Hirnnervs

Hirnnervs, or cranial nerves, are twelve paired nerves that arise directly from the brain and brainstem. They transmit sensory information and provide motor control to structures of the head and neck, with several fibers also carrying autonomic or special sensory functions. In humans, they are designated CN I through CN XII. The olfactory (CN I) and optic (CN II) nerves originate from the forebrain, while the remaining ten pairs emerge from the brainstem.

Functions and components: CN I olfactory—smell; sensory. CN II optic—vision; sensory. CN III oculomotor—controls most extraocular muscles, eyelid elevation, and pupil constriction; motor and parasympathetic. CN IV trochlear—controls the superior oblique muscle; motor. CN V trigeminal—facial sensation and muscles of mastication; both. CN VI abducens—lateral rectus muscle; motor. CN VII facial—facial expression; taste from the anterior two-thirds; lacrimal and salivary glands; both. CN VIII vestibulocochlear—hearing and balance; sensory. CN IX glossopharyngeal—taste and swallowing, parotid secretion; both. CN X vagus—parasympathetic innervation of thorax and abdomen, taste, and control of palate, pharynx, larynx; both. CN XI accessory—sternocleidomastoid and trapezius muscles; motor. CN XII hypoglossal—tongue movements; motor.

Exit routes: CNs I and II pass through the skull via the olfactory foramina and the optic canal; CN III–VI traverse the cavernous sinus and superior orbital fissure; CN VII and VIII pass through the internal auditory meatus; CN IX–X and XI exit via the jugular foramen; CN XII exits via the hypoglossal canal.

Clinical relevance: Deficits map to the affected nerve, aiding diagnosis; testing includes smell, visual tests, pupillary reflexes, facial movement, corneal reflex, hearing, tongue and throat functions, and gag reflex.