GI system

• Most of the GI tract has a distinct structure made up of four basic types of tissue layers surrounding the lumen of the tract
• Mucosa is the inner lining of the digestive tract, contains cells that absorbs nutrients and is one of the main 4 tissues surrounding the lumen
• Epithelium has cels that create barrier in the small intestine and aids in absorption
• Lamina propria contains connective tissue with small blood vessels, nerve cells, and exocrine and endocrine cells
• Muscularis mucosa is smooth muscle
• Submucosa is a portion of the interic nervous system. Contains Meissner's (submucosal) plexus which is a network of nerve fibers that sense pressure and stretch and has larger blood vessels
• Muscularis externa is circular and longitudinal muscles
• Auerback's (myenteric) plexus: between the 2 muscles, receives nervous system signals...both para and sympathetic...fight or flight
• Serosa seperates tissue layers and the surroundings
• Intrinsic Enteric Nervous System (ENS) is embedded in the submucosa and muscularis externa.
• ENS contains cholinergic neurons that are stimulated by acetyl choline that regulate GI motility/secretions
• ENS is a receiver and transmitter of brain-gut communication. It contains the sympathetic and parasympathetic nervous system
• Afferent Signaling (gut-brain): regulates food intake, controlled by norepinephrine, from the sympathetic nervous system
• Efferent Signaling (brain-gut): facilitates digestion, controlled by Acetylcholine released by the vagal nerve, from the parasympathetic nervous system
• Lower esophageal sphincter: opens with acetylcholine contractions
• GERD is caused by weakening of LES
• Stomach: gastric mucosa has pits which are entrances to gastric glands
• Gastric glands are lined with various cell types that secrete "gastric juice"
• Surface Mucous Cell: produces mucus that protects the lining of the stomach
• Mucous Neck Cell: secretes mucus that protects the lining of the stomach
• Parietal Cell: secretes hydrochloric acid and intrinsic factor, part of chemical digestion
• Chief Cell: secretes pepsinogen and gastric lipase
• Pepsinogen: inactive enzymes, once active, breaks down proteins
• Gastric lipase: enzyme for lipids
• G Cell: secretes the hormone gastrin
• In stomach, mainly protein is digested
• Enteroendocrine cells (EECs) secrete hormones that regulate HCl secretion
• Gastrin hormone (in stomach) stimulates HCl secretion by stimulating histamine release
• Somatostatin inhibits HCl secretion by acting directly upon parietal cells, and inhibiting the release of gastrin and histamine
• Oblique Muscle Layer of stomach helps to exposure food to all cells
• Gastrin and cholecystokinin (CCK) compete so that chyme is released slowly
• CCK is released by the small intestine and inhibits gastric emptying
• Most of the digestion and uptake of nutrients occurs in the small intestine
• the SI is divided into 3 sections: duodenum, jejunum, and Ileum
• Secretin (hormone) is secreted by cells in the duodenum
• Secretin stimulates pancreas to flood lumen with bicarbonate. Which neutralizes the acid from stomach
• Pancrease is not releasing bicarbonate
• Most nutrients are absorbed in the Jejunum
• Duodenum absorbs Fe and Ca. Ileum absorbs B12 and bile acids.
• Enterocytes are present in the SI to absorb nutrients