Spinal nerves divide in the vertebral canal into two branches: the dorsal root and the ventral root. The dorsal root, which is the posterior branch, contains the axons of sensory neurons that bring information to the spinal cord. The ventral root, which is the anterior branch, contains the axons of motor neurons that carry commands to muscles or glands. Therefore, each spinal nerve is considered a mixed nerve with both sensory and motor neurons.
Anatomy of spinal nerves: 1 = Anterior cutaneous branches; 2 = Ventral ramus; 3 = Sympathetic ganglion; 4 = White ramus; 5 = Dorsal ramus; 6 = Spinal cord; 7 = Sympathetic nerve; 8 = Gray ramus; and 9 = Lateral cutaneous branches. (From Stedman’s Medical Dictionary. 27th Ed. Baltimore: Lippincott, Williams & Wilkins, 2000.)