The Cochlea of the Inner Ear

1. Hearing is best developed in tetrapods, specifically mammals and birds
2. The cochlea is a spiral-shaped tube consisting of two connected canals; the upper vestibular canal and the lower tympanic canal that are continuous at the apex of the cochlea
1. These two canals are filled with perilymph
3. The middle canal is the cochlear duct, and is filled with endolymph
1. The organ of Corti is located in the cochlear duct, and is composed of hair cells resting on a basilar membrane
2. The basilar membrane separates the cochlear duct from the tympanic canal
3. The tectorial membrane lies above the hair cells
4. In humans, sound waves cause the tympanic membrane to vibrate; the three middle ear bones (malleus, incus, and stapes) transmit and amplify the vibration to the oval window, which transmits the vibration to the perilymph in the vestibