Gray ramus communicans
Each spinal nerve receives a branch called a gray ramus communicans from the adjacent paravertebral ganglion of the sympathetic trunk. The gray rami communicantes contain postganglionic nerve fibres of the sympathetic nervous system.
Preganglionic sympathetic fibres from the intermediolateral cell column (lateral grey horn) of the spinal cord are carried in the white rami communicantes to the paravertebral ganglia of the sympathetic trunk. At a paravertebral ganglion, preganglionic sympathetic fibres may either:
- Synapse in the paravertebral ganglion to postganglionic nerve fibres, or;
- Pass through the paravertebral ganglion without synapsing, and therefore continue as preganglionic nerve fibres since they are yet to synapse.
The gray rami communicantes are the branches responsible for carrying both these types of fibres to their target destinations. Postganglionic nerve fibres from the paravertebral ganglia are carried directly to their effector organs through spinal nerves, while preganglionic nerve fibres are carried to the prevertebral ganglia. At the prevertebral ganglia the preganglionic nerve fibres carried in the grey rami communicantes synapse to postganglionic nerve fibres responsible for innervation of the pelvic viscera.
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.
- Some Cervical ganglia and Thoracic ganglia send gray rami communicantes directly to the heart. 
- 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.
- The pelvic ganglia form grey rami communicantes whose lateral (postganglionic) branches supply the pelvic wall and lower limb.
- F. Netter, Autonomic Nervous System: Schema, Back and Spinal Cord, Plate 153
- gray+rami+communicantes at eMedicine Dictionary
- Atlas image: n3a6p1 at the University of Michigan Health System - "Autonomic Connections of the Spinal Cord"
- Overview and diagram