Digestive System

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Anatomy and Cell Biology
ACB 221
Keith Russell

The Digestive System The digestive system consists of a muscular tube, called the digestive tract (i.e., mouth, pharynx, esophagus, stomach, small intestine, and large intestine), and various accessory organs (i.e., teeth, tongue, and various glandular organs salivary glands, liver, pancreas). o Digestion is basically the mechanical and chemical breakdown of complex food into small molecules that can be absorbed by the epithelium that lines the digestive tract for transfer into the circulating blood. Muscle layers and movement o The digestive tract contains visceral smooth muscle tissue. Being comprised of smooth muscle, the digestive system is under involuntary muscle control. The initial activation stimulus can be from a neighbouring motor neuron, local response to chemicals, hormones, the concentration of O2and CO ,2or physical factors such as extreme stretching or irritation. However, smooth muscle can maintain a stretched position without contraction (or until a contraction signal is sent) this ability to tolerate extreme stretching is called plasticity. These smooth muscle cells are electrically connected to one another via gap junctions. Thus, when one visceral smooth muscle cell contracts, the contraction spreads in a wave that travels throughout the tissue. o Peristalsis Muscle contractions that create waves along the digestive tract so as to move the bolus (mass of food) along its length this occurs in the esophagus, as well as in the small and large intestines. Longitudinal muscles contract, shortening the adjacent segments of the digestive tract Circular muscles contract next causing the materials (i.e., bolus) to move in the desired direction. Longitudinal muscle Circular muscle The Digestive System o Segmentation Muscle contractions that occur in most areas of the small intestine and some regions of the large intestine. These movements churn and fragment the digestive materials, mixing the contents with intestinal secretions. They do not produce any net movement in a particular direction they are just for mixing. o Segmentation and peristalsis may be triggered by pacesetter cells (found in the muscularis mucosae and externa; undergo spontaneous depolarization triggering contractions). Segmentation and peristalsis may also be triggered by hormones, chemicals, physical stimulation, or by afferent and efferent fibers within the glossopharyngeal, vagus, and/or pelvic nerves that synapse on the myenteric plexus. Peritoneum o The peritoneum is the serous membrane that forms the lining of the abdominal cavity. The peritoneal membrane is a semi-permeable membrane that lines the abdominal wall (parietal peritoneum) and covers the abdominal organs (visceral peritoneum). o The peritoneal lining continually produces a watery peritoneal fluid that lubricates the peritoneal surfaces. The Digestive System o Mesenteries Fused, double sheets of serous membrane that supports and stabilizes the position of an organ in the abdominopelvic cavity and provides a route for the associated blood vessels, nerves, and lymphatics. The mesentery, within the peritoneal cavity, is separated into two regions: Dorsal mesentery AKA greater omentum - Located along the greater curvature of the stomach and extends inferiorly between the body wall and the anterior surface of the small intestines. - The loose connective tissue within the mesentery of the greater omentum usually contains a thick layer of adipose tissue. The lipids in this adipose tissue are an important energy reserve as well as an important insulator that reduces heat loss across the anterior abdominal wall. Ventral mesentery AKA lesser omentum - Located on the lesser curvature of the stomach, between the stomach and the liver. Omentum literally means fat skin. Lesser Omentum Greater Omentum Oral cavity o The oral cavity (or buccal cavity), is lined by oral mucosa (non-keratinized stratified squamous epithelium) that protects the mouth from abrasion during the ingestion of food. o The roof of the oral cavity is formed by the hard and soft palates, while the tongue dominates the floor of the oral cavity.
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