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Home , Myenteric plexus, Pancreatic juice, Submucosal plexus, S cell

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 control contraction/motility of the GI tract, and effector motor neurons of the 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

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): à ~ 10 inches à ~ 12 inches and holds ~ 1-2 L (full) up to ~ 3-4 L (distended) à first 10 inches of the à next 3 feet of small intestine (when smooth muscle tone is lost upon death, extends to 8 feet) 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, , Muscularis Externa, and Serosa

Mucosa à epithelium, lamina propria (areolar connective tissue), & 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 ); not found in esophagus (replaced by superficial layer known as )

Stomach Cells (little absorptive and little digestive capabilities; enteroendocrine activity):

Cell Type/Name Location Function Surface Mucous Cell Surface epithelium Secretes Cell Epithelium lining gastric pit Secretes mucus (exocrine) Epithelium of gastric gland Secretes HCl & (exocrine) Chief Cell Epithelium of gastric gland Secretes pepsinogen & gastric lipase (exocrine) Epithelium of gastric gland Secretes (endocrine) D Cell Epithelium of gastric gland Secretes hormone (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 & absorbs nutrients Surface epithelium Secretes mucus (exocrine) Epithelium of Secretes hormone (endocrine) CCK Cell Epithelium of intestinal gland Secretes hormone (endocrine) K Cell Epithelium of intestinal gland Secretes glucose-dependent insulinotropic peptide (endo) 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 (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 (containing NaHCO3 and digestive enzymes)

Digestive Enzymes (see Table 24.5)

Secreted Origin Function Salivary Salivary glands Chemical digestion of polysaccharides (starches & glycogen) Lingual lipase Lingual glands (tongue) Chemical digestion of lipids (triglycerides) * 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 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 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 (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 to release insulin