cranial nerve, ninth See glossopharyngeal nerve.

cranial nerves A series of 12 nerve pairs connecting directly to the brain, emerging through openings in the skull and then dividing into several major branches. All but two of the cranial nerve pairs connect with the brain stem (the lowest section of the brain). one nerve from each pair serves one side of the body, while the other nerve serves the other side.

The cranial nerves, known by a numbering system invented by the early Greek physician galen, include the olfactory nerve (first cranial nerve); optic nerve (second cranial nerve); oculomotor nerve (third cranial nerve); trochlear nerve (fourth cranial nerve); trigeminal nerve (fifth cranial nerve); abducens nerve (sixth cranial nerve); facial nerve (seventh cranial nerve); vestibulocochlear nerve (eighth cranial nerve); glossopharyngeal nerve (ninth cranial nerve); vagus nerve (10th cranial nerve); spinal accessory nerve (11th cranial nerve); hypoglossal nerve (12th cranial nerve).

Each of the cranial nerves plays a part in at least one of the following operations: carrying motor and sensory information to regions of the head and neck; serving as the basic wiring system of the sense of vision, hearing and balance, taste and smell; and transmitting autonomic information to special glands and organs.

Some of the cranial nerves play a variety of roles. For example, the trigeminal nerve wears several hats, carrying sensations from the face, teeth and sinuses to the brain as well as controlling several important muscles, including the strong jaw muscles used for chewing. The facial nerve sends branches to the important facial muscles of expres-sion—those used in smiling, frowning, raising one's eyebrows, and so on. The olfactory and optic nerves carry visual and olfactory input from the eyes and nose to the brain for processing. The oculomotor nerve controls eye movements and also helps focus images on the retina. The vagus nerve, in addition to performing sensory and motor functions, sends nerve fibers to the heart, stomach, and intestines to help regulate autonomic activity.

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