The cranial nerves are an important collection of nerves, all of which travel directly to the brain rather than through the spinal cord, like most other nerves. The cranial nerves have several functions critical for day-to-day life, so they become very important to physicians, as well as patients impacted by disorders of cranial nerve function.
- The Olfactory Nerve The olfactory nerve is the first cranial nerve, responsible for transmitting everything we smell to the brain. Disruptions to this nerve can cause anosmia, an inability to detect scents. This also dramatically impacts our sense of taste.
- The Optic Nerve The optic nerve transmits electrical signals from the eye to the brain, which transforms these signals into an image of what we see in the world around us. Disorders of the optic nerve, such as optic neuritis, can lead to visual disturbances and even blindness.
- The Oculomotor Nerve The oculomotor nerve has two main functions. First, the oculomotor nerve transmits signals that allow the eyes to move in every direction not controlled by cranial nerves IV and VI. Second, the oculomotor nerve carries parasympathetic fibers to the iris, causing the iris to constrict when you're in bright light. A lesion in the oculomotor nerve can cause not only double vision (diplopia), but can also cause a "blown pupil" — a pupil that cannot constrict. Due to its location, the oculomotor nerve is susceptible to damage by elevated intracranial pressure, and a blown pupil can be a sign of serious neurological trouble.
- The Trochlear Nerve Cranial Nerve IV is the trochlear nerve, which controls a muscle that moves the eyeball down and out. A lesion of this nerve can cause diplopia, which can be improved by tilting the head away from the affected eye.
- The Trigeminal Nerve The trigeminal nerve is primarily a sensory nerve, meaning that it relays sensation from the face to the brain. In addition, the trigeminal nerve controls some facial muscles important for chewing. One of the worst complications of trouble with Cranial Nerve V is trigeminal neuralgia, an extreme form of facial pain.
- The Abducens Nerve This nerve controls the nerve that moves the eye away from the nose. A lesion of the abducens nerve causes double vision, in which one image is directly next to the other. Sometimes the abducens nerve can be impacted on both sides in cases of increased intracranial pressure, such as pseudotumor cerebri.
- The Facial Nerve The facial nerve is complicated. Not only does it control most muscles of the face; this nerve also transmits taste signals from the front of the tongue, conveys parasympathetic fibers that make the eyes tear and mouth salivate, and is responsible for a little bit of sensation around the ear. It also helps modulate hearing through control of the stapedius muscle. This is why inflammation of the facial nerve, such as in Bell's palsy, can lead to more problems than just facial weakness, though such weakness is usually the most obvious symptom.
- The Vesibulocochlear Nerve This nerve has two main components: the cochlear component relays acoustic information to the brain so that we can hear, and the vestibular portion sends signals regarding balance and movement. Problems with the vestibulocochlear nerve, then, can cause either hearing loss or vertigo, and often cause both.
- The Glossopharyngeal Nerve Cranial Nerve IX has a bunch of odd jobs. This nerve is responsible for taste from the back of the tongue, sensation from a small portion of the ear and parts of the tongue and throat, the innervation of one muscle important for swallowing (the stylopharyngeus), and salivation by the parotid gland. It also receives important information about blood pressure from chemoreceptors and baroreceptors in the carotid body. Irritation of the glossopharyngeal nerve can lead to glossopharyngeal neuralgia, a condition in which it is very painful to swallow.
- The Vagus Nerve This nerve controls the pharynx (for swallowing) and larynx (for speaking), as well as sensation from the pharynx, part of the meninges and a small portion of the ear. Like the glossopharyngeal nerve, the vagus nerve detects taste (from the throat) and also detects special signals from chemo and baroreceptors near the heart (in the aortic arch). Furthermore, the vagus nerve relays parasympathetic fibers to the heart, the signals from which can slow the heart's beating. Because of its relationship to the heart, disorders of the vagus nerve could be very dangerous. On the other hand, stimulation of the vagus nerve has been shown to be potentially useful in a wide array of disorders, including epilepsy.
- The Spinal Accessory Nerve Cranial Nerve XI is less complicated than its immediate predecessors. It only has one main function: to cause the contraction of the sternocleidomastoid muscle and the trapezius in order to help move the head or shoulder. Disorders of this nerve diminish the ability to use these muscles.
- The Hypoglossal Nerve Last but not least, the hypoglossal nerve controls all the movements of the tongue. Difficulty speaking (dysarthria) is one potential consequence of a damaged hypoglossal nerve.
Unless you're a medical professional, it's not usually necessary to know all the details about each individual nerve. However, the information above may help you better understand the source of particular problem that you've been having, and guide you towards more information.
Hal Blumenfeld, Neuroanatomy through Clinical Cases. Sunderland: Sinauer Associates Publishers 2002
Ropper AH, Samuels MA. Adams and Victor's Principles of Neurology, 9th ed: The McGraw-Hill Companies, Inc., 2009. McCabe MP, O'Connor EJ.