Module 06 The Digestive System

MODULE 06: THE DIGESTIVE SYSTEM Target Outcomes At the end of the lesson, you are expected to: Differentiate ruminant and monogastric gastrointestinal tract; Identify factors of digestion; Enumerate functions of saliva and its importance in the ruminant animals; Understand the basic unit of salivary secretion; and Explain the mechanism of salivary secretion, its control and importance in animals. Abstraction PHYSIOLOGY OF DIGESTION AND ABSORPTION Digestion is the process of breakdown of complex food into simpler form by the activities of the alimentary tract and glandular secretions for absorption of nutrients and the rejection of their residues. Food Food is a complex mixture of substances like carbohydrates proteins, fats, vitamins, inorganic salts and water to meet the nutritive requirements of an animal. Herbivorous animals (Cattle, sheep, horse, goat etc.,) derive their nutritive requirements from plant sources. Carnivorous animals (dog, cat etc.,) obtain their food from animal sources. Omnivorous animals (man, pig, etc.,) get their foods from both animals and plants sources. Alimentary tract It extends from the lips, mouth, pharynx, esophagus, stomach, small intestine (Duodenum, jejunum and ileum), large intestine (Cecum, colon and rectum) and anus. In carnivores, alimentary tract is small and simple. The simple stomach herbivores (horse, rabbit), are also referred as hindgut digesters because they have relatively simple stomach portion, but the large intestine is much more complex and voluminous than the carnivores. In ruminants (cattle, sheep and goat), the stomach is extensively large and complex, whereas the large intestine is relatively small, hence they are known as foregut digesters. Functions of GI tract To assist in the acquisition of nutrients. To prepare nutrients for digestion. To digest the nutrients. To felicitate absorption of products of digestion. Absorption of water. As excretory organs to help in elimination of waste products. As an endocrine gland to influence digestion and other metabolic functions. Motility of the GI tract. Prehension of food Prehension is the seizing and conveying of food into the mouth. In bipeds (Primates) the hands are the prehensile organs. Dogs and Cats hold their pray with the forelimbs which is passed into the mouth by the head and jaw movements. In horse, upper lip, tongue and the incisor teeth are the main prehensile organs to collect the food. The clefted upper lip in sheep favors close grazing, on contrast the unclefted upper lip in goat. In cattle, large strong, rough protrudable tongue and incisor teeth of the lower jaw are the prehensile organs. The pointed lower lip functions as a prehensile organ in swine. FACTORS OF DIGESTION Mechanical factors Mastication (Grinding the food) Deglutition (Swallowing) Regurgitation (Return of the cud from stomach to mouth) Rumination (Chewing of cud) Remastication (Regrinding) Redeglutition (Reswallowing) Intestinal motility Eructation (Elimination of gases) Defecation (Elimination of fecal matters) Chemical factors Digestive enzymes and hormones Non-enzymatic chemicals: HCl, HCO3 Microbiological factors Bacteria Protozoa Fungi MASTICATION Definition Extensive chewing of the feed causes mechanical reduction in the size of the food. The finely divided food particles provide greater surface area for enzymatic action and proper mixing with saliva for easy swallowing. This act is achieved by deduction and occlusion of both the upper and lower molar teeth. Temporomandibular joint is involved. Hence, lateral forward and backward movement is possible. Most of the animals masticate on one side at a time due to the anatomical arrangement of the dental table (wider upper jaw and narrower lower jaw). Molar teeth are chisel shaped with the arrangement of inner most lower teeth and outer most teeth. Both are sharp edged. Importance of mastication In herbivores mastication is of greater significance due to the coarse bulky nature of food. In ruminants, only during remastication the food is thoroughly ground. In herbivores, mastication of food material is by lateral movement of the lower jaw and to-andfro movement. In herbivores, the upper jaw is wider than the lower jaw and mastication of food occur on only one side at a time. The molar teeth are chisels shaped with sharp edged lower teeth pointed towards inner side and that of upper teeth towards outer side. Incisor teeth are used to cut and lacerate the food In ruminants, the upper incisors are absent, but modified as dental pad and in the lower jaw they are loosely and obliquely placed in the sockets. Grazing Traditional grazers (sheep, cattle) Grass and roughage eaters. Mostly graze on monocotyledons Use their small mouth and long tongue to grasp grass and hay. Tongue is curved around the grass Browser ruminants (deer, giraffe and moose) Graze on dicotyledons Have large mouth opening and use their incisors to gnaw on fruits and