus enhancing the secretion of insulin Motilin- • Duodenal and jejunum cells secrete motilin during the postabsorptive state • Motilin stimulates GI motility thereby moving intestinal contents toward the end of the ileum

29. Motility • Different types of contraction- • Tonic contractions • Sustained • Smooth muscle sphincters and stomach • Phasic contractions • Last a few seconds • Peristalsis moves bolus forward • Segmentation mixes GI Smooth muscle contract spontaneously- • The smooth muscle of the gastrointestinal tract is excited by almost continual slow, intrinsic electrical activity along the membranes of the muscle fibers. • This activity is simillar to what happens in myocardial muscle and the electrical wave is referred to as -slow waves

31. Patterns of Contractions in the GI Tract • The basic propulsive movement of the gastrointestinaltract is peristalsis, illustrated in Figure. • A contractile ring appears around the gut(in the circular muscle layer) and then moves forward. • The usual stimulus for intestinal peristalsis is distention of the gut. That is, if a large amount of food collects at any point in the gut, the stretching of the gut wall stimulates the enteric nervous system to contract • the gut wall 2 to 3 centimeters behind this point, and a contractile ring appears that initiates a peristaltic movement.

32. Motility • local intermittent constrictive contractions occur every few centimeters in the gut wall. • These constrictions usually last only 5 to 30 seconds, then new constrictions • occur at other points in the gut, thus • “chopping” and “shearing” the contents first here and • then there. • Clinical correlation-esophageal spasm, • Delayed gastric emptying, constipation, diarrhea, • Irritable bowel syndrome.