Lesions of the vestibulocochlear n. can impair hearing, balance, or both. A lesion of its cochlear portion produces sensorineural hearing loss (impairment of sound perception), which must always be differentiated from conductive hearing loss (impairment of sound conduction, usually due to blockage of the external auditory canal by cerumen, or to a disease process in the middle ear). A lesion of the vestibular portion causes disequilibrium and vertigo. Vestibular vertigo usually occurs in a particular direction and is accompanied by autonomic symptoms and nystagmus. Common vestibular disorders causing vertigo include vestibular neuritis and benign paroxysmal positioning vertigo. A vestibular lesion, however, is only one possible cause of vertigo; many other disorders must be included in the differential diagnosis.
The eighth cranial nerve (vestibulocochlear n.) conducts auditory and vestibular information to the central nervous system.
• Auditory impulses arise in the organ of Corti in the cochlea and travel by way of the cochlear n. to the cochlear nuclei of the brainstem and then onward in the auditory pathway to the auditory cortex in the temporal lobe.
• Vestibular impulses arise in the ampullae and in the macula statica of the saccule and utricle, the organ of equilibrium; they then travel by way of the vestibular n. to the vestibular nuclei, from which they are conducted to multiple areas of the brain, including the cerebellum.
These anatomical relations are depicted in Fig. 3.11 and discussed on p. 24.
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