Cranial nerves

The cranial nerves are twelve pairs of nerves that emerge from the underside of the brain, the organ that fills the skull. They mainly serve the head and neck.

They do not arise where other nerves do. Spinal nerves arise from the spinal cord. Cranial nerves, by contrast, are tied to structures in the cortex and to nuclei in the brainstem, which are clusters of neurons in the part of the brain that connects to the spinal cord.

Based on function, the traditional classification sorts them into three groups, and it helps to think of streets: some run one way toward the brain, others run one way outward, and others carry two-way traffic. Nerves I, II and VIII are sensory and only bring information in: from smell, from the retina, which is the layer of the eye that receives light, and from the inner ear. Nerves III, IV, VI, XI and XII are motor: they only carry movement commands out to the neck, the tongue and the orbit, which is the socket that holds the eye. And nerves V, VII, IX and X, that is, the trigeminal, facial, glossopharyngeal and vagus nerves, are mixed: they do both.