Na/na

GI Physiology Review

I. SPHINCTERS: Divides GI tract into functional segments; areas of high intraluminal P to prevent reflux of contents; controlled by CNS, “one-way” valves

|Sphincter |Muscle Type |Function |
|Upper Esophageal |Striated |Separates mouth/pharynx from esophagus|
|Lower Esophageal |Smooth |Separates esophagus from stomach |
|Pyloric |Smooth |Separates stomach from duodenum |
|Sphincter of Oddi |Smooth |Separates pancreatobiliary ducts from duodenum |
|Ileocecal |Smooth |Separates smallinstestine from large intestine |
|Internal Anal |Smooth |Control defecation |
|External Anal |Striated |Control defecation |

II. NERVES:
- SNS & PNS synapse on ENS plexuses (interneurons) which integrate the commands & puts them into action
- Neuralcontrol of gut is HIERARCHIC: GI stimuli ( enteric NS ( prevertebral ganglia ( PNS & SNS ( higher brain centers

A. PNS – generally excitatory to secretion & motor function of the gut; extrinsic innervation (long reflexes)
• VAGUS nerve: ~ 80% of vagal fibers are afferents (transmit sensory info from gut)
• PELVIC NERVES
B. SNS – celiac, superior, &inferior mesenteric; generally have an inhibitory effect on secretory & motor function in gut (paralysis of motor function w/ reduced splanchnic blood flow); up to ~50% of SNS fibers are afferents; extrinsic innervation (long reflexes)

C. ENTERIC NERVOUS SYSTEM: “little brain” in gut to coordinate & program GI function; embedded in wall of gut; long & short reflexes (loops) that respond toluminal stimuli

** ENS has both excitatory and inhibitory neurotransmitters that can be used to coordinate functions
• MYENTERIC (AUERBACH’S) PLEXUS: control of MOTOR activity in gut (smooth muscle function)
• SUBMUCOSAL (MEISSNER’S) PLEXUS: MUCOSAL functions (secretion & absorption; & regulates local blood flow)
- VAGOVAGAL REFLEX: moment to moment adjustmentsrequired for optimal digestive functions in upper GI tract
- Pain inhibits gastric emptying

III. GI ENDOCRINOLOGY
- APUD cells line GI, contain receptors that are “sensitive” to specific luminal stimuli (contents)
- Families of peptides: (1) bind to same receptor & (2) same 2nd messenger
|Hormone |Origin |Respond to|Action |
|Gastrin |G cells in antrum |Distension of stomach or presence of amino acids / |↑ acid secretion |
|(gastrin family) | |peptides in antrum |(effects similar to CCK) |
|Secretin|S cells in duodenum |Dudodenal acidity | ↑ bicarbonate secretion |
|(Secretin family) | | | |
|Cholecystokinin (CCK– |I cells in duodenum |Fat & protein digestion products induodenum |Contracts gall bladder |
|Gastrin family) | | |↑ enzyme secretion |
| | | |(similar effects to gastrin) |...