Gray communicating branch of spinal nerve

The gray communicating branch of spinal nerve (a.k.a. gray ramus communicans, pl. gray rami communicantes) is a neuronal structure which connects the autonomic sympathetic trunk with the anterior ramus of a spinal nerve and allows for the passage of post-ganglionic sympathetic neurons. The gray rami communicantes–along with the white communicating branches–offer organization for linking the sympathetic autonomic nervous system with the sensorimotor outflow tracts of spinal nerves.

The sympathetic motor pathways, as you know, comprise of preganglionic and postganglionic neurons. Many of the pre-ganglionic neurons synapse with the cell bodies of the post-ganglionic sympathetic neurons, within the ganglia of the sympathetic chain. Even though, the white rami communicans are absent above the T1 and below the L2 vertebral outflows, the gray communicating branches exist at all levels. This provides the axons of post-ganglionic sympathetic neurons, the liberty of re-entering the anterior rami of ALL spinal nerves–cervical, thoracic, lumbar, sacral and coccygeal–via their gray communicating branches. Once reaching back the anterior rami, the post-ganglionic sympathetic neurons innervate various parts of the body, as they travel through the posterior or the anterior ramus of each spinal nerve.

The communicating branches allow the autonomic sympathetic motor neurons to channelize through the spinal nerves and control visceral motor tone in smooth muscles, cardiac tissue and glands in different body regions.