flowers, trees. Intermediary – opportunistic feeders (goat) Adapt to both the pattern of feeding. They prefer low fiber dicotyledons. Preferred food can be sought by climbing Bovine Keratinized papillae of the bovine help in the passage of food in the oral cavity. As the tongue pushes the cell grass between the lower incisors and upper dental pad, simultaneous mandibular severing reduces the size of the particles. As rumination is the predominant activity, they spend less time in initial mastication. Sheep and goat Mandibular grinding Horse Incisors thrust the feed into the oral cavity with the propulsive movement of mandible, maceration of grass at its base is achieved. Masticate thoroughly prior to swallowing. Omnivores occupy a position between herbivores and carnivores in the aspect of timing. Pig Use incisors, tongue and propulsive movements of head for mastication. Rodents have the unique property of mastication due to their ability to shift their teeth in a proplineal pattern. In carnivores, mastication is imperfectly performed by vertical movement of the lower jaw. Dog and cat Mastication is done by teeth with food in oral cavity by propulsive head and mandibular movements. In cats, papillae of the tongue (dorsal lingual spicules) help in pushing the feed into the oral cavity. In carnivores and omnivores, the upper and lower jaws are equal width and the teeth are relatively simple. Drinking Dogs and cats Fluid is drawn into the oral cavity by the rapid extension and retraction of the tongue by making the free end of the tongue as ladle-like structure to convey the liquid into the mouth. Horse, Bovine, Sheep and Goat Drinking of fluid is by suction by creating negative pressure by keeping closed mouth beneath the fluid and tongue. Suckling It is affected by creating negative pressure in the mouth largely by the backward movement of base of the tongue. Milk is forced from the teat into the mouth due to pressure gradient. FUNCTION AND MECHANISM OF MASTICATION Function Physical breakdown increases the surface area of food, which improves the microbial digestion. Assist in appreciating the flavor of the food. Improves salivary and gastric activities. Initial digestion of carbohydrates is facilitated. Contributes to dilution and buffering of ruminoreticular fluid. Mechanism Reflex activity under the control of CNS. Mastication is basically a voluntary act but usually takes place involuntarily. Mastication reflex or chewing reflex is the rhythmic movements of mandible. Lowering pf mandible due to extension of tongue (mainly in bovine) is known as LINGUO MANDIBULAR REFLEX. Stimuli for masticatory reflex is the presence of food in the oral cavity. These are receptors in tongue and oral mucosa which initiates mastication by the sense of food. Sensory impulses are carried via trigeminal, facial, and glossopharyngeal nerves to brain stem. Efferent via trigeminal supplied to masticatory muscles. This in turn causes rhythmic movement of the mandible in relation to the maxilla. This results in shearing and crushing of food. The main masticatory muscles include temporalis with the assistance from masseter muscle for shearing and masseter with the assistance from pterygoid muscles for grinding. DEGLUTITION (SWALLOWING) Deglutition is the act of passage of food from the mouth to the stomach through pharynx and esophagus. It starts, as a voluntary act then becomes an involuntary reflex during its execution. The afferent nerves originate from receptors located in the posterior part of the mouth and pharynx. Deglutition center is situated in the medulla. It is the motor activity involving the integrated movement of muscles of tongue, pharynx and esophagus. (including the peristaltic movement) It is under direct neural control of brain. Complex action of tongue, to place the bolus centrally between the tongue and the hard palate. Pressure of the bolus in the pharynx stimulates pharyngeal pressure receptors to initiate swallowing reflex. Act of swallowing It takes place in three phases From mouth to pharynx (voluntary act) From pharynx into the esophagus (reflex mechanism) From the esophagus into the stomach (reflex mechanism) Contraction of mylohyoid and hypoglossal muscles. Pressing of the tongue against the hard palate and pulling the root of the tongue backwards. The tongue acts like a plunger driving bolus towards pharynx. Elevated soft palate is to cut off communication with nasal passage. Forward pulling of hyoid bone and the larynx, cause opening of the entrance of esophagus. Closure of the larynx by epiglottis make the bolus to enter directly into the esophagus. STAGES OF SWALLOWING Pharyngeal stage - Pharyngoesophageal sphincter Esophageal stage - Significance of lower esophageal sphincter. Pharyngeal stage Relaxation of pharyngoesophageal sphincter. Peristaltic contractions of the pharynx to propel the food bolus from the base of the tongue into laryngopharynx. Propulsion of the bolus via relaxed