GIT & Saliva Lecture Notes PDF

GIT & Saliva Objectives At the end of lecture, students will be able to: Differentiate between the mesenteric and submucosal plexuses in terms of structure and function. Explain the role of the autonomic nervous system in regulating gastrointestinal functions Discuss the composition of s...

GIT & Saliva Objectives At the end of lecture, students will be able to: Differentiate between the mesenteric and submucosal plexuses in terms of structure and function. Explain the role of the autonomic nervous system in regulating gastrointestinal functions Discuss the composition of saliva and the mechanisms involved in its secretion.  Enteric nervous system Confined within the wall of the GUT Extends from oral part of the Esophagus to the Internal anal sphincter. 2 sets of intercommunicating nerve plexus – MYENTERIC / AUERBACH plexus – SUBMUCOUS / MESSINER plexus Both plexus are composed of intricate network of fine un-myelinated nerve fibers intermingled with clusters of neuronal cell bodies arranged in the form of ganglia 3  MYENTERIC PLEXUS AUERBACH’s plexus Located b/w the outer longitudinal and the inner circular layers of smooth muscles extend from the upper esophagus to the anus. Multipolar Motor function Controls mainly the gastrointestinal movements 4  SUBMUCOSAL PLEXUS MEISSNER’s plexus Lies b/w the inner circular smooth muscle and the muscularis mucosae and runs from the stomach to the anus. Bipolar/ unipolar Sensory function Multipolar interneurons. Controls mainly gastrointestinal secretions & local blood flow 5  MYENTERIC PLEXUS SUB-MUCOSAL PLEXUS Controlling muscle activity Controlling function within the along the length of the gut with inner wall of each minute specific effects:- segment of intestine such as:- – Increased muscle tone – Local intestinal secretion – Increased intensity of the – Local absorption rhythmical contractions – Local contraction of – Slightly increased rate of sub- mucosal muscle the rhythm of contraction That causes various degrees of – Increased velocity of in- folding of G I mucosa. conduction of excitatory waves along gut wall, Composed of mainly causing more rapid excitatory neurons movement of gut peristaltic waves Composed of excitatory as well as inhibitory neurons 6 Role of Neurotransmitters Enteric neurons secrete an intimidating array of neurotransmitters. One major neurotransmitter produced by enteric neurons is acetylcholine. In general, neurons that secrete acetylcholine are excitatory, stimulating smooth muscle contraction, increases in intestinal secretions, release of enteric hormones and dilation of blood vessels.  Norepinephrine is also used extensively for neurotransmission in the gastrointestinal tract, but it derives from extrinsic sympathetic neurons; the effect of norepinephrine is almost always inhibitory and opposite that of acetylcholine. 9 Role of autonomic nervous system The enteric nervous system can and does function autonomously, but normal digestive function requires communication links between this intrinsic system and the central nervous system. These links take the form of parasympathetic and sympathetic fibers that connect either the central and enteric nervous systems or connect the central nervous system directly with the digestive tract. Through these cross connections, the gut can provide sensory information to the CNS, and the CNS can affect gastrointestinal function. Con’t Connection to the central nervous system also means that signals from outside of the digestive system can be relayed to the digestive system: for instance, the sight of appealing food stimulates secretion in the stomach. In general, sympathetic stimulation causes inhibition of gastrointestinal secretion and motor activity, and contraction of gastrointestinal sphincters and blood vessels. Conversely, parasympathetic stimuli typically stimulate these digestive activities. 12 ENS works independently to maintain Peristaltic Secretory movements functions Can even function Without the participation of ANS ** 13 Gastro-intestinal reflex pathway 14 SALIVARY GLANDS The principal glands of salivation are: 1. Parotid glands 2. Submandibular (Submaxillary) glands 3. Sublingual glands 4. Smaller glands in mucosa of tongue, palate, etc. Daily secretion of saliva = 800-1500 mL (average value of 1000 mL) with pH = 6-7 15 SALIVARY GLANDS 16  Saliva Composition Composition of Saliva: Is a hypotonic solution of 99% water and other solutes PH of 6.8 to 7.0 Solutes in saliva: Salivary amylase- an enzyme that begins starch digestion Lingual lipase- activated by stomach acid and digest fat after the food is swallowed Mucus- binds and lubricate the food mass and aids in swallowing Lysozymes- kills bacteria IgA- inhibit bacterial growth Electrolytes- including sodium, potassium, chloride, phosphate and bicarbonate ions. Saliva Functions Moistens the Mouth Digest a little starch and fat Cleanse the teeth Inhibits bacterial growth Dissolves molecules that they can stimulate taste buds Dilute and buffer food Moistens food and binds particles together to add in swallowing Secretion of saliva Serous secretion contains ptyalin for starches digestion Saliva contains high concentration of potassium and bicarbonate ions and low concentration of sodium and chloride ions. Parasympathetic nervous signals mainly control salivation  Secretion of Saliva Simple Reflex: occurs when chemoreceptors and pressure receptors within the oral cavity respond to the presence of food. Conditioned or acquired reflex: salivation occurs without oral stimulation e.g mouth watering. Saliva Secretion Characteristics ▪Saliva contains two major types of secretion: 1. Aqueous fluids (a serous secretion) Water, ions and enzymes such as ptyalin (an α- amylase) Parotid, Submandibular and Sublingual glands 2. Mucus secretion (mucin) Submandibular and Sublingual glands 21  Cont. Salivary glands produce serous secretions and mucous secretions or mixed secretions. Serous cells: Secreted by serous cells which are specialized for the synthesis, storage and secretions of the proteins. Mucous Cells: The secretory product of the mucous cells differ from the serous cells in two important respect. 1. They have little or no enzymatic activity and only serve the purpose of lubrication. 2. The ratio of carbohydrate is greater to that of the protein with large amount of sialic acid. 24 References Guyton and Hall 15th Edition. Sherwood Physiology Internet 2 6