Autonomic Nervous System
  Pg 1: Overview | Pg 2: Divisions | Pg 3: Parasympathetic | Pg 4: Sympathetic | Pg 5: Dual Innervation
Sympathetic Nervous System

The first motor neuron in the sympathetic division is in the intermediolateral cell column. The IML lies in the lateral horn of the gray matter in the spinal cord, from C8, T1 to L4,5 spinal cord segments. It sends its axon (preganglionic fiber) out a ventral root to the spinal nerve and then down the ramus communicans to the sympathetic trunk where it either synapses on a 2nd motor neuron of the paravertebral (sympathetic) ganglia, or continues on through a splanchnic nerve to synapse on a 2nd motor neuron of the prevertebral (splanchnic) ganglia. It uses acetylcholine as its transmitter.

The second motor neuron is in the paravertebral (sympathetic trunk) ganglia, which include the superior cervical, middle cervical, and cervicothoracic (or stellate) ganglia of the upper end of the sympathetic trunk, and other paravertebral ganglia along the thoracic, lumbar, sacral, and coccygeal regions of the sympathetic trunk; or in the prevertebral ganglia, which lie adjacent to origins of abdominal arteries. It generally uses norepinephrine (sometimes epinephrine or acetylcholine), but is also sensitive to acetylcholine, monoamines, and peptides.

Note that the abdominal and pelvic viscera have a special set of sympathetic ganglia, the prevertebral ganglia, that lie adjacent to abdominal branches of the aorta. The prevertebral ganglia are connected by splanchnic nerves to the sympathetic trunk. The thoracic viscera use paravertebral ganglia, particularly the middle cervical, cervicothoracic and upper thoracic paravertebral ganglia for their source of postganglionic fibers. These fibers pass from the sympathetic trunk to the thoracic viscera in cardiac nerves (even those going to the respiratory organs).

The sympathetic division innervates all vascular smooth muscle (vasomotor function for blood pressure control and distribution control), all sweat glands (sudomotor function - core temperatue control) and all arrector pili smooth muscle of hair follicles (pilomotor function - temperature control). The postganglionic neurons reside mostly in the paravertebral ganglia of the sympathetic trunk and pass to their targets either via the dorsal or ventral rami of the spinal nerves. There is no parasympathetic innervation of these targets.


Copyright © 2006 - University of Pennsylvania School of Veterinary Medicine, All rights reserved.
Faculty: Dr. Richard Miselis, Dr. Peter Hand
Students: Natalie Fickel V'12, Ryan Donahue V'09, Cara Catanese V’08, Rebecca Wolf V'06, Keith Javic V'03, Ken Bixel V'99, Russel Howe-Smith V'97