pharyngoesophageal sphincter to the esophagus. Pharyngoesophageal sphincter It is also known as upper esophageal sphincter formed by the cricopharyngeal muscle and esophageal circular muscle. As the bolus proceeds to upper esophagus, simultaneous contractions of this sphincter to a closed state to initiate next stage of swallowing reflex. The bolus travels from cranial esophagus to caudal esophagus via gastro esophageal junction. Gastro esophageal junction is guarded by a lower esophageal sphincter. Reflex, contraction of the esophageal muscles to propel the bolus down. This reflex is completed by vagus. Accumulation of several boluses in the esophagus causes local myogenic stimulus initiates more powerful peristalsis to push the food to gastro esophageal junction. Esophageal stage Lower esophageal sphincter It differentiates esophagus and stomach. Physiologically is in a high-pressure zone. The incoming bolus exert force to open this sphincter. This is an active reflex mediated by vagus to increase esophageal pressure. Synchronized act of relaxation of lower esophageal sphincter with esophageal wave propel the bolus into the stomach. SWALLOWING REFLEX Swallowing center is located in the brain stem (medulla oblongata). Stimulation of the receptors in the soft palate, pharynx (posterior wall) and epiglottis (dorsal surface) by food material. Sensory fibers emerge through vagus, glossopharyngeal and hypoglossal to mylohyoid and hypoglossal muscles. Contraction of mylohyoid and hypoglossal muscles press the tongue against hard palate. Backward pulling of the tongue elevates soft palate. The tongue forces the bolus in the opened esophagus. Pulling action of hyoid bone and larynx opens the esophagus. Larynx is closed by epiglottis. This reflex involves internucial neurons from reticular formation. Swallowing center also activates the neighboring respiratory regulatory neurons. This interrupts respiration during swallowing to avoid aspiration of food particles into respiratory passages. Mechanism of regurgitation Common in ruminants. Initiated with closed glottis and raised palate. Drop in intrathoracic and intraesophageal pressure due to inspiratory effort of the tongue. Opening of cardia, clearing of cardia. Extra reticular contraction (regurgitation contraction) pushed ruminoreticular cud to the mouth via esophagus. ESOPHAGUS AND STOMACH Esophagus is a muscular tube-like structure extends from the pharynx to the stomach. In dog, cattle and sheep, the muscular layer is striated throughout the length of the esophagus, whereas in pigs and horse, it begins as striated but becomes smooth muscles at caudal esophagus. The pharyngoesophageal junction is normally closed by esophageal sphincter. Vagus is the main motor nerve regulates the motility of the esophagus. During swallowing, the peristaltic wave travels from pharyngoesophageal sphincter towards cardiac sphincter which is located at gastroesophageal junction. Reverse peristalsis/antiperistalsis is involved in bringing the gastric contents into the esophagus during belching and regurgitation. Peristaltic waves, buccopharyngeal pressure and gravity are responsible for the movement of food bolus through esophagus, of which buccopharyngeal pressure is important for the passage of liquids. Cardia The point of opening of esophagus into the stomach is called cardia. It is provided with a sphincter muscle known as cardiac sphincter. It prevents back flow of food from stomach to esophagus. Cardia is ordinarily closed except during swallowing and regurgitation. The cardiac sphincter is well developed and powerful in horse. The activity of cardiac sphincter is under the control of CNS. Stomach It functions as a reservoir of food. Actively involved in grinding the food to reduce their size Initiates enzymatic digestion of food materials Controls the rate of passage of food to the small intestine for final digestion and absorption. Produces the intrinsic factor for the absorption of vitamin B12 from the intestine, which involves in hematopoiesis. Based on structure and function of stomach, domestic animals fall into two general classes: Non-ruminants/ simple stomach animals – Horse, cat, dog, and pig. Ruminants – Cattle, sheep, goat, camel and buffalo The stomach of nonruminants is simple consisting of only one compartment, whereas the stomach of ruminants is complex, consisting of four compartments (rumen, reticulum, omasum and abomasum) of which only abomasum secretes the gastric juice. The stomach is a hollow, sac like organ made up of four layers - serous, muscular, submucosa and mucosa from outside to inside. The stomach mucosa of simple stomach animals, is divisible into esophageal (glandless) region and glandular region. The glandular area includes cardiac, fundic (parietal) and pyloric regions. In horse, esophageal region is extensive up to 1/3 to 1/5 of the surface area of stomach. In the glandular region, the cardiac gland zone is very narrow, while the fundic