Evolutionary changes in the cochlea and labyrinth: Solving the problem of sound transmission to the balance organs of the inner ear.
This review article examines the evolutionary adaptations in the vertebrate inner ear that allow selective activation of auditory or vestibular hair cells, although both are housed in the same bony capsule. The problem of separating acoustic stimuli from the vestibular end organs in the inner ear has recently reemerged with the recognition of clinical conditions such as superior canal dehiscence syndrome and enlarged vestibular aqueduct syndrome. In these syndromes, anatomical defects in the otic capsule alter the functional separation of auditory and vestibular stimuli and lead to pathological activation of vestibular reflexes in response to sound. This review demonstrates that while the pars superior of the labyrinth (utricle and semicircular canals) has remained fairly constant throughout evolution, the pars inferior (saccule and other otolith, macular, and auditory end organs) has seen considerable change as many adaptations were made for the development of auditory function. Among these were a relatively rigid membranous labyrinth wall, a variably rigid otic capsule, immersion of the membranous labyrinth in perilymph, a perilymphatic duct to channel acoustic pressure changes away from the vestibular organs, and different operating frequencies for vestibular versus auditory epithelia. Even in normal human ears, acoustic sensitivity of the labyrinth to loud clicks or tones is retained enough to be measured in a standard clinical test, the vestibular-evoked myogenic potential test.