Functions and Regulation of GI Tract

Question 1

describe the fluid balance of the digestive system

Answer 1

• 2 L of food and drink
  
  • increased addition of saliva, bile, gastric secretions, etc
• 7.5 L absorbed from small intestine
• 1.4 L absorbed from large intestine
• 0.1 L excreted in feces

Question 2

describe the function of Meissner’s plexus vs Auerbach’s plexus

Answer 2

Meissner’s: stimulated endocrine cells to secrete hormones

Auerbach’s/myenteric: regulates local muscle/sphincter tone

Question 3

describe smooth muscle in the GI

Answer 3

• found in walls of hollow organs/tubes
• single unit, spindle shaped
• bundles of small cells electrically coupled via gap junctions
• force of contraction is a fxn of Ca entry (not the recruitment of more cells)
  
  • smooth muscle contracts when [Ca]_in = 10^-7 M

Question 4

describe smooth muscle contraction

Answer 4

• increase in intracellular Ca caused by channels or SR store
• Ca binds to calmodulin and forms complex
• Ca-calmodulin complex that activates MLCKs
• MLCK activates myosin and causes cross-bridging

Question 5

describe relaxation of smooth muscle

Answer 5

• MLCK is inactivated
• MLC phosphatase is activated
• myosin dephosphorylated
• muscle relaxes

Question 6

describe the location of tight vs leaky epithelia

Answer 6

• tight: junctions are high resistance
  
  • distal colon, distal renal tubule
  • significiant ionic gradients and transepithelial voltage
  • highy regulated
• leaky: junctions are low resistance
  
  • small intestine, prox. renal tubule, gallbladder and choroid plexus
  • higher water permeability
  • smaller ionic and voltage gradients

Question 7

name apical membrane transporters

Answer 7

• ENaC Na channels
• Na-nutrient transporters
• Na-Cl cotransporter
• Na-H antiport
• Cl-HCO3 antiport

Question 8

describe basolateral membrane transporters

Answer 8

• Na-K ATPase pump
• Cl channel
• K channel

Question 9

name the ions secreted in the GIT

Answer 9

• stomach: HCl secretion
• pancreas: bicarb
• duodenum: bicarb
• small intestine: NaCl

Question 10

describe the effect of PNS on the gut

Answer 10

• PNS: promotes secretion and motility via vagus nerve
  
  • Vagus: upper GIT
  • S2-S4 (pelvic nerves): Lower GIT
• trasmitters:
  
  • ACh: contracts GIT smooth muscle
  • Vasoactive Intestinal peptide (VIP): relaxes GIT smooth muscle
• sensory afferents: receptors detect nausea and distension
• vagovagal reflexes exist: adjust the upper GIT in fed states

Question 11

describe sympathetic innervation of GIT

Answer 11

• antagonizes PNS: vasoconstriction of blood vessels, contraction of sphincters
• thoracolumbar: preganglionic efferent fibers synpase on ganglions; post ganglionic fibers end mainly on ENS
• transmitters:
  
  • NE acts on presynaptic a2 receptors to inhibit ACh release from PS nerves
• sensory afferents: nausea, pain
• vagovagal reflexes exist: adjust the upper GIT in fed states

Question 12

describe the location of gut peptide secretions

Answer 12

Question 13

describe gastrin and CCK

Answer 13

Question 14

describe secretin and ghrelin

Answer 14

Question 15

describe motilin

Question 16

describe the incretin effect

Answer 16

• The total amount of insulin secreted is greater when glucose is administered orally than when administered intravenously
  
  • this enhanced response to oral glucose is an indication that the presence of glucose in the GI tract stimulates hormonal release and other effects that enhance the sensitivity of the beta cell to glucose

Question 17

describe the function of GIP and GLP-1

Answer 17