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Enteric Nervous System (ENS): 1) Myenteric (Auerbach) Plexus & 2

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Enteric Nervous System (ENS): 1) Myenteric (Auerbach) Plexus & 2 Enteric Nervous System (ENS): 1) Myenteric (Auerbach) plexus & 2) Submucosal (Meissner’s) plexus à both triggered by sensory neurons with chemo- and mechanoreceptors in the mucosal epithelium; effector motors neurons of the myenteric plexus control contraction/motility of the GI tract, and effector motor neurons of the submucosal plexus control secretion of GI mucosa & organs. Although ENS neurons can function independently, they are subject to regulation by ANS. Autonomic Nervous System (ANS): 1) parasympathetic (rest & digest) – can innervate the GI tract and form connections with ENS neurons that promote motility and secretion, enhancing/speeding up the process of digestion 2) sympathetic (fight or flight) – can innervate the GI tract and inhibit motility & secretion by inhibiting neurons of the ENS Sections and dimensions of the GI tract (alimentary canal): Esophagus à ~ 10 inches Stomach à ~ 12 inches and holds ~ 1-2 L (full) up to ~ 3-4 L (distended) Duodenum à first 10 inches of the small intestine Jejunum à next 3 feet of small intestine (when smooth muscle tone is lost upon death, extends to 8 feet) Ileum à final 6 feet of small intestine (when smooth muscle tone is lost upon death, extends to 12 feet) Large intestine à 5 feet General Histology of the GI Tract: 4 layers – Mucosa, Submucosa, Muscularis Externa, and Serosa Mucosa à epithelium, lamina propria (areolar connective tissue), & muscularis mucosae Submucosa à areolar connective tissue Muscularis externa à skeletal muscle (in select parts of the tract); smooth muscle (at least 2 layers – inner layer of circular muscle and outer layer of longitudinal muscle; stomach has a third layer of oblique muscle under the circular layer) Serosa à superficial layer made of areolar connective tissue and simple squamous epithelium (a.k.a. visceral peritoneum); not found in esophagus (replaced by superficial layer known as Adventitia) Stomach Cells (little absorptive and little digestive capabilities; enteroendocrine activity): Cell Type/Name Location Function Surface Mucous Cell Surface epithelium Secretes mucus Neck Cell Epithelium lining gastric pit Secretes mucus (exocrine) Parietal Cell Epithelium of gastric gland Secretes HCl & intrinsic factor (exocrine) Chief Cell Epithelium of gastric gland Secretes pepsinogen & gastric lipase (exocrine) G Cell Epithelium of gastric gland Secretes hormone gastrin (endocrine) D Cell Epithelium of gastric gland Secretes hormone somatostatin (endocrine) Small Intestine Cells (high absorptive and high chemical/enzymatic digestive capabilities; enteroendocrine activity): Cell Type/Name Location Function Absorptive Cell Surface epithelium Brush-border digestive enzymes & absorbs nutrients Goblet Cell Surface epithelium Secretes mucus (exocrine) S Cell Epithelium of intestinal gland Secretes hormone secretin (endocrine) CCK Cell Epithelium of intestinal gland Secretes hormone cholecystokinin (endocrine) K Cell Epithelium of intestinal gland Secretes glucose-dependent insulinotropic peptide (endo) Paneth Cell Epithelium of intestinal gland Secretes lysozyme (bactericidal activity) Large Intestine Cells* (moderate absorptive capabilities; no enteroendocrine activity): Cell Type/Name Location Function Absorptive Cell Surface epithelium Absorbs primarily water; no enzymes secreted & Epithelium of intestinal gland Goblet Cell Epithelium of intestinal gland Secretes mucus (exocrine) * - Chemical digestion (fermentation and amino acid conversion) is carried out by bacteria that inhabit the lumen; bacteria also synthesize vitamins (B’s and K) that are absorbed Liver Cells (form clusters of exocrine glandular epithelium called acini) Cell Type/Name Location Function Hepatocyte Hepatic laminae Secretes bile (exocrine) that is stored in the gallbladder Kupffer Cell* Hepatic sinusoid Phagocytic breakdown of worn out blood cells & bacteria * - also go by the name of hepatic macrophages or stellate reticuloendothelial cells; breakdown hemoglobin from RBCs into heme and globin components à globin is broken into amino acids that are recycled for use in new protein synthesis, and heme is further processed to remove Fe (for storage in the liver and eventual transfer to red bone marrow as new RBCs are generated during erythropoiesis) from heme ring which is then converted to bilirubin (bile component) and ultimately converted from bilirubin to stercobilin by bacteria in the large intestines (gives feces its pigmentation). Pancreas Cells (form clusters of exocrine glandular epithelium called acini) Cell Type/Name Location Function Pancreatic Cell Pancreatic acini Secretes pancreatic juice (containing NaHCO3 and digestive enzymes) Digestive Enzymes (see Table 24.5) Secreted Enzyme Origin Function Salivary amylase Salivary glands Chemical digestion of polysaccharides (starches & glycogen) Lingual lipase Lingual glands (tongue) Chemical digestion of lipids (triglycerides) Pepsin* Gastric glands (chief cells) Chemical digestion of protein (* - pepsinogen becomes pepsin in acid) Gastric Lipase Gastric glands (chief cells) Chemical digestion of lipids (triglycerides) Pancreatic amylase Pancreatic acini Chemical digestion of polysaccharides (starches & glycogen) Trypsin Pancreatic acini Chemical digestion of proteins Chymotrypsin Pancreatic acini Chemical digestion of proteins Elastase Pancreatic acini Chemical digestion of proteins Carboxypeptidase Pancreatic acini Chemical digestion of peptides Pancreatic lipase Pancreatic acini Chemical digestion of lipids (triglycerides) Nucleases Pancreatic acini Chemical digestion of nucleic acids (RNA & DNA) Brush-border Enzyme Origin Function α-Dextrinase Small intestine Chemical digestion of starch & glycogen fragments Maltase Small intestine Chemical digestion of maltose Sucrase Small intestine Chemical digestion of sucrose Lactase Small intestine Chemical digestion of lactose Enterokinase Small intestine Chemical activation of trypsinogen to trypsin Aminopeptidase Small intestine Chemical digestion of peptides Dipeptidase Small intestine Chemical digestion of dipeptides Nucleosidase Small intestine Chemical digestion of nucleotides Phosphatase Small intestine Chemical digestion of nucleotides Digestive Hormones (see Table 24.8) Hormone Origin Function Gastrin G cells (stomach) Promotes gastric juice secretion Somatostatin D cells (stomach) Inhibits release of gastrin Secretin S cells (small intestine) Promotes pancreatic juice secretion Cholecystokinin (CCK) CCK cells (s. intestine) Promotes pancreatic juice secretion & bile release Glucose-dependent Insulinotropic Peptide (GIP) K cells (s. intestine) Stimulates beta cells of pancreas to release insulin .
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