D Transmission

(1) The sound stimulus is transferred from the stapes to the perilymph of the scala vestibuli. Here the stimulus is transmitted as a pressure pulse in the fluid.

(2) In response, the basilar membrane of the cochlea vibrates (mechanically oscillates). Only selected portions of the basilar membrane vibrate at any one time, depending on the frequency of the sound stimulus.

(3) The hair cells of the organ of Corti at that particular location are mechanically stimulated. This stimulation is transferred to the neurons of the acoustic nerve (cranial nerve VIIIa). The acoustic nerve passes out of the modiolus into the internal auditory meatus of the temporal bone. From here, it enters into the cranial cavity and goes to the brain.

Figure 11-14. Diagram of the scalae.

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