White communicating branch of spinal nerve

The white communicating branch of spinal nerve (a.k.a. white ramus communicans, pl. white rami communicantes) is a neuronal structure which connects the anterior ramus of a spinal nerve with the autonomic sympathetic trunk and allows for the passage of pre-ganglionic sympathetic neurons. The white rami communicantes–along with the gray 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. The cell bodies of preganglionic sympathetic neurons reside in the lateral horns of the thoracolumbar regions of spinal cord. Their axons emerge though the anterior motor roots and reach the anterior rami of thoracolumbar (T1–L2) spinal nerves. From here, the preganglionic sympathetic neurons traverse through the white rami communicantes of anterior rami of T1–L2 spinal nerves to reach the paravertebral sympathetic trunk.

Many of the pre-ganglionic neurons synapse with the cell bodies of the post-ganglionic sympathetic neurons, within the ganglia of the sympathetic chain. Please note that the white communicating branches (a.k.a. white rami communicans) are absent above the T1 and below the L2 vertebral outflows. Therefore, when inside the sympathetic trunk, the preganglionic sympathetic neurons can take one of three courses:

 1-They could synapse with the ganglia at T1–L2 levels, or

2-Without synapsing, the axons could ascend or descend within the sympathetic trunk to synapse into a ganglion of a different level (for instance, at cervical or sacral levels), or

3-Without synapsing, the axons could travel through the–greater, lesser, least or lumbar–splanchnic nerves and synapse in a peripheral ganglion close to the visceral organ to be innervated

Unlike the white communicating branches, the gray communicating branches (a.k.a. gray rami communicantes) exist at all levels, including above T1 and below T12. Therefore, the post-ganglionic axons have the liberty of re-entering the anterior rami of ALL spinal nerves–cervical, thoracic, lumbar, sacral and coccygeal–via their gray communicating branches. From here onwards, 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.