Exam 3 Review:  Chapter 23:  Stomach Histology & Physiology

gastric pits - The numerous small indentations in the mucous membrane of the stomach which are the mouths of the gastric glands.

gastric glands - Branched tubular glands located in the mucosa of the stomach wall; they contain parietal cells which secrete HCL and intrinsic factor, chief cells which secrete pepsin(ogen) and mucous neck cells which secrete mucin; their secretion is regulated by gastrin release from G cells in the stomach and by gastric inhibitory peptide = GIP secreted by enteroendocrine cells from the mucosal epithelial cells of the duodenum and by parasympathetic autonomic motor impulses.

--lumen---gastric pits--------mucous neck cells------parietal cells are\\\///chief cells are\\\///---------------muscularis mucosae is -------------------------------------------------------large and pale pink///\\\small and dark purple///\\\------the thin vertical pink band

mucous neck cells - The secretory cells which produce an acidic solution containing the protein mucin; they are located primarily in the upper, more superficial portions of the gastric glands; they are believed to be the stem cells for all the other cells in the mucosal glands of the stomach.

chief cells - One of the two glandular epithelial cell types located in the basal half of gastric glands; they synthesize and secrete pepsinogen, the inactive precursor to the proteolytic enzyme pepsin; they appear smaller and with a more darkly staining granular cytoplasm than the neighboring parietal cells.

parietal cells - One of the two glandular epithelial cell types located in the basal half of gastric glands; they synthesize and secrete HCl and intrinsic factor, which is necessary for vitamin B₁₂ absorption in the small intestine; they appear larger and with a pale staining cytoplasm than the neighboring chief cells.

CC = Chief Cell    and     PC = Parietal Cell

gastric juice - The clear, viscous, strongly acid secretion of the gastric glands, consisting of enzymes (pepsin(ogen) in adults, and, in neonates, additionally, rennin and gastric lipase) produced by the chief cells, HCL and intrinsic factor produced by the parietal cells, and an acidic mucus fluid secreted by the mucous neck cells; it continues the chemical digestion of food already acted on by saliva.

pepsinogen - The inactive precursor to the proteolytic enzyme pepsin, synthesized and secreted by the chief cells of the gastric glands of the stomach; it is converted to pepsin in the lumen of the stomach by autocatalysis in the presence of gastric hydrochloric acid.

pepsin - The active proteolytic enzyme, synthesized as an inactive precursor precursor, pepsinogen, and secreted by the chief cells of the gastric glands of the stomach; it is activated in the lumen of the stomach by autocatalysis in the presence of gastric hydrochloric acid; it catalyzes the hydrolysis of most large proteins into somewhat smaller units (peptones).

gastric lipase - The enzyme present in gastric juice which catalyzes the hydrolysis of neutral fats (mono-, di-, and triglycerides) into glycerol and free fatty acids in the stomach; it is synthesized and secreted by the chief cells of the gastric glands of the stomach of neonates to contribute to the digestion of milk's butterfat; synthesis is regulated off soon after the infant is weaned.

mucin - Any of a group of fibrous glycoproteins found in the secretions of mucous membranes, and in the matrix of some connective tissues; when mixed with the water of glandular secretions it forms the major constituent of mucus and gives it its peculiar ropy character and its viscosity; it serves to lubricate mucous membranes; it is also sticky and helps trap microorganisms.

hydrochloric acid = HCl - A solution of hydrogen chloride gas in water; it is a highly corrosive, strong mineral (inorganic) acid which is secreted by the parietal cells of gastric glands; it creates a low pH environment in the stomach which serves a protective function by destroying most microorgaisms, it contributes in a minor way to chemical digestion of foods with a general hydrolysis of chemical bonds, and it activates the conversion of pepsinogen to pepsin.

intrinsic factor - A transport glycoprotein which is secreted by the parietal cells  of the gastric glands of the mucous membrane of the stomach and is essential for the absorption of vitamin B₁₂ in the intestines; hereditary failure to secrete it is a major cause of pernicious anemia.

rennin - An enzyme, synthesized and secreted by the chief cells of the gastric glands of the stomach of neonates which catalyzes the coagulation of milk; synthesis is regulated off soon after the infant is weaned.  [Note:  it is also obtained from the gastric juice of the fourth stomach of young ruminants and used in making cheeses and junkets (where it is called rennet); a similar microbial rennin enzyme is also used in cheese production.]

G cells - Enteroendocrine cells, located in the gastric glands in the mucous membrane of the stomach, which synthesize and secrete the digestive hormone gastrin, a peptide hormone; they secrete gastrin in response to gastric distension or to a rise in gastric pH of the stomach, which then stimulates the production of HCl by parietal cells; it also promotes gastric mucosal growth.

