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Cranial Nerves
Cranial Nerves
What are cranial nerves?
Think back to your last wellness check-up at your medical provider’s office. You might remember him/her shining a light in your eyes, asking you to look around to different corners of the room, or making you shrug your shoulders. What’s the point to all of these little exercises?
Testing cranial nerve (CN) functions! CNs are 12 nerves that stem directly from the brain to exchange information between the brain and regions of the head, neck, shoulders, and parasympathetic nervous system. In the picture above, the number is placed directly over where each associated CN is located.
When CNs function properly, they result in things like vision, smell, eye movements, tongue movements, face sensation, hearing, and shrugging. That’s why we think of CNs as falling into either sensory (e.g., hearing, smelling, vision), motor (e.g., shoulder shrugging, tongue/eye movements), or mixed categories.
When a CN is not working properly, we call it a CN “palsy”. This palsy can either be partial or complete loss of function to the CN. There are many reasons why a palsy might occur, including head trauma, aging, stroke, infection, tumor, etc. Sometimes, people are born with a palsy.
Cranial Nerve Functions & Palsies
Smell: CN 1
The technical term for "smell" is "olfaction." Not surprisingly, then, the olfactory nerve (i.e., CN 1) is a special sensory nerve that is the only CN involved in smell. CN 1 palsy results in anosmia, or the inability to smell, and tends to stem from central nervous system infections, traumatic brain injury that damages CN 1, or metabolic diseases.
Visual Acuity: CN2
If you look at the "Vision" page, you'll notice that visual inputs pass through the optic nerve (i.e., CN 2) as part of the visual system. The optic nerve is a special sensory nerve that passes along information from the retina to the rest of the visual system. Damage to CN 2 can result in total eye blindness. When your healthcare provider asks you to stare at a focal point and indicate whether you see his/her finger wiggling in the periphery, (s)he is testing your visual system.
Pupil Reactions: CN 2,3
When the lighting around you changes, your pupils shrink (i.e., constrict) or widen (i.e., dilate) to help with visual acuity. Your optic and oculomotor nerves (i.e., CN 3) are responsible for this process. CN 3 is considered a motor nerve for its role in pupil constriction (i.e., visceral motor) and eye movements (i.e., somatic motor). When your healthcare provider shines a light in your eyes, (s)he is testing these CNs to make sure your pupils react properly.
Eye Movements: CN 3,4,6
There are 3 important CNs for eye movements, including the oculomotor nerve, the trochlear nerve (i.e., CN 4; somatic motor), and the abducens nerve (i.e., CN 6; somatic motor). These nerves are uniquely connected to 6 muscles that surround the eye to result in eye movements. When your healthcare provider asks you to track his/her finger with your eyes only, or look around to different corners of the room, (s)he is testing this set of CNs.
Face Sensations/Jaw Movement/Corneal Reflex: CN 5
The trigeminal nerve (i.e., CN 5) has mixed sensory and motor functions. Pain, touch, and temperature sensations are transmitted through three subparts, including the ophthalmic division, the maxillary division, and the mandibular division. The main motor function is jaw movement. When CN 5 is damaged, a person might have difficulty with functions like corneal reflex (or your reaction of closing your eyelid when your cornea is touched by something), intense pain in the face (e.g., trigeminal neuralgia), or facial numbness.
Non-Eye/Jaw Face Movements: CN 7
The facial nerve (i.e., CN 7) is also involved in mixed sensory and motor functions of the face. It helps to move/supply sensation to the parts of the face that are not associated with CN 5, and also contributes to taste perception. A common palsy associated with CN 7 is called "Bell's palsy" and happens when one side of the face becomes temporarily paralyzed as a result of things like infection, high blood pressure, or head trauma.
Hearing/Balance: CN 8
The vestibulocochlear nerve (i.e., CN 8) is important for both hearing (i.e., cochlear) and balance (i.e., vestibular). Your healthcare provider might test the function of this nerve by putting a vibrating tuning fork next to your ear while you have your eyes closed. Damage to CN 8 can lead to things like vertigo (or dizziness), hearing loss, or nystagmus (i.e., repetitive, uncontrolled eye movements).
Taste/Swallow/Gag Reflex: CN 7, 9
The facial and glossopharyngeal (i.e., CN 9) nerves are important for taste. Further, CN 9 is a mixed sensory and motor nerve that helps you with eating functions like swallowing, or gag reflex if the thing you are eating is potentially harmful. When your healthcare provider asks you to swallow or cough, (s)he is testing your CN 9.
Speech: CN 5,7,10,12
Speech is made possible by a variety of CNs, including CNs 5, 7, 10 (i.e., vagus nerve), and 12 (i.e., hypoglossal nerve). The vagus nerve has many diverse sensory and motor functions, supplying smooth muscle of you parasympathetic system and vocal cords, esophagus, and trachea. The hypoglossal nerve has only motor functions and is important for tongue movement.
Shoulder Shrug: CN 11
The accessory nerve (i.e., CN 11) allows you to shrug your shoulders and move your head from side-to-side.
References
[1] US NIH, “Cranial Nerves”
[2] Dharani, K. (2014). The Biology of Thought: A Neuronal Mechanism in the Generation of Thought-a New Molecular Model. Academic Press.
[3] American Association for Pediatric Ophthalmology and Strabismus, “Cranial Nerve Palsy”
[4] Adams, R. D., Victor, M., & Ropper, A. H. (1997). Principles of Neurology. 6th. Edition. Singapore, McGraw-Hıll Book Co, 603-605.
[5] Walker, H. K., Hall, W. D., & Hurst, J. W. (1990). Cranial Nerves III, IV, and VI: The Oculomotor, Trochlear, and Abducens Nerves.
[6] Sanders, R. D. (2010). The trigeminal (V) and facial (VII) cranial nerves: head and face sensation and movement. Psychiatry (Edgmont), 7(1), 13.
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