There are 12 paired cranial nerves that exit the skull under the brain's surface that extend to various parts of the head, neck, chest and abdomen. The cranial nerves comprises three nerve types: motor nerves that send an impulse signal to a muscle; sensory nerves that transmit sensations from the body back to the brain; and autonomic nerves with both motor and sensory components that monitor and control visceral functions such as salivation, heart rate and intestinal movement. The cranial nerves are numbered one through 12 and are referred to by either their name or number.
1 - The Olfactory Nerve
The olfactory nerve is a sensory nerve that receives and transmits the sense of smell from the nose and sinuses back to the brain. It is located under the frontal lobes of the brain and perforates the skull at the cribiform plate.
2 – The Optic Nerve
The optic nerve is a sensory nerve responsible for vision. It transmits signals from the retina in the eye back to the visual cortex in the posterior lobes of the brain traversing the skull through the optic canal.
3 – The Oculomotor Nerve
The oculomotor nerve traverses the skull through the superior orbital fissure and is both a motor and autonomic nerve. The motor portion transmits signals from the brain that result in eye movements. The autonomic portion controls constriction and dilation of the pupil and prevents the upper eyelid from drooping.
4 – The Trochlear Nerve
The trochlear nerve is a motor nerve that sends signals from the brain causing the eye to move in the downward and inward directions. The trochlear nerve also traverses the skull through the superior orbital fissure.
5 – The Trigeminal Nerve
The trigeminal nerve is the largest of the 12 cranial nerves and is both a motor and sensory nerve. The motor portion of the trigeminal nerve is responsible for jaw movement and chewing, while the sensory portion of the nerve provides the sensation of touch over the face as well as on the surface of the eye. There are three major branches of the trigeminal nerve--the ophthalmic branch, the maxillary branch and the mandibular branch, which each traverses the skull in different locations.
6 – The Abducens Nerve
The abducens nerve is a motor nerve that is responsible for lateral or outward eye movement. It traverses the skull through the superior orbital fissure.
7 – The Facial Nerve
The facial nerve traverses the skull through the internal auditory canal and has all three nerve type components–motor, sensory and autonomic. The motor portion is responsible for facial movements and expression, as well as some muscles deep in the neck. The sensory portion is responsible for registering taste on the anterior two- thirds of the tongue. The autonomic portion monitors and controls moisture of the eyes as well as salivation.
8 – The Auditory Nerve
The auditory nerve is also known as the vestibulocochlear nerve and has both sensory and autonomic nerve characteristics. The cochlear portion is sensory, innervating the inner ear and is responsible for hearing. The vestibular portion is autonomic, innervating a different portion of the inner ear and is responsible of the sense of balance. The auditory nerve also exits the skull via the internal auditory canal.
9 – The Glossopharyngeal Nerve
The glossopharyngeal nerve exits the skull via the jugular foramen carrying motor, sensory and autonomic nerve types. The motor portion innervates muscles of the neck responsible for swallowing and speech. The sensory portion transmits taste and touch from the posterior one-third of the tongue as well as sensation of a portion of the ear. The autonomic portion monitors and controls dilatation of a portion of the carotid artery in the neck and thereby has an influence on blood pressure.
10 – The Vagus Nerve
The vagus nerve exits the skull through the jugular foramen as well and is also composed of motor, sensory and autonomic nerve types. The motor portions innervate muscles of the throat that aid in swallowing and speech. The vagus nerve supplies sensory information from the throat and is responsible for the gag reflex. The autonomic portion extends nerves to regions of the aorta in the chest that monitor blood pressure and to nerves in the abdomen that monitor and control bowel function.
11 – The Spinal Accessory Nerve
The spinal accessory nerve is a motor nerve that innervates and causes movement of the sternocleidomastoid muscle in the neck and the trapezius muscle in the upper back resulting in shrugging. It also exits the skull via the jugular foramen.
12 – The Hypoglossal Nerve
The hypoglossal nerve is a motor nerve that innervates the muscles of the tongue responsible for tongue movement. It traverses the skull through the hypoglossal canal.