A cranial nerve root is the initial or proximal segment of one of the twelve pairs of cranial nerves leaving the central nervous system from the brain stem or the highest levels of the spinal cord. A spinal nerve root is the initial or proximal segment of one of the thirty-one pairs of spinal nerves leaving the central nervous system from the spinal cord. Each spinal nerve is formed by the union of a sensory dorsal root and a motor ventral root,[1] meaning that there are sixty-two dorsal/ventral root pairs, and therefore one hundred and twenty four nerve roots in total, each of which stem from a bundle of nerve rootlets (or root filaments).