control of GI tract

neural

extrinsic

intrinsic = enteric nervous system ("gut brain")

submucosal plexus (Meissner's plexus): controls secretion and epithelium function

myenteric plexus (Auerbach's plexus): controls movement of circular muscles and longitudinal muscles of GI tract

parasympathetic innervation stimulates digestive tract for activity (Ach = neurotransmitter)

function: coordinate movement of materials along the digestive tract

sympathetic innervation inhibits gastrointestinal activity

sensory neurons (sense stretch and pH change) which they communicate with enteric plexus, spinal cord, and CNS

control of movement

pacesetter cells (interstitial cells of Cajal)

smooth muscle cells

peristalsis

located: in myenteric plexus

cause oscillating depolarization and repolarization of membrane potential that brings transmembrane potential of SMCs close to threshold but doesn't directly cause them to contract

weak contraction

Ca2+/K+

spike potential

linked via gap junctions

slow Ca2+ channels open which initiates contraction

waves of muscular contractions

intestinal contractions

segmentation contractions: serve to mix chyme and expose it to pancreatic enzymes

peristaltic contractions: serve to propel the bolus forward from the small intestine to the large intestine

behind bolus: circular muscles contract, long. muscles relax

ahead of bolus: circular muscles relax, long. muscles contract

hormones

CCK (cholecystokinin)

trigger: arrival of lipids in the duodenum

functions

opens hepatopancreatic sphincter (Oddi) which causes release of bile into duodenum

contractions in walls of the gallbladder

inhibits gastric contraction (keeps the stomach from emptying)

secretin

function: release pancreatic and biliary bicarbonate

trigger: decreased pH and fatty acid arrival to duodenum

gastrin inhibiting peptide (GIP)

trigger: glucose, amino acids, fatty acids

function: simulate insulin release

gastrin

function: stimulate parietal cells to produce acid and stimulation of gastric motility