Digestion Part 2 PDF

The Regulation of Digestive Activities. The major factors responsible for regulating digestive activities: neural mechanisms; hormonal mechanisms; local mechanisms. Neural Mechanisms The movement of materials along your digestive tract, as well as many secretory functions, is controlled primarily b...

The Regulation of Digestive Activities. The major factors responsible for regulating digestive activities: neural mechanisms; hormonal mechanisms; local mechanisms. Neural Mechanisms The movement of materials along your digestive tract, as well as many secretory functions, is controlled primarily by neural mechanisms. These reflexes are also called myenteric reflexes , and the term enteric nervous system is often used to refer to the neural network that coordinates the myenteric reflexes along the digestive tract. Short reflexes control activities in one region of the digestive tract. The control may involve coordinating local peristalsis and triggering the secretion of digestive glands. 9 Sensory information from receptors in the digestive tract is also distributed to the CNS, where it can trigger long reflexes , which involve interneurons and motor neurons in the CNS. Long reflexes may involve parasympathetic motor fibers in the glossopharyngeal, vagus, or pelvic nerves that synapse in the myenteric plexus. Hormonal Mechanisms The sensitivity of the smooth muscle cells to neural commands can be enhanced or inhibited by digestive hormones. Your digestive tract produces at least 18 hormones that affect almost every aspect of digestive function, and some of them also affect the activities of other systems. The hormones ( gastrin, secretin , and others), which are peptides produced by enteroendocrine cells in the digestive tract, reach their target organs by distribution in the bloodstream Local Mechanisms Prostaglandins, histamine, and other chemicals released into interstitial fluid may affect adjacent cells within a small segment of the digestive tract. For example, the release of histamine in the lamina propria of the stomach stimulates the secretion of acid by cells in the adjacent epithelium. The Oral Cavity, or buccal cavity The Oral Cavity. (a) anterior view (b) A sagittal section. We can summarize the functions of the oral cavity as follows: (1) analysis of material before swallowing; (2) mechanical processing through the actions of the teeth, tongue, and palatal surfaces; (3) lubrication by mixing with mucus and salivary gland secretions; and (4) limited digestion of carbohydrates and lipids. 10 The posterior margin of the soft palate supports the uvula, a dangling process that helps prevent food from entering the pharynx prematurely. The Tongue functions of the tongue are (1) mechanical processing by compression, abrasion, and distortion; (2) manipulation to assist in chewing and to prepare material for swallowing; (3) sensory analysis by touch, temperature, and taste receptors, and (4) secretion of mucins and the enzyme lingual lipase. We can divide the tongue into an anterior body , or oral portion , and a posterior root , or pharyngeal portion. Your tongue contains two groups of skeletal muscles: (1) intrinsic tongue muscles and (2) extrinsic tongue muscles. All gross movements of the tongue are performed by the relatively large extrinsic muscles. The smaller intrinsic muscles change the shape of the tongue and assist the extrinsic muscles during precise movements, as in speech. Both intrinsic and extrinsic tongue muscles are under the control of the hypoglossal nerve (XII). Salivary Glands Three pairs of salivary glands secrete into the oral cavity The Salivary Glands. (a) A lateral view, showing the relative positions of the salivary glands and ducts on the left side of the head. For clarity, the left ramus and body of the mandible have been removed. (b) The submandibular gland secretes a mixture of mucins, produced by mucous cells, and enzymes, produced by serous cells. The large parotid salivary glands lie inferior to the zygomatic arch deep to the skin that covers the lateral and posterior surface of the mandible. Each gland has an irregular shape, extending from the mastoid process of the temporal bone across the outer surface of the masseter muscle. 11 The parotid salivary glands produce a thick, serous secretion containing large amounts of salivary amylase , an enzyme that breaks down starches (complex carbohydrates). The sublingual salivary glands are covered by the mucous membrane of the floor of the mouth. These glands produce a watery, mucous secretion that acts as a buffer and lubricant. Numerous sublingual ducts ( Rivinus' ducts ) open along either side of the lingual frenulum. The submandibular salivary glands are situated in the floor of the mouth along the inner surfaces of the mandible within a depression called the mandibular groove. The submandibular glands secrete a mixture of buffers, glycoproteins called mucins , and salivary amylase. The submandibular ducts ( Wharton's ducts ) open into the mouth on either side of the lingual frenulum immediately posterior to the teeth Saliva Your salivary glands produce 1.0–1.5 liters of saliva each day. Saliva is 99.4 percent water, and the remaining 0.6 percent includes an assortment of electrolytes (principally and ), buffers, glycoproteins, antibodies, enzymes, and