Which structure is NOT innervated by CN 8?

The correct answer is that the tensor tympani muscle is not innervated by cranial nerve VIII (CN 8). Cranial nerve VIII, also known as the vestibulocochlear nerve, is primarily responsible for auditory and balance functions.

The cochlea, vestibule, and semicircular canals are all integral parts of the auditory and vestibular systems and are directly innervated by CN 8. The cochlea is responsible for hearing, converting sound waves into nerve impulses. The vestibule and semicircular canals play key roles in maintaining balance and spatial orientation, with the semicircular canals detecting rotational movements of the head and the vestibule responding to changes in head position.

In contrast, the tensor tympani muscle, which is located in the middle ear, is innervated by the mandibular branch of the trigeminal nerve (CN V) instead. This muscle functions to dampen loud sounds by tensing the tympanic membrane, which is a protective mechanism not associated with CN 8. Thus, recognizing the specific innervations of cranial nerves helps clarify the roles and pathways of auditory and vestibular functions in the body.