As the eyes rotate upward, the upper eyelid reflexively retracts. Which cranial nerve is primarily ...

Ans:
B

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The CN III (oculomotor) nucleus, which extends through a considerable part of the midbrain, contains clusters of lower motor neurons for each of the five eye muscles it innervates. Because of its plane of attachment, the inferior oblique rotates the eye in the frontal plane pulling the top of the eye laterally. In other words, as the eyes rotate upward, the upper eyelid is reflexively retracted, and in the downward gaze, it is lowered, restricting exposure of the conjunctiva to air and reducing the effects of drying. CN I is involved in olfactory function; CN VI (abducens nerve) innervates the lateral rectus, which abducts the eye. CN IV (trochlear nerve) innervates the superior oblique muscle, which depresses, rotates laterally, and intorts the eyeball.