At 31 places along the spinal cord the dorsal and ventral roots come together to form spinal nerves. Spinal nerves contain both sensory and motor fibers, as do most nerves. Spinal nerves are given numbers which indicate the portion of the vertebral column in which they arise.t 31 places along the spinal cord the dorsal and ventral roots come together to form spinal nerves. Spinal nerves contain both sensory and motor fibers, as do most nerves. Spinal nerves are given numbers which indicate the portion of the vertebral column in which they arise.