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LEC.1 17/4/2016 Dr.Baybeen Alselevany Gastrointestinal (GI) System Objectives: Identify structures and organs that make up the GI system Identify functions of GI system identify the tissue layers that compose the majority of the GI system. identify basic electrical activity of GI smooth muscle. Neural control of GI system. Structures of gastrointestinal system The gastrointestinal system is a muscular tube about 9 meters long.GI system is a group of interconnected organs and glands that run continuously from mouth to anus .Its divided into two parts: 1. gastrointestinal tract (GIT) or Alimentary canal or tubular alimentary tract extending from the mouth to the anus .(mouth , pharynx , esophagus , stomach , small intestine , large intestine , and anal canal(anus).. GIT is subdivided into: .upper GIT includes the mouth, esophagus and stomach. Lower GIT is composed of small intestine, large intestine, rectum and anus. 2- Outlying glands or accessory or associated organs such as: Salivary glands, Pancreas, liver and Gallbladder .These accessory organs release their secretion into GIT. Functions of GIT The gastrointestinal system or gut is the portal through which nutritive substances, vitamins, minerals, and fluids enter the body. Proteins, fats, and complex carbohydrates, are broken down into absorbable units (digested). Ingestion: intake, in this case in the form of food and fluids. 1 Digestion: the breakdown of foods into smaller basic parts, to be used by body for growth and repair. Its occurs principally in the small intestine. Mechanical digestion is the process of changing the physical structure of food while chemical digestion which occurs by digestive enzymes in GIT system and food particles are converted into simpler forms appropriate for absorption. absorption :is the mechanisms by which the digestive end products as well as water , electrolytes , vitamins and minerals cross the mucosa and enter the blood stream and ultimately to tissue and cells. Elimination: removal of waste from the body. Tissue layers that compose the majority of the GI system Physiologic Anatomy of the Gastrointestinal Wall Walls of GIT have various layers, beginning at esophagus and extending to the anus .The GIT is composed of the five basic tissue layers 1. Mucosa (innermost layer) .2.submucosa containing blood and lymph vessels .3. Circular muscle layer 4. Longitudinal muscle layer. 5. Serosa. Gastrointestinal smooth muscle functions as a "syncytium" Basic Electrical Activity of GIT smooth muscle The smooth muscle of the GIT has spontaneous rhythmic fluctuations in membrane potential between about -50 and -60 mV.The smooth muscle of the GIT is excited by continual slow, intrinsic electrical activity along the membranes of muscle fibers 1. Slow wave's potential or basic electrical rhythm (BER) or pace-maker waves. .This BER) is initiated by the specialized cells, called interstitial cells of Cajal, that are believed to act as electrical pacemakers for smooth muscle cells.. The interstitial cells of Cajal undergo cyclic changes in membrane potential. The intensity of BER varies between 5 and 15 mv and their frequency ranges in different parts of the human GIT from 3-12 pulse per minute.. In the stomach about 3 pulse / minute, duodenum 12 pulse / minute , 2 terminal ileum 8 pulse / minute, Cecum 9 pulse /minute, and Sigmoid 16 pulse /minute. Therefore the rhythm of contraction of the: body stomach is 3 contractions /min. ,Duodenum 12 contractions /min, Terminal ileum 8 contractions / min. , Cecum 9 contractions /min sigmoid 16 contractions/ min. 2- Spike potentials (SP) or action potential (AP) In GIT smooth muscle fibers the channels responsible for (AP) are: calcium sodium channels, they allow large numbers of calcium ions to enter along with smaller numbers of sodium ions. These channels are much slower to open and close. 3- Changes in voltage of the resting membrane potential (RMP): Under normal conditions the RMP averages -56mv, multiple factors can change these levels which are: A- Depolarization of the membrane: Depolarization is due to calcium ion influx. Factors that depolarize the membrane increases the tension of intestinal smooth muscle are:. 1- Stretching of the muscle. 2- Stimulation by acetylcholine (ACH). 3- Stimulation by Parasympathetic that secret acetylcholine at their never endings.4- Cold (colic pain). 5- Stimulation by several specific gastrointestinal hormones. B- Hyperpolarization of the membrane: hyperpolarizing portion is due to K+ efflux. Factors that hyperpolarization membrane and decreases the tension of intestinal smooth muscle are: .1- Norepinephrine (NE) or epinephrine .2.Heat 3- Stimulation of sympathetic nerves that secrete mainly NE at their never endings. 3 NEURAL CONTROL 1. Enteric (intrinsic) nervous system. 2. Autonomic (extrinsic) nervous system. 1. ENTERIC NERVOUS SYSTEM (ENS): Enteric nervous system is important in controlling GIT motility and secretion. The GIT has a nervous system all its own called the enteric nervous system. It lies entirely in the wall of the gut beginning in the esophagus and extending all the way to the anus .There are two major networks of nerve fibers that are intrinsic to GIT: a- myenteric plexus (Auerbach’s plexus which controls mainly the GIT movements (Motility) or peristalsis. b- an inner plexus called submucosal or Meissner’s plexus .This plexus controls GI secretion, local blood flow, and local absorption. Together these two plexuses constitute the intramural (within the wall) plexuses or ENS. Neurotransmitters secreted by ENS: different neurotransmitter substances those are released by the nerve endings of different types of enteric neurons. : (1) ACH which excites GI activity. (2) NE almost inhibits GI activity. Epinephrine secreted from adrenal medulla and secreted into the circulation which reaches GIT by the way of the blood. 3-serotonin 4- Dopamine. 5. Cholecystokinin (CCK) 8- somatostain. 9-gases: nitric oxide (No) and carbon monoxide (CO) .10- Vasoactive intestinal peptide (VIP). Which inhibits GI activity.( receptive relaxation ) 2. Autonomic control of GIT or Extrinsic innervations The intestine receives dual extrinsic innervations nervous system (A.N.S) . from the autonomic 1. Parasympathetic nervous system 2. Sympathetic nervous system Nerve fiber from both sympathetic and parasympathetic enters the GIT and synapse with both plexuses of enteric nervous system. A. parasympathetic innervations: The parasympathetic supply to the gut is divided the: 1. cranial outflow. 2. Sacral outflow. Cranial outflow, which innervating salivary. About 75% of all parasympathetic nerves fibers are in the vagus nerve which innervates esophagus, stomach, pancreas, liver, gallbladder, less to the small intestine, and proximal half of the colon. 4 2- Sacral outflow: originated in the second, third, and forth sacral segments of spinal cord and pass through the pelvic nerves to the distal half of large intestine and all the way to the anus. These fibers function especially to excite the defecation reflexes. The parasympathic fibers are cholinergic (release acetylcholine). . So stimulation of parasympathetic nerves causes increase in the activity (motility& secretion). .b. Sympathetic outflows. In general stimulation of the sympathetic will inhibit motility and secretion. 5