gland zone is very wide. In pig, the esophageal region of the stomach is limited to a small area around the cardia. The cardiac gland zone is very extensive, whereas the fundic and pyloric gland zones are similar to those of horse. In dogs, the esophageal region is absent, the cardiac glands are found as a narrow zone scattered along the lesser curvature of the stomach around the cardia. The fundic glandular zone is extensive occupying about 2/3 of gastric mucous membrane. RUMINANT STOMACH Ruminants are animals capable of regurgitating their food from their stomach and remasticate them. Capacity of the stomach varies with age and size of the animal. All herbivorous animals have spacious compartments in their G.I tract. It favors retention of bulky fibrous plant for soaking, mixing and microbial fermentation. In ruminants and kangaroo, stomach (Rumen) provides an additional space for microbial fermentation. Bacteria, protozoa and fungi in the rumen are responsible for extensive fermentative digestion in the rumen. It is supported by the mechanical activity of the three compartments (rumen, reticulum and omasum). Only the abomasum, the true stomach secretes gastric enzymes and HCl. Ruminants are animals which can regurgitate and remasticate. Abomasum is the largest compartment in new born ruminants. As age advances, the rumen and reticulum grow at a faster rate than abomasum. In adults, rumen and reticulum occupies 69%, omasum 8% and abomasum 23% of the stomach portion. Omasum is not well developed in sheep and goat, it is absent in Camel and Llama (Tylapoda) Esophagus opens into the rumen through cardia. Rumen has two sacs, dorsal and ventral sacs that are freely communicating with each other. Rumen is connected to reticulum by ruminoreticular folds. Reticulum is communicated with omasum through reticulomasal orifice. Esophageal/reticular groove extends from the cardia to reticulomasal orifice. It is a gutter like invagination. This groove is more functional in young ruminants. During suckling the receptors in the pharynx and mouth get stimulated causes reflex closure of reticular groove to conduct liquid and milk directly from the esophagus into reticulomasal orifice, bypassing rumen and reticulum. In calves, reticular groove/esophageal groove acts as a bypass route for the passage of milk directly from the esophagus into the omasum and abomasum. Closure of this groove is mediated by behavioral, psychological response and also by chemicals. Administration of chemicals like NaCl, NaHCO3, CuSO4 and sugar solutions reflex close this groove, but CuSO4 is less effective in calf and older ruminants, but more effective in sheep. Sodium salts like NaHCO3 (60ml of 10% solution) stimulates closure in calves. OTHER STRUCTURES Liver Liver and gall bladder represent accessory structures of the digestive system. Metabolic organ, participate in digestion, metabolism and also in circulatory/immune system. Special and unique organ as it receives double blood supply, seat of metabolism and is of diagnostic purpose. Pancreas Representing one of the accessory structures of the digestive system secretes clear, colorless liquid representing water, enzymes and salts like sodium bicarbonate. Small Intestine Has 3 portions Duodenum, Jejunum and Ileum. Concerned with digestion and absorption. Large Intestine Has 3 major portions: cecum, colon and rectum. Main function includes formation of certain vitamins, completion of absorption, formation of dung and its expulsion. Gastrointestinal Motility The walls of GI tract are muscular and capable of generating slow waves of electrical depolarization which are originated by the presence of food/ingestion as a stimulus from various areas of gut lumen. These motility aims to propel ingesta from one area to the other, to promote digestion and to aid absorption. Movement popularly known as motility are of propulsive, reventire or mixing. SALIVARY GLAND AND ITS SECRETION Salivon is the functional salivary unit. It begins most proximally as several Acini which converge into a intercalated duct. Numerous intercalated ducts unite to form a striated duct. A group of these ducts directly drain their content into glandular tubule which pour their content into fewer excretory ducts. Finally, the excretory ducts converge to form a single excretory duct, which leads to oral cavity. Salivary gland and secretion Saliva is the mixed secretion of three pair of main salivary glands, namely parotid, submaxillary or submandibular and sublingual and also many small glands found in the mucous membrane of the mouth. Glands in general are divided into serous, mucous and mixed types. Serous glands give rise to thin, watery secretion containing protein without mucin, mucous glands produce a secretion containing the glycoprotein in mucin; mixed glands produce both serous and mucous secretions. Cells that gives rise to a serous secretion also secrete enzymes. Zymogens granules are the precursors of enzyme s