Learning Modules - Medical Gross Anatomy
Introduction to Autonomics, Part 2 - Page 4 of 12


Presynaptic sympathetic fibers reach the prevertebral ganglia in the abdomen by way of splanchnic nerves: the greater, lesser, least thoracic splanchnic nerves and the lumbar splanchnic nerves. Sacral splanchnic nerves, predominantly postsynaptic, enter pelvic autonomic plexi to reach pelvic viscera. You will encounter the words "splanchnic" or "visceral" in many contexts. "Splanch"is a Greek word meaning "organ" in English or "viscera" in Latin. The thoracic and lumbar splanchnic nerves enter the prevertebral ganglia, located on the major branches of the abdominal aorta: the celiac, aorticorenal, superior mesenteric and inferior mesenteric ganglia. Within these ganglia, all the presynaptic sympathetic fibers synapse (except those going to the suprarenal gland). Some fibers from these splanchnic nerves may enter a more superior ganglion and pass through it to a more inferior ganglion before synapsing. The postsynaptic sympathetic fibers leaving these ganglia form perivascular plexuses along the branches of the aorta and ride these vessels to their destinations in abdominal organs. Sympathetic innervation to the digestive tract decreases peristalsis and blood flow to allow other structures to receive more blood, as well as contracts the internal anal sphincter to prevent defecation. Sympathetic innervation to the liver and gallbladder promotes the breakdown of stored glycogen to provide more energy to the body. Sympathetic innervation to the urinary tract results in slowed urine production and contraction of the internal sphincter of the bladder to prevent urination.

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