Cranial nerve is any nerve of the 12 pairs of nerves that connect directly to the brain. Most nerves connect to the spinal cord, which relays nerve signals to and from the brain. Cranial nerves, however, pass through openings in the skull, linking the brain directly to other parts of the body. Most cranial nerves connect to parts of the head and neck. Like other nerves, cranial nerves can be sensory, motor, or mixed. Sensory cranial nerves carry information gathered by the senses to the brain. Motor cranial nerves transmit motor signals from the brain, enabling it to control the movement of muscles. Mixed cranial nerves contain both sensory and motor nerve fibers.
Anatomists (scientists who study the structures of the body) traditionally designate the cranial nerves with Roman numerals, starting with the cranial nerve farthest forward in the head. The cranial nerves are the: (I) olfactory, (II) optic, (III) oculomotor, (IV) trochlear, (V) trigeminal, (VI) abducens, (VII) facial, (VIII) vestibulocochlear, (IX) glossopharyngeal, (X) vagus, (XI) spinal accessory, and (XII) hypoglossal nerves.
The olfactory nerve
conveys sensory information about smell to the brain. Its fibers arise from specialized receptor cells in the mucous membranes of the nose. Odors cause these cells to generate nerve signals. The olfactory nerve transmits the signals to the olfactory bulb, a structure on the underside of the brain associated with smell. See also Smell (How odors are detected) .
The optic nerve
brings visual information to the brain. Its fibers begin in the retina, the light-sensitive tissue at the back of the eye. The two optic nerves—one from each eye— meet at the base of the brain at a point called the optic chiasm. At the optic chiasm, approximately half the nerve fibers from each eye cross over, joining the fibers from the other eye. The crossover enables each side of the brain to receive visual signals from both eyes. See also Eye .
The oculomotor, trochlear, and abducens nerves
govern the six extraocular muscles, muscles that move the eyeball. The oculomotor nerve controls four extraocular muscles that turn the eye up, down, and toward the nose. It also commands the muscles that control the size of the pupil, muscles that help shape the lens, and muscles used to move the upper eyelid. The trochlear nerve controls one extraocular muscle responsible for moving the eye downward and away from the nose. The abducens nerve transmits impulses to an extraocular muscle that pulls the eye away from the nose.
The trigeminal nerve
carries somatosensory information from the face and head to the brain. Somatosensory information includes sensory signals related to touch, pressure, pain, and temperature. The trigeminal nerve, which is a mixed nerve, ranks as the largest of all the cranial nerves. It has three major branches: (1) the ophthalmic nerve, (2) the maxillary nerve, and (3) the mandibular nerve.
The ophthalmic nerve links the brain with the skin of the eyelid, the scalp, the tip of the nose, the forehead, and the cornea, the transparent covering of the eyeball. The maxillary nerve serves the skin of the face and the side of the nose, the upper teeth and gums, and the cheek. The mandibular nerve transmits impulses from the lower teeth and lips, the skin of the chin, the outer ear and the scalp, and the joint between the lower jaw and the temple. Somatosensory information from the front of the tongue and the floor of the mouth also travels through the mandibular nerve, along with some of the motor impulses involved in chewing.
The facial nerve
transmits taste information from the front of the tongue and the floor of the mouth. This mixed nerve also conveys somatosensory information from the ear, helps control the flow of saliva, and carries impulses to the muscles used in facial expression.
The vestibulocochlear nerve
is a sensory nerve that consists of two branches that control hearing and balance. One branch, the vestibular nerve, arises in the semicircular canals of the inner ear and provides the brain with information related to balance and head movement. The other branch, the cochlear nerve, originates in the cochlea, a spiral-shaped cavity of the inner ear. The cochlear nerve transmits signals related to hearing. See also Ear .
The glossopharyngeal nerve
also conveys taste information. This mixed nerve carries sensations from the back of the tongue, the tonsils, and the pharynx, the cavity at the back of the mouth. In addition, the glossopharyngeal nerve controls some muscles used in swallowing and salivation.
The vagus nerve
is the only cranial nerve that extends beyond the head and neck. This mixed nerve connects the brain with the heart, larynx, trachea, lungs, esophagus, stomach, spleen, kidney, aorta, colon, and liver. A branch of the vagus nerve also conveys taste information from taste buds in the back of the mouth.
The spinal accessory and hypoglossal nerves
are both motor nerves. The spinal accessory nerve controls the sternocleidomastoid and trapezius muscles that move the head and neck. The hypoglossal nerve transmits motor impulses to the tongue.
See also Nervous system ; Senses .