G cells stained brown with special stain	G cell (enteroendocrine cell) stained dark red under very high power

cephalic phase - The first stage of regulation of digestive system activity, particularly in terms of the activities of the stomach, duodenum, pancreas, and gall bladder; sensory information about food (visual, olfactory, gustatory = taste) create conditioned reflexes that trigger some parasympathetic output to the stomach; other stimuli include the actions of chewing and swallowing (mastication and deglutition) and the recognition of hypoglycemia by the hypothalamus; digestive system responses occur within minutes but are temporary if no food actually enters the stomach; during this period small amounts of gastric secretions are produced, particularly H⁺ from the parietal cells; this stage is mediated by parasympathetic autonomic motor impulses and may involve some secretion of gastrin from G cells.

gastric phase - The second stage of regulation of digestive system activity, particularly in terms of the activities of the stomach, duodenum, pancreas, and gall bladder; visceral sensory stimulation of mechanoreceptors caused by distension of the stomach and detection of protein digestion in the bolus (the presence of peptones, peptides, and amino acids); during this period large amounts of gastric secretions are produced; it is mediated by parasympathetic autonomic motor impulses and by gastrin release from G cells and secretin release from enteroendocrine cells of the mucosal epithelium cells of the duodenum.

intestinal phase - The third stage of regulation of digestive system activity, particularly in terms of the activities of the stomach, duodenum, pancreas, and gall bladder; this period is directly related to control of gastric emptying; during this period negative feedback inhibition of further gastric secretions occur; the presence of chyme in the duodenum brings about neural and endocrine responses which first stimulate and later inhibit gastric acid secretion by the stomach; when gastric chyme is at a pH above 3, during early gastric emptying, gastric acid is stimulated; later, when the buffering capacity of gastric chyme is exhausted and the pH of chyme falls below a pH of 2, gastric secretion is inhibited; during this third stage, gastric secretion is first stimulated by the presence of peptides and amino acids in the distended duodenum; G cells in the duodenum and upper jejunum release gastrin in response to peptides and amino acids; this gastrin is released into the blood and can stimulate parietal cells of the stomach to secrete gastric acid; later in this third stage, the enterogastric reflex takes effect, during which, sensory signals from distension of the small intestine and the detection of gastric HCl in the duodenal chyme inhibit gastric motility and gastric secretion; it is mediated by parasympathetic autonomic motor impulses and by gastrin release from G cells and by gastric inhibitory peptide = GIP and secretin release from enteroendocrine cells of the mucosal epithelium cells of the duodenum.

 3. the three phases of digestion in the stomach (Can also be diagramed).  Describe the neural and hormonal mechanisms involved in each stage, including the stimuli for each.

Phase of Gastric Digestion Regulation cephalic phase Neuronal:  Sensations of smell and taste or even thoughts or memories of food stimulate cortical centers which activate parasympathetic impulses delivered by the Vagus nerve (X) to initiate gastric secretion.  Hormonal:  none [Certain negative emotional states will decrease parasympathetic impulses and, therefore, gastric secretion.] gastric phase Neuronal:  Ingestion and deglutition (swallowing) of food expand the stomach.  Proprioceptive stretch receptors in the stomach wall trigger parasympathetic autonomic reflexes which stimulate both gastric secretion and gastric motility (churning). Hormonal:  G cells (enteroendocrine cells) respond to sensory information indicating the presence of nutrients, especially peptides, or to drugs (e.g., caffeine), or to a rising pH (due to gastric acid dilution by food or beverage) with gastrin release which stimulates gastric secretion. [(1)  Excessive gastric acidity (pH < 2) or certain negative emotional states will decrease gastrin release and parasympathetic impulses and, therefore, gastric secretion and motility. (2)  Proprioceptive stretch receptors in the intestinal wall trigger sympathetic autonomic reflexes which inhibit both gastric secretion and gastric motility (churning) while encouraging gastric emptying.] intestinal phase Neuronal:  As chyme is passed intermittently to the duodenum, the duodunum stretches, triggering proprioceptive stretch receptors in the intestinal wall.  These impulses trigger parasympathetic autonomic reflexes which reduce gastric secretion and regulate gastric emptying. Hormonal:  Distension of the duodenum and detection by chemoreceptors of acidic hypertonic chyme containing digested fatty acids and amino acids triggers release of intestinal enteroendocrine substances (cholecystokinin (CCK), secretin, gastric inhibitory peptide (GIP), and vasoactive intestinal peptide (VIP) which reduce gastric secretion and regulate gastric emptying. [Note:  these same intestinal enteroendocrine substances (CCK, secretin, GIP, and VIP) also stimulate the liver, gall bladder, pancreases and intestinal glands and smooth muscle to coordinate digestive activities in the small intestine.]