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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:
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.