Gastrointestinal Physiology Part 1.pdf

Gastrointestinal Function I. INTRODUCTION A. Food must be broken down through digestion to molecular size before it can be absorbed by the digestive system and used by the cells. II. OVERVIEW OF THE DIGESTIVE SYSTEM A. Organs of the digestive sys...

Gastrointestinal Function I. INTRODUCTION A. Food must be broken down through digestion to molecular size before it can be absorbed by the digestive system and used by the cells. II. OVERVIEW OF THE DIGESTIVE SYSTEM A. Organs of the digestive system B. C. The functional segments of the GI tract include the mouth, pharynx, esophagus, stomach, small intestine, and large intestine and sphincters between segments. D. The accessory structures that contribute to the food processing include the teeth, tongue, salivary glands, liver, gallbladder, and pancreas. E. Digestion includes six basic processes. a. Ingestion is taking food into the mouth (eating). b. Secretion is the release, by cells within the walls of the GI tract and accessory organs, of water, acid, buffers, and enzymes into the lumen of the tract. c. Mixing and propulsion result from the alternating contraction and relaxation of the smooth muscles within the walls of the GI tract. d. Digestion a. Mechanical digestion consists of movements of the GI tract that aid chemical digestion. 1  b. Chemical digestion is a series of catabolic (hydrolysis) reactions that break down large carbohydrate, lipid, and protein food molecules into smaller molecules that are usable by body cells. e. Absorption is the passage of end products of digestion from the GI tract, through enterocytes, into blood or lymph for distribution to other cells in the body. f. Defecation is emptying of the rectum, eliminating indigestible substances from the GI tract. F. These digestive processes are regulated by neural, hormonal and local factors. III. LAYERS OF THE GI TRACT A. The basic arrangement of layers in the gastrointestinal tract from the inside outward includes the mucosa, submucosa, muscularis, and serosa (visceral peritoneum). B. The mucosa consists of an epithelium, lamina propria, and muscularis mucosa. 1. The epithelium consists of a protective layer of non-keratinized stratified squamous cells in the mouth, pharynx and esophagus and simple columnar cells for secretion and absorption in the stomach and intestines. Other cells include mucus secreting cells as well as some enteroendocrine cells that secrete hormones that help regulate the digestive process. 2. The lamina propria consists of three components 1) loose connective tissue that adheres the epithelium to the lower layers, the system of blood and lymph vessels through which absorbed food is transported, 2) nerves, and 3) sensors. 2  a. The lymph system is part of the mucosa-associated lymph tissues (MALT) that monitor and produce an immune response to pathogens passing with food through the GI tract. b. It is estimated that there are as many immune cells associated with the GI tract as in all the rest of the body. 3. The muscularis mucosa causes local folding of the mucosal layer to increase surface area for digestion and absorption. C. The submucosa 1. The submucosa consists of areolar connective tissue. It is highly vascular, contains a part of the submucosal nerve plexus (Meissner’s), and contains glands and lymphatic tissue. D. Muscularis 1. The muscularis of the mouth, pharynx, and superior part of the esophagus contains skeletal muscle that produces voluntary swallowing. Skeletal muscle also forms the external anal sphincter. 2. Through the rest of the tract, the muscularis consists of smooth muscle in an inner sheet of circular fibers and an outer sheet of longitudinal fibers. IV. NEURAL INNERVATION OF THE GI TRACT A. The Enteric Nervous system (ENS) (intrinsic control) 1. Is entirely within the wall of the GI tract. 2. Can be regulated by autonomic nervous system (ANS), but can function independently of ANS too. Contains excitatory or inhibitory neurotransmitters. a. The submucosal nerve plexus (plexus of Meissner) a. It regulates movements of the mucosa, vasoconstriction of blood vessels, secretion by secretory cells of mucosal/other glands and can help absorption. b. It is stimulated by touch (contact), distension or irritation of muscosa. b. The myenteric nerve plexus (plexus of Auerbach). This plexus mostly controls GI tract motility (contraction and propulsion of food). Also decrease sphincter tone (relax sphincters, stay open) and allow propulsion of food. 3  c. Neurotransmitters of the ENS: a. Excitatory - acetylcholine, substance P, serotonin b. Inhibitory – VIP (vasoactive intestinal peptide), Nitric Oxide c. Afferent neurons stimulated by distension (stretch) of the GI wall, irritation, pain and chemicals. B. Autonomic Nervous System (extrinsic control) 1. In general, stimulation of the parasympathetic nerves that innervate the GI tract secrete Acetylcholine, causes an increase in GI secretion and motility by increasing the activity of ENS neurons. The ENS is actually the postganglionic parasympathetic neurons. 