Eachspinal nerve receives a branch called agray ramus communicans (pl.:rami communicantes) from the adjacentparavertebral ganglion of thesympathetic trunk. The gray rami communicantes containpostganglionic nerve fibers of thesympathetic nervous system and are composed of largely unmyelinated neurons. This is in contrast to thewhite rami communicantes, in which heavily myelinated neurons give the rami their white appearance.^[1]

Preganglionic sympathetic fibers from theintermediolateral nucleus in thelateral grey column of the spinal cord are carried in thewhite ramus communicans to the paravertebral ganglia of the sympathetic trunk. Once the preganglionic nerve has traversed a white ramus communicans, it can do one of three things.

• The preganglionic neuron can synapse with a postganglionic sympathetic neuron in the sympathetic paravertebral ganglion at that level. From here, the postganglionic sympathetic neuron can travel back out the grey ramus communicans of that level to the mixed spinal nerve and onto the effector organ.
• The preganglionic neuron can travel superiorly or inferiorly to a sympathetic paravertebral ganglion of a higher or lower level where it can synapse with a postganglionic sympathetic neuron. From here, the postganglionic sympathetic neuron can travel back out the grey ramus communicans of that level to the mixed spinal nerve and on to an effector organ.
• The preganglionic neuron can pass through the paravertebral ganglion without synapsing, and therefore continue as a preganglionic nerve fiber (Splanchnic nerves) until it reaches a distant collateral ganglion anterior to the vertebral column (Prevertebral ganglia). Once inside the prevertebral ganglia, the individual neurons comprising the nerve synapse with their postganglionic neuron. The postganglionic nerve then proceeds to innervate its targets (pelvic visceral organs) It will generally be responsible for the innervation of the pelvicviscera.

Ganglionic influence can be specifically targeted to end organs as well as becoming part of a summating systemic response. If the response is the whole body response, as in sympathetic fight or flight, the signals are distributed to other spinal nerves by way of gray rami communicantes which serve as conduits between the spinal nerves.

Somecervical ganglia andthoracic ganglia send gray rami communicantes directly to the heart.^[2]

Thoracic ganglia send grey rami communicantes to their adjacent body wall. They supply blood vessels, sweat glands, and arrectores pilorum muscles.

Lumbar ganglia have grey rami communicantes that rejoin the appropriate spinal nerves to supply the abdominal wall and lower limbs.

Thepelvic ganglia form grey rami communicantes whose lateral (postganglionic) branches supply the pelvic wall and lower limb.

This article incorporates text in thepublic domain from the 20th edition ofGray's Anatomy(1918)

Wilson-Pauwels, Linda; Stewart, Patricia A.; Akesson, Elizabeth J. (January 1997).Autonomic Nerves. Canada:B. C. Decker, Inc. pp. 71–104.ISBN 978-1-55009-030-7.

• 
  Deep and superficial dissection of the lumbar plexus.
• 
  Thoracic portion of the sympathetic trunk.

• Atlas image: n3a6p1 at the University of Michigan Health System - "Autonomic Connections of the Spinal Cord"
• Overview and diagram

• Nerves of the torso
• Thoracic nerves

• Wikipedia articles incorporating text from the 20th edition of Gray's Anatomy (1918)