There are 12 cranial nerves that pass directly from the brain through openings in the skull. Each of these nerves serves a unique function: some provide sensory information to the brain; some allow messages to pass from the brain to muscles, causing voluntary movement; and some are part of the autonomic nervous system which regulates the function of the internal organs. A neurological assessment, following methods such as those suggested in the book
"Clinical Methods: An Overview of the Nervous System," will evaluate each of the 12 cranial nerves.
Olfactory Nerve
The olfactory nerve is also know as the first cranial nerve. It passes from the brain in to the nasal cavities and functions to help the brain detect various odors. In order to test the olfactory nerve, the physician has the patient hold a nostril shot, close the eyes and let the doctor know when he smells a test odor. The process is then repeated on the other side.
Optic Nerve
The optic nerve is the second cranial nerve; it carries sensory information from the eye to the brain, enabling vision. The function of the optic nerve is tested through techniques similar to those an eye doctor uses to evaluate visual acuity.
Oculomotor, Trochlear and Abducent Nerves
These three cranial nerves, numbered three, four and six, respectively; all have motor components that serve the muscles that move the eye. They are tested by evaluating the patient's ability to visually track a moving object.
In addition to supplying eye muscles, the oculomotor nerve, the third cranial nerve, functions in enabling the pupil to dilate and constrict. The oculomotor nerve is tested by observing pupil dilation in response to changes in light.
Trigeminal Nerve
The trigeminal nerve is the fifth cranial nerve, and it detects sensory stimulation over most of the face. It also has a motor function, supplying muscles that enable jaw movement. This nerve is tested by evaluating the patient's ability to sense touch over the forehead, cheek and jaw, as well as her ability to move and clench the jaw.
Facial Nerve
Cranial nerve seven is the facial nerve which supplies the muscles of facial expression. While evaluating the patient's facial movements for any asymmetry, the physician will ask the patient to demonstrate a series of expressions such as smiling and blinking the eyes. This nerve also supplies the tip of the tongue, which is tested by asking the patient to identify the flavor of a substance smeared on the tongue.
Vestibulocochlear Nerve
The vestibulocochlear nerve, the eighth cranial nerve, is a special sensory nerve used in the detection of sound and the ability to maintain balance. To evaluate this nerve, the patient is given a hearing test.
Glossopharyngeal and Vagus Nerves
The glossopharyngeal nerve is the ninth cranial nerve, and helps with swallowing and speech, as does the vague nerve, the tenth cranial nerve. These nerves are tested via evaluating the gag response, as well as the patient's voice and ability to swallow a glass of water.
Accessory Nerve
The eleventh cranial nerve is a motor nerve called the accessory nerve. It supplies the sternocliedomastoid muscles which facilitate the ability to turn the head, and the trapezius muscles which enable an individual to shrug his shoulders. The subject is tested for his ability to perform these movements against resistance.
Hypoglossal Nerve
The hypoglossal nerve is the twelfth and last cranial nerve. It is a motor nerve that allows the tongue to move. It is evaluated by observing the tongue for abnormal placement or shrinkage, and by asking the patient to move her tongue in various positions.