waste products. The glycoproteins, called mucins , are primarily responsible for the lubricating action of saliva. About 70 percent of saliva originates in the submandibular salivary glands, 25 percent in the parotids, and the remaining 5 percent in the sublingual salivary glands. Buffers in the saliva keep the pH of your mouth near 7.0 and prevent the buildup of acids produced by bacterial action. In addition, saliva contains antibodies (IgA) and lysozymes that help control populations of oral bacteria. The saliva produced when you eat has a variety of functions, including: Lubricating the mouth. Moistening and lubricating materials in the mouth. Dissolving chemicals that can stimulate the taste buds and provide sensory information about the material. Initiating the digestion of complex carbohydrates before the material is swallowed. The enzyme involved is salivary amylase , which is also known as ptyalin or alpha–amylase. Although the digestive process begins in the oral cavity, it is not completed there, and no absorption of nutrients occurs across the lining of the cavity. Saliva also contains a small amount of lingual lipase that is secreted by the glands of the tongue. The mumps virus most often targets the salivary glands, especially the parotid salivary glands, although other organs can also become infected. Infection typically occurs at 5–9 years of age. The first exposure stimulates the production of antibodies and, in most cases, confers permanent 12 immunity; active immunity can be conferred by immunization. In post– adolescent males, the mumps virus can also infect the testes and cause sterility. Infection of the pancreas by the mumps virus can produce temporary or permanent diabetes; other organ systems, including the central nervous system, are affected in severe cases. Control of Salivary Secretions Salivary secretions are normally controlled by the autonomic nervous system. Each salivary gland receives parasympathetic and sympathetic innervation. The parasympathetic outflow originates in the salivatory nuclei of the medulla oblongata and synapses in the submandibular and otic ganglia. Any object in your mouth can trigger a salivary reflex by stimulating receptors monitored by the trigeminal nerve (V) or by stimulating taste buds innervated by cranial nerves VII, IX, or X. Parasympathetic stimulation accelerates secretion by all the salivary glands, resulting in the production of large amounts of saliva. For example, chewing with an empty mouth, the smell of food, or even thinking about food will initiate an increase in salivary secretion rates; that is why chewing gum is so effective at keeping your mouth moist. The presence of irritating stimuli in the esophagus, stomach, or intestines will also accelerate the production of saliva, as will the sensation of nausea The Pharynx serves as a common passageway for solid food, liquids, and air. The Esophagus 13 The Esophagus. (a) A transverse section through the esophagus. (b) The esophageal mucosa. (c) A color– enhanced SEM of the transition between the esophageal and gastric mucosae at the lower esophageal sphincter. The Esophagus is a hollow muscular tube with a length of approximately 25 cm (1 ft) and a diameter of about 2 cm (0.75 in.) at its widest point. The primary function of the esophagus is to carry solid food and liquids to the stomach. The esophagus begins posterior to the cricoid cartilage, at the level of vertebra From this point, where it is at its narrowest, the esophagus descends toward the thoracic cavity posterior to the trachea. It passes inferiorly along the dorsal wall of the mediastinum and enters the abdominopelvic cavity through the esophageal hiatus, an opening in the diaphragm. The esophagus then empties into the stomach anterior to vertebra. The esophagus is innervated by parasympathetic and sympathetic fibers from the esophageal plexus. Resting muscle tone in the circular muscle layer in the superior 3 cm (1 in.) of the esophagus normally prevents air from entering your esophagus. A comparable zone at the inferior end of 14 the esophagus normally remains in a state of active contraction. This condition prevents the backflow of materials from the stomach into the esophagus. Histology of the Esophagus The mucosa of the esophagus contains a nonkeratinized, stratified squamous epithelium similar to that of the pharynx and oral cavity. The mucosa and submucosa are thrown into large folds that extend the length of the esophagus. The muscularis mucosae consists of an irregular layer of smooth muscle. The submucosa contains scattered esophageal glands , which produce a mucous secretion that reduces friction between the bolus and the esophageal lining. : Swallowing , or deglutition , is a complex process whose initiation can be voluntarily controlled, but that proceeds automatically once it begins. Although you are consciously aware of, and voluntarily control, swallowing when you eat or drink, swallowing can also occur unconsciously, as saliva collects at the back of the mouth. Each day you swallow approximately 2400 times. We can divide swallowing into buccal, pharyngeal, and esophageal phases 15 The Swallowing Process. This sequence, based on a series of X rays, shows the stages of swallowing and the movement of materials from the mouth to the stomach 16