The 12 Cranial Nerves and Their Functions

The olfactory nerve is responsible for transmitting scent information to the areas of the brain responsible for smell and memory. Of the twelve cranial nerves, the olfactory nerve is the smallest. It also does not emanate from the brainstem. Damage to the olfactory nerve can result in loss of smell, changes in taste, and distortion in the sense of smell.

Like the olfactory nerve, the optic nerve does not emanate from the brainstem. It is responsible for transmitting visual information. Additionally, the optic nerve conducts visual impulses for two neurological reflexes. The accommodation reflex is the swelling of the lens that occurs when looking at a near object. The pupillary light reflex is the constriction of the pupils in the presence of light. Injuries to the optic nerve can create varying levels and types of blindness, while inflammation may impact vision sharpness or color detection.

Three separate nerves work together to move the eyes. The first is the oculomotor nerve, which controls all muscles of the eyes except for the oblique and the lateral rectus muscles. It also assists the optic nerve with the pupillary light reflex. Cranial nerve III originates in the midbrain, which is part of the brainstem. Damage to the oculomotor nerve may cause double vision, eyelid drooping, pupil dilation, and an inability to coordinate both eyes.

The trochlear nerve is the second nerve that provides eye movement. It controls the oblique muscle and therefore allows the eyes to look down and inward. Like the oculomotor nerve, it originates from the midbrain. If either this nerve or the oblique muscle is damaged, the eye may not be able to move downwards properly. This nerve is the only cranial nerve to exit the brainstem from the rear.

The final nerve for eye movement is the abducens nerve, which controls the lateral rectus muscle. It is this muscle that allows the eye to look outward. Impairment of the lateral rectus muscle or the abducens nerve can cause double vision. This nerve leaves the brainstem where the pons and medulla meet.

The trigeminal nerve is the largest of the cranial nerves and has three divisions:

• Ophthalmic: Sends sensory information from the upper section of the face
• Maxillary: Sends sensory information from the middle part of the face
• Mandibular: Sends sensory information from the lower part of the face and controls the muscles within jaw and ear

This nerve provides sensation to the skin of the face and controls the muscles for chewing and similar actions. Injuries affecting the trigeminal nerve can cause loss of sensation in the respective area of the face.

The facial nerve controls the muscles responsible for facial expression, as well as supplying the sensation of taste for the front two-thirds of the tongue, and controlling the stapedius muscle. It also supplies salivary, tear-producing, and other glands in the head and neck. Plus, it can communicate some sensations from the outer parts of the ear. The facial nerve also has one of the more complex paths, originating as two separate roots in the brainstem that eventually merge.

To supply information relating to both hearing and balance, the vestibulocochlear nerve has two branches: the vestibular and cochlear. The former supplies sensation for areas in the inner ear, including information about balance. This enables the vestibulo-ocular reflex, which stabilizes the head and allows the eyes to track objects in motion. The cochlear branch transmits information from the cochlea, which is how we hear sounds.

Oral sensation, taste, and salivation all partially stem from the glossopharyngeal nerve that sends sensory information from the back part of the tongue, the back of the throat, sections of the inner ear, and the sinuses. Additionally, it provides the sense of taste for the back of the tongue. The glossopharyngeal nerve is also responsible for the voluntary movement of the stylopharyngeus muscle in the throat. It originates in the medulla oblongata.

Perhaps the most diverse of the cranial nerves, the vagus nerve provides both sensory and parasympathetic supply to various structures in the neck and most of the organs in the chest and abdomen. It enables motor control of the throat muscles and stimulates the muscles of the organs in the chest. The vagus nerve also transmits information from the ear canal, throat, chest organs, and abdominal organs. It has the longest pathway of the cranial nerves, originating from the medulla.

The accessory nerve supplies the sternocleidomastoid and trapezius muscles in the neck. These muscles allow for most of the movements of the neck and some shoulder motions. The accessory nerve has two sections: cranial and spinal. The cranial portion begins in the medulla oblongata while the spinal section originates from the spinal cord. Damage to this nerve can weaken the neck and shoulders.

The hypoglossal nerve supplies the intrinsic muscles of the tongue, which allow for its many intricate movements. It originates from the medulla oblongata and travels into the jaw. Unlike many other nerves, motor cortices of both hemispheres of the brain supply the hypoglossal nerve. Any damage can result in atrophy of the tongue muscles, causing weakness of tongue movement.