Anatomy And Physiology

Cranial Nerves

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The cranial nerves are the major nerves running into the brain from the brainstem, or originating from the brain itself. There are twelve recognized cranial nerve pairs, numbered with roman numerals. These nerves serve the face and the five traditional senses, running directly into the respective sensory areas of the cerebrum. Many of the nerves overlap in function and location, with several serving the sense of taste and involuntary reflexes of the throat and brainstem.

The olfactory nerve (I) transmits impulses from the olfactory bulb, which is located in the upper nasal cavity, along the ethmoid bone. This nerve handles the sense of smell, and the impulses are carried to the temporal lobe of the brain for processing.

The optic nerves (II) pass out of the back of the eyeballs, crossing at the optic chiasma to enter the occipital lobe at the back of the brain, where vision processing occurs. The oculomotor (III), trochlear (IV), and abducens (VI) nerves handle the actual movement of the eyeball, operating different muscles for up/down, side to side, and rotational movements. Cranial nerve III also controls the constriction of the pupil in bright light or for near vision.

Another sensory cranial nerve is the acoustic nerve (VIII), known as the vestibulocochlear nerves, which handle hearing and equilibrium. The nerves originate in the inner ear and travel to the auditory areas of the temporal lobes.

The trigeminal nerve (V) transmits the sensations for the face, scalp, and teeth. Chewing is made possible by the signals from this nerve. The contraction of other facial muscles relies on the facial nerve (VII), which also transmits signals for the mouth saliva secretion and taste. Taste is determined by chemoreceptors on the tongue, the taste buds. They feed into not only cranial nerve VII but also the glossopharyngeal nerve (IX), which is responsible for involuntary blood pressure reflexes, cardiac and respiratory sensing, and contraction of the pharynx, the swallowing reflex. However, movement of the tongue is directed by the hypoglossal nerve (XII).

Overlapping with cranial nerve IX is the vagus nerve (X), which is responsible for transmitting signals about the cardiac, respiratory, and blood pressure reflexes, as well as digestive secretions, peristalsis of the esophagus, speaking (larynx sensory and motor responses), and decreases in heart rate. The accessory nerve (XI) is also involved in larynx motor signals for speaking, but it is responsible for the contraction of neck and shoulder muscles as well.

Whereas the majority of the cranial nerves are sensory and/or motor for their respective areas, the vagus nerve branches much farther into the nervous system. As mentioned above, this nerve has many reflex functions necessary for life - breathing, heart rate, and gastrointestinal. In fact, the word "vagus" means "wanderer."

Reference: Scanlon and Sanders. Essentials of Anatomy and Physiology, 4th edition. Saunders.

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