Digestion Lecture 27

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Smooth Muscle

-modulates luminal pressure/tension
-tissue is present in sheets, bundles or sheaths around tissues: cardiovascular system, respiratory system, digestive system, urinary system, reproductive system

Unitary SM cells

-Walls of the digestive tract, gall bladder, urinary bladder
-May not have direct contact with any motor neuron
-Autorhythmic: adjacent cells connect via gap junctions -> transmission of APs from one fiber to another
-Cell membranes adhere to one another -> transmission of force
-Slow synchronized "graded" contraction
-Entire sheet of muscle contracts - "functional syncytium"

diffuse junctions

SM have numerous varicosities that release neurotransmitter into --- in the general area of the SM fiber

Protein Digestion

beings in the stomach
-HCl denatures (unfolds) protein molecules
-HCl activates pepsinogen into pepsin, an enzyme that breaks peptide bonds between certain amino acids

gastric lipase

Fat digestion continues --- splits the triglycerides in milk fat, although this is most effective at pH 5-6 (infant stomach)

nutrients

Absorption of --- by the stomach: Water, electrolytes, some drugs & alcohol, fat content in the stomach slows the passage of alcohol to the intestine, where absorption is more rapid, alcohol is absorbed more slowly if taken with a meal

Cephalic Phase

--- - Anticipation
-Prepare the mouth and stomach for food
-Cerebral cortex - sight, smell, taste & thoughts of food stimulate the prasympathetic nervous system to stimulate
-> Salivation (facial and glossopharyngeal)
-> Gastric glands to secrete gastric juice (vagus)

Gastric Phase

--- Breakdown of Material

Neural influences

over stomach activity
-stretch receptors & chemoreceptors (pH) signal bolus entry
-vigorous peristalsis and gastric gland secretions
-chyme perioically released into the duodenum

endocrine influences

over stomach activity
-distention and presence of food in stomach cause G cells to secrete gastrin into the bloodstream; gastrin increases gastric gland secretions and motility, and causes pyloric sphincter relaxation

gastrin

Gastric emptying increased as bolus of food enters the stomach
-distention of the stomach and presence of undigested contents increase the secretion of --- hormone & vagal nerve impulses
-this stimulates contraction of the lower esophageal sphincter and the stomach, as well as relaxation of the pyloric sphincter for emptying

Intestinal Phase

--- controlled release of chyme
-entry of chyme into duodenum slows gatric activity and increases intestinal activity

Neural influences

distention of the duodenum and chemical contents of the chyme activate sympathetic nerves, which slow gastric activity (enterogastric reflex)

Endocrine Influences

distension of duodenum and contents of chyme trigger hormonal release from enteroendocrine cells in the duodenum

secretin

hormone decreases stomach secretions

cholecystokinin

CCK - decreases stomach emptying

Enterogastric reflex

regulates the amount of chyme released into the duodenum
-initiated by distension of duodenum & contents of the chyme
-sensory impulses sent to the medulla inhibit parasympathetic stimulation of the stomach
-they also cause cholecystokinin and secretin release from the duodenum and stimulate sympathetic impulses, both of which inhibit gastric empyting

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