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## BRS Physiology ### Structure of the gastrointestinal tract - Epithelial cells, endocrine cells, and receptor cells - Lamina propria - Muscularis mucosae - Circular muscle - Longitudinal muscle - Serosa ### **Sympathetic nervous system** - Is usually inhibitory on the functions of the GI tract....

## BRS Physiology ### Structure of the gastrointestinal tract - Epithelial cells, endocrine cells, and receptor cells - Lamina propria - Muscularis mucosae - Circular muscle - Longitudinal muscle - Serosa ### **Sympathetic nervous system** - Is usually inhibitory on the functions of the GI tract. - Fibers originate in the spinal cord between T8 and L2. - Preganglionic sympathetic cholinergic fibers synapse in the prevertebral ganglia. - Postganglionic sympathetic adrenergic fibers leave the prevertbral ganglia and synapse in the myenteric and submucosal plexuses. - Direct postganglionic adrenergic innervation of blood vessels and some smooth muscle cells also occurs. - Cell bodies in the ganglia of the plexuses then send info to the smooth muscle, secretory cells, and endocrine cells of the GI tract. ### **Intrinsic innervation (enteric nervous system)** - Coordinates and relays information within the GI tract. - Uses *local reflexes* to relay information within the GI tract. - Controls most functions of the GI tract, especially motility and secretion, even in the absence of extrinsic innervation. #### **Myenteric plexus (Auerbach plexus)** - Primarily controls the motility of the GI smooth muscle. #### **Submucosal plexus (Meissner plexus)** - Primarily controls secretion and blood flow. - Receives sensory information from chemoreceptors and mechanoreceptors in the GI tract. ## **Regulatory substances in the gastrointestinal tract** ### **GI hormones** - Are released from endocrine cells in the GI mucosa into the portal circulation, enter the general circulation, and have physiologic actions on target cells. - Four substances meet the requirements to be considered "official" GI hormones: - Gastrin - Cholecystokinin (CCK) - Secretin - Glucose-dependent insulinotropic peptide (GIP) ### **Gastrin** - Contains 17 amino acids ("little gastrin"). - Little gastrin is the form secreted in response to a meal. - All of the biologic activity of gastrin resides in the four C-terminal amino acids. - "Big gastrin" contains 34 amino acids, although it is not a dimer of little gastrin.