Which clinical finding or findings would be caused by a...

Which clinical finding or findings would be caused by a complete paralysis of cranial nerve III?
(A) Dilated pupil with downward and outward deviation and ptosis.
(B) Afferent pupillary defect.
(C) Impaired corneal reflex.
(D) Difficulty abducting the globe.

Key Concepts

Levator Palpebrae Superioris Function and Ptosis

The levator palpebrae superioris muscle, which is innervated by cranial nerve III, is crucial for elevating the upper eyelid. Paralysis of this nerve causes ptosis, or drooping of the eyelid, further highlighting the nerve's role in maintaining normal ocular and eyelid function.

Pupillary Control and Parasympathetic Innervation

The parasympathetic fibers of cranial nerve III mediate pupillary constriction. When these fibers are paralyzed, the imbalance leads to unopposed sympathetic stimulation, resulting in a dilated pupil. This underscores the key role of the oculomotor nerve in the autonomic regulation of pupil size.

Cranial Nerve III (Oculomotor Nerve)

This nerve is primarily responsible for innervating most of the extraocular muscles, including the medial rectus, superior rectus, inferior rectus, and inferior oblique, as well as the levator palpebrae superioris. It also provides parasympathetic fibers to the pupil, enabling constriction. Therefore, its complete paralysis results in characteristic eye deviations and a lack of pupillary constriction.

Extraocular Muscle Innervation and Ocular Deviation

The oculomotor nerve controls muscles that move the eye upward, inward, and downward. Loss of its function leaves the lateral rectus (controlled by CN VI) and superior oblique (controlled by CN IV) unopposed, leading to an eye that is deviated downward and outward. This demonstrates the importance of coordinated extraocular muscle control in eye alignment.

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