What Are the Major Cranial Nerves?
Twelve pairs of cranial nerves branch off the brain or brainstem and innervate many different places in the body. Some of them have motor functions, some have sensory functions, and a few have both. Here are the cranial nerves in numerical order:
CN I: Olfactory nerve: Located in the olfactory epithelium, this nerve leaves the cranium at the foramina in the cribiform plate of the ethmoid bone and enters the olfactory bulb. Fibers in the olfactory tract convey sensory impulses to the cerebral cortex. It’s a sensory nerve responsible for the sense of smell.
CN II: Optic nerve: This nerve starts at the retina and passes through the optic canal and forms the optic chiasm. In the optic chiasm, the fibers from the medial part of the retina cross to the optic tract of the opposite side. The fibers from the lateral portion of the retina travel to the optic tract of the same side. This crossing is necessary for binocular vision. The majority of fibers in the optic tract synapse with neurons in the lateral geniculate nucleus of the thalamus. The axons of cells from the lateral geniculate nucleus travel to the visual cortex of the cerebral hemispheres. CN II is a sensory nerve necessary for vision.
CN III: Oculomotor nerve: Starting in the midbrain and leaving the cranium at the superior orbital fissure, this motor nerve innervates the levator palpebrae superioris, the muscle that raises the eyelid. It also innervates the superior rectus, medial rectus, inferior rectus, and inferior oblique, the muscles that turn the eyeball superiorly, medially, and inferiorly. This nerve also provides the parasympathetic input that constricts the pupil and affects the lens.
CN IV: Trochlear nerve: This motor nerve leaves the posterior side of the midbrain and exits with the oculomotor nerve at the superior orbital fissure. It innervates the superior oblique muscle.
CN V: Trigeminal nerve: This large nerve emerges from the pons and contains both a sensory and motor root. The cell bodies of the sensory neurons are located in the large trigeminal ganglion (grouping of nerve cells). The sensory fibers from the trigeminal ganglion form three nerves that provide the primary sensory innervation of the head. The motor fibers are distributed with the mandibular nerve:
CN V1: The ophthalmic nerve is a sensory nerve that emerges from the trigeminal ganglion and leaves the cranium at the superior orbital fissure. It transmits sensory impulses from the cornea, skin of the forehead, and scalp, eyelids, nose, nasal cavity, and paranasal sinuses.
CN V2: The maxillary nerve is a sensory nerve that also emerges from the trigeminal ganglion and exits the cranium at the foramen rotundum. It receives sensory input from the skin over the maxilla, upper lip, maxillary teeth, nasal mucosa, maxillary sinuses, and palate.
CN V3: The mandibular nerve has both sensory and motor fibers. The sensory fibers exit the trigeminal ganglion, and the motor fibers branch off the pons. All fibers leave the cranium through the foramen ovale. It receives sensory input from the skin over the chin, lower lip, side of the head, mandibular teeth, temporomandibular joint, inside of the mouth, and the anterior two-thirds of the tongue. Its motor fibers innervate muscles of mastication, mylohyoid, anterior belly of the digastric, tensor veli palatini, and the tensor tympani.
CN VI: Abducent nerve: This motor nerve starts in the pons and leaves the cranium through the superior orbital fissure. It innervates the lateral rectus muscle that moves the eyeball laterally.
CN VII: Facial nerve: This nerve has both sensory and motor fibers. The sensory fibers start at the geniculate ganglion and exit the cranium at the internal acoustic meatus. The facial nerve transmits sensory impulses from the skin of the external acoustic meatus of the ear. It also carries the sense of taste from the anterior two-thirds of the tongue. The motor fibers of CN VII start at the pons and leave the cranium at the internal acoustic meatus. They innervate muscles of facial expression, the stapedius muscle of the inner ear, and the stylohyoid and digastric muscles.
CN VIII: Vestibulocochlear nerve: This sensory nerve has two divisions:
The vestibular nerve starts in the vestibular ganglion and leaves the cranium via the internal acoustic meatus. It’s needed for equilibrium.
The cochlear nerve starts at the spiral ganglion and leaves the cranium via the internal acoustic meatus. It’s needed for hearing.
CN IX: Glossopharyngeal nerve: This nerve branches from the medulla oblongata and has both sensory and motor fibers. It leaves the cranium via the jugular foramen. It provides parasympathetic input to the parotid salivary gland and somatic motor fibers that innervate the stylopharyngeaus muscle for swallowing. The sensory nerves serve the pharynx, external and middle parts of the ear, the carotid body and sinus, and general sense and taste from the posterior part of the tongue.
CN X: Vagus nerve: This nerve originates from the medulla oblongata. It has both motor and sensory fibers and exits the cranium through the jugular foramen. The vagus nerve provides motor innervation to the muscles of the palate, pharynx, larynx, and esophagus for swallowing. It’s the source of presynaptic parasympathetic fibers that innervate the trachea, bronchi, digestive tract, and heart. It also transmits taste sensations from the palate and epiglottis and somatic sensation from the external acoustic meatus and auricle.
CN XI: Spinal accessory nerve: This motor nerve starts on the spinal cord and leaves the cranium through the jugular foramen. It innervates the sternocleidomastoid and trapezius muscles.
CN XII: Hypoglossal nerve: This motor nerve starts on the medulla and exits the cranium at the hypoglossal canal. It innervates the tongue.