Cranial nerves are the nerves that emerge directly from the brain, in contrast to spinal nerves. 10 of 12 of the cranial nerves originate in the brainstem, Cranial nerves relay information between the brain and parts of the body, primarily to and from regions of the head and neck. Spinal nerves emerge sequentially from the cord with the spinal nerve closest to the head emerging in the space above the first cervical vertebra. The cranial nerves, however, emerge from the nervous system above this level. Each cranial nerve is paired and is present on both sides, depending on definition in humans there are twelve or thirteen cranial nerves pairs, which are assigned Roman numerals I–XII, sometimes also including cranial nerve zero. The numbering of the nerves is based on the order in which they emerge from the brain. The terminal nerves, olfactory nerves and optic nerves emerge from the cerebrum or forebrain, and the ten pairs arise from the brainstem. Most typically, humans are considered to have pairs of cranial nerves. Cranial nerves are generally named according to their structure or function, for example, the olfactory nerve supplies smell, and the facial nerve supplies motor innervation to the face. The trigeminal nerve is named in accordance with its three components, and the nerve is named for its wandering course. Cranial nerves are numbered based on their position, when viewing the brain. If the brain is removed from the skull the nerves are typically visible in their numeric order. Cranial nerves have paths within and outside of the skull, the paths within the skull are called intracranial and the paths outside the skull are called extracranial. There are many holes in the skull called foramina by which the nerves can exit the skull, all cranial nerves are paired, which means that they occur on both the right and left sides of the body. The muscle, skin, or additional function supplied by a nerve on the side of the body as the side it originates from, is referred to an ipsilateral function. If the function is on the side to the origin of the nerve. The cell bodies of many of the neurons of most of the nerves are contained in one or more nuclei in the brainstem. These nuclei are important relative to cranial nerve dysfunction because damage to these nuclei such as from a stroke or trauma can mimic damage to one or more branches of a cranial nerve, the fibers of these cranial nerves exit the brainstem from these nuclei
View of the human brain from below showing the cranial nerves on an autopsy specimen
A case with unilateral hypoglossal nerve injury in branchial cyst surgery.
Various deviations of the eyes due to abnormal function of the targets of the cranial nerves