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Biological Sciences
Peter A.Bowler

e109 11/30/12 lec 24 Anatomy of digestive tract (=GI tract, gastrointestinal tract) -Food and drink enter oral cavityesophagusstomachsmall intestinelarge intestinerectum Digestive processes can be divided into three “phases”: 1. cephalic phase: head 2. gastric phase: stomach 3. intestinal phase: intestine (not just a passive conveyor belt!) Cephalic phase of digestion -thinking about food -food entering the mouth, moving down esophagus -until food enters stomach, then cephalic phase ends -“feedforward” -signal to salivary glands, enteric nervous system (stomach, intestine, accessory glands) Remember Fig 11-5 (autonomic sympathetic and parasympathetic pathways) -vagus nerve: important parasympathetic neurons to digestive system and heart and lungs; regulate and control the response of the digestive system from information coming from the brain (medulla oblongata); vagus nerve takes the output from the brain to the digestive organs; 75% of parasympathetic neurons are found in the vagus nerve In the cephalic phase, the sight, smell, taste, and thought of food initiate long reflexes that prepare the stomach. Before food even arrives, digestive activity in the stomach begins with the long vagal reflex of the cephalic phase In the cephalic phase, the sight, smell, taste, tactile sensation of food in the mouth, and thoughts of food send nervous long reflexes to the medulla oblongata. These reflexes cause parasympathetic neurons via the vagus nerves to stimulate secretion of hydrochloric acid and pepsin in the stomach. The parasympathetic stimulation also results in secretion of gastrin from the lower part of the stomach. This hormone travels through the bloodstream and further stimulates hydrochloric acid and pepsin secretion in the upper and middle parts of the stomach. Fig 21-13 still: once food enters the stomach, stimuli in the gastric lumen initiate a series of short reflexes that constitute the gastric phase of digestion. In gastric phase reflexes, distension of the stomach and the presence of peptides or amino acids in the lumen send sensory input to the enteric plexus that activate endocrine cells and enteric neurons. Hormones, neurocrine secretions, and paracrine molecules then influence effector cells to influence secretion and motility. In the gastric phase, food has entered and distended the stomach. This distension activates a parasympathetic reflex via the medulla oblongata, and also has a direct stimulatory effect on the gastric glands. The result is continued secretion of hydrochloric acid and pepsin. In the intestinal phase of gastric secretion, chyme has entered the duodenum, so gastric secretion is no longer needed. When the chyme contains lipids from the digestion of fats or contains enough hydrocholoric acid to bring its pH below 2, gastric secretion is inhibition. Salivary glands -exocrine glands: dump saliva OUTSIDE of the body into the lumen of the digestive tract -secrete saliva: water, mucus, enzymes; lubrication and dissolving of nutrients -not connected to the vagus nerve Secretion and Fluid balance in/out of digestive system Fig 21-1 9 L of body fluid that needs to be reabsorbed into the intestine (7 L of that fluid is made by the body, and 2 L of it is ingested) -compare Fig 20-2 and Fig 20-1 -Fig 21-1 is the fluid input into the digestive system (9 L total input into the lumen) must equal the fluid output from the lumen [secretion into to the digestive system, and absorption and excretion out of the digestive system] -Fig 20-2 is the water gain (2.2 L/day) + metabolic production (0.3 L/day) – output (2.5 L/day) = 0 are must be in balance Next stop for food: the stomach—now entering the gastric phase -Food mixed with acid, digestive enzymes released in the stomach (sensory laden muscular bag) -Mixture is call “chyme” (pronounced “kime”) -Rugae: surface folding increases area -stomach is being monitored by sensors, and when ready, chyme is released -motility: material being pushed downstream by coordinate muscular movements in the system -Both the esophagus and pyloric valve have valves associated with them +esophagus doesn’t let food go back up +pyloric valve doesn’t let the food go into the small intestine too early In the stomach, surface area is increased by invaginations called gastric glands -Mucosa: lines the lumen and directly in contact with the chyme layer/lumen of the stomach; lined with an epithelium and have arteries and veins that send branches up into the pockets; capillary beds at all the terminations of the folds; lymph vessels send connections to the mucosa -different types of muscles: oblique, circular, longitudinal Gastric mucosa: outermost layer of the stomach wall -has many different types of sensory receptors (distension or peptides and amino acids initiate short reflexes)—information is sent to the enteric plexus (enteric nervous system) +can sense if the stomach is full or not and send input to the enteric nervous system Fig 21-15 GASTRIN is the gastric peptide hormone selected for emphasis/example -Parietal cells secrete the HCl (hydrochloric acid) (how cells make HCL**on exam)—the parietal cells stimulus for release include acetylcholine, gastrin, and histamine; function of secretion include activating pepsin and killing bacteria -G cells secrete Gastrin (gastric peptide hormone selected for emphasis/example); Gastrin acts on the Parietal cells to secrete HCL(gastric acid) -Chief cells secrete pepsin(ogen), where it becomes activated in the lumen of the stomach (acidic HCL-adds H+ to it), which digest proteins. It was initially secreted as an inactive/safe form, until it becomes activated in the stomach. -Chief cells secrete gastric lipase, which digests fats -Mucus: lubricates the epithelial cells from the acidic conditions of the digestive tract; mucus lines the stomach to prevent the
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