2. In general, the sympathetic nerves that supply the GI tract cause a decrease in GI secretion and motility by secreting Norepinephrine and inhibiting the neurons of the ENS C. Gastrointestinal reflexes 1. Local reflexes from afferent gut neurons terminate on the ENS and regulate GI secretion and motility. 2. Long loops have their afferents from GI tract, enter prevertebral ganglia and then influence motility of another part of the GI tract. Usually named from origin to destination which sort of hints at the functions. a. Gastrocolic reflex: stomach signals the colon to increase motility. b. Gastroenteric reflex: stomach signals intestines to increase motility. c. Enterogastric reflex: Intestines signal the stomach to inhibit motility. d. Vagovagal reflex: From stomach through vagus, to brainstem, back to stomach (vagus) to inhibit stomach motility and secretion. 4  e. Intestine-intestinal reflex: distension inhibits bowel movement. f. Gastroileal reflex: presence of food in stomach relaxes the ileal sphincter (opens), and food moves on to cecum. g. Defecation reflex: From stretch of colon/rectum to spinal cord and back to increase motility of rectum, relax internal anal sphincter. V. HORMONES OF THE GI TRACT: A. Here is a table showing GI hormones and functions Hormone Stimulus for secretion Secreted by Main Function Gastrin Protein digestion products in G cells of stomach Gastric acid (HCl) secretion from parietal cells stomach and duodenum Some growth (trophic effects) of stomach Distension of stomach mucosa. Cholecystokinin Fatty acids, monoglycerides, I cells in Contract gall bladder-deliver bile into small (CCK) peptides, proteins, Amino duodenum intestines acids in small intestines Relax sphincter of Oddi (fats and proteins) Potent stimulator of enzyme secretions from pancreas Also some bicarbonate secretion Secretin Acid chyme in duodenum S cells of Stimulate bile and pancreatic bicarbonate rich duodenum secretions from ducts Nature’s Also fat antacid Inhibit acid secretion in stomach Stimulate pepsin secretion in stomach GLIP (Gastric All major foods, carbs, K cells od Stimulates insulin secretion from pancreas dependent proteins, fatty acids duodenum insulinotropic peptide) Motilin Under neural control- M cells of Stimulates upper GIT motility secreted every 90-100 duodenum minute intervals Accounts for the MMCs (migrating motility complex) housekeeping contractions to keep Also by fat and acid low bacterial counts. Paracrine These below are secreted and do not enter blood-act hormones on their neighboring cells 5 Somatostatin acid Stomach, Inhibit secretion of all Gut hormones, duodenum, especially gastrin pancreas Histamine Gastrin (hormonal) and ECL Cause acid secretion in stomach. Histamine Acetylcholine (neural) (enterochromaffin blockers decrease acid secretion (eg., zantac) cells) in stomach Neurocrines These below are secreted from neurons VIP (vasoactive Esp ENS Gut mucosa and Relaxation of gut smooth muscles intestinal Smooth muscles peptide) (SM) (inhibit movement) Nitric oxide ENS Gut mucosa and Relaxation of gut smooth muscles (NO) Smooth muscles (SM) (inhibit movement) GRP Gastric mucosa Increase Gastrin Release (Bombesin) Enkephalins Gut mucosa and Increase smooth muscle tone (decrease SM motility) VI. GI SMOOTH MUSCLES: A. Unitary (act as one single unit), are connected through gap junctions, which propagate action potentials between muscles fibers and so can contract as 1 unit. B. Autorhythmic, but can be modulated to contract faster/slower by neural and hormonal/local influences. Slow waves caused by pacemaker of the GI tract, cells called the interstitial cells of Cajal. Slow waves allow only entry of sodium ions. Slow waves by themselves do not cause any contraction, except in the stomach. C. True action potentials occur in GI because of spike waves. These last longer but still slower than in neurons outside the GI (20 msec, compared to 1msec in other nerves). In other neurons in the body, sodium influx cause depolarization, through voltage gated sodium channels. In the GI smooth muscles, it is the influx of mostly calcium ions through calcium-sodium voltage channels that cause depolarization and contraction. This channels open and close slowly, giving slow, long lasting potentials and contractions. 6 VII. MOVEMENTS OF THE GI: A. 2 types: 1. Propulsive movements-food moves along the tract. Peristalsis is the main propulsive movement, towards the anus (caudad). 2. Mixing movements-segmental-keeps food mixing all the time. 3. Main stimulus for peristalsis is distension of gut. Orad (proximal) contraction with downstream (distal) receptive relaxation = “Law of the Gut”. Parasympathetic stimulation, and irritation of gut mucosa will also cause peristalsis. Peristalsis requires a functional myenteric plexus and secretion of the neurotransmitter Acetylcholine. 7 VIII. SPLANCHNIC CIRCULATION: A. Note: All blood from the GI, pancreas and spleen drain through the liver via the hepatic portal vein. B. The liver sinusoids filter the blood and the blood then goes via the hepatic vein to the Vena cava. Kupffer cells in the liver remove microorganisms. The liver also extracts about ½ of the nutrients. C. Blood flow to the GI increases in direct proportion to increased gut or gut metabolic activity-just like everywhere else-same principles of hemodynamics. 1. Increased submucosal blood flow occurs with increased absorption. 2. Increased muscle blood flow occurs due to increased motility. 3. Following a meal, blood flow increases and stays high for about 2 hours. 4. Vasodilator hormones - gastrin, secretin, CCK also increase blood flow 5. Vasodilator kinins (kalinin, bradykinin) also are powerful local vasodilators. 6. Low oxygen (high adenosine) will also increase blood flow. 7. Parasympathetic stimulation increases gut activity and blood flow. D. Countercurrent blood flow in villi: 8  1. Normally in the villi, most blood is shunted from artery to vein but it does not affect function. 2. However, in case of circulatory shock or other conditions, GI blood flow can decrease significantly and the villi can undergo ischemic death. IX. MOUTH A. Salivary Glands: complex glands 1. There are three pairs of salivary glands: parotid, submandibular (submaxillary), and sublingual glands. a. 2. Saliva lubricates and dissolves food and initiates the chemical digestion of carbohydrates (contains alpha-amylase/ptyalin). 3. Chemically, saliva is 99.5% water and 0.5% solutes such as salts, mucous. It is rich in K+ (7x in plasma), and HCO3- (alkaline). Also contains lysozyme that destroys bacteria. 4. Salivation is entirely under nervous control (parasympathetic stimulation). The inferior and superior salivatory centers in the brainstem stimulate salivation. 5. Primary aldosteronism will result in almost no sodium in saliva and a lot of potassium in saliva. B. Tongue and teeth 1. The tongue is composed of skeletal muscle and is important for tasting, chewing, swallowing and in speech. 2. The teeth project into the mouth and are adapted for mechanical digestion. 3. Teeth are composed primarily of dentin which is covered by enamel, the hardest substance in the body, which protects the tooth from the wear of chewing. C. Mechanical and Chemical Digestion in the Mouth 1. Through mastication (chewing), food is mixed with saliva and shaped into a bolus that is easily swallowed. Mastication is a reflex. Food in mouth, muscles relax, 9  stimulate muscle spindles, then reflex contraction of muscles through Trigeminal nerve (V). 2. The enzyme salivary amylase (ptyalin) converts polysaccharides (carbs, starches) to disaccharides (maltose). This is the only chemical digestion that occurs in the mouth. 3. Mucous here and throughout the gut is slightly alkaline (has more HCO3- ions), helps form a good bolus, lubricates and protects the gut from digestion. X. ESOPHAGUS A. The esophagus connects the pharynx to the stomach. B. Physiology of the Esophagus 1. The esophagus contains an upper. (UES) and a lower esophageal sphincter (LES). 2. Upper 1/3 of esophagus has striated/skeletal muscles, lower 2/3 has smooth muscles. XI. DEGLUTITION A. Deglutition (swallowing), moves a bolus from the mouth to the stomach. It is facilitated by saliva and mucus and involves the mouth, pharynx, and esophagus. B. Deglutition consists of 3 phases: 1. Voluntary state (voluntary)- we initiate swallowing process. 2. Pharyngeal stage (involuntary)- A reflex-stimulated by presence of food in pharynx, excites sensory receptors, and then food passes through pharynx into esophagus through UES (upper esophageal sphincter). Fast and interrupts respiratory center (breathing). 3. Esophageal stage: 2 phases (involuntary): a. Primary peristalsis: (skeletal muscles) continuation of pharyngeal phase, coordinated by swallowing center. b. Secondary peristalsis (smooth muscles): Stimulated by distension, and involves ENS and also swallowing center. 10 4. Receptors in the oropharynx stimulate the deglutition center in the brain stem (in or around nucleus tractus solitarius-also taste center). During swallowing, LES and fundus relax - receptive relaxation (low pressure). Vagal input is inhibitory. Nitric oxide is transmitter for receptive relaxation. 11