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Tympanic duct (scala tympani)

Hair cells /

Basilar membrane

Pineal Gland Activation

Figure 12.15

(a) A micrograph of the organ of Corti and the tectorial membrane (75x micrograph enlarged to 300x). (b) A scanning electron micrograph of hair cells in the organ of Corti, looking down on the "hairs" (bright yellow) (6,700x).

have neurotransmitter-containing vesicles in the cytoplasm near its base. In the presence of calcium ions, some of these vesicles fuse with the cell membrane and release neurotransmitter to the outside. The neurotrans-mitter stimulates the ends of nearby sensory nerve fibers, and in response they transmit nerve impulses along the cochlear branch of the vestibulocochlear nerve (cranial nerve VIII) to the brain.

Although the ear of a young person with normal hearing is able to detect sound waves with frequencies varying from about 20 to 20,000 or more vibrations per second, the range of greatest sensitivity is between 2,000 and 3,000 vibrations per second (fig. 12.16).

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