Anterior ramus of spinal nerve

Anterior ramus of spinal nerve is one of the branches of the spinal nerve. While exiting the vertebral canal through the intervertebral foramen, each spinal nerve splits up into anterior and posterior rami. Since, the spinal nerve is a mixed fascicle comprising of motor and sensory neurons, therefore, the anterior ramus of spinal nerve also contains both motor as well as sensory neurons. The neurons passing through the anterior ramus contribute to all major neural plexuses–cervical, brachial, lumbosacral–and are responsible for majority of body’s sensorimotor innervation.

The anterior rami of spinal nerves are connected with the autonomic sympathetic trunk through their white and gray communicating branches, which link the 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 communicating branches of anterior rami of T1-L2 spinal nerves to reach the paravertebral sympathetic trunk, where many synapse with the cell bodies of the post-ganglionic sympathetic neurons.

Please note that even though the white rami communicating branches (a.k.a. white rami communicans) are absent above the T1 and below the L2 vertebral outflows; the gray communicating branches exist in ALL the regions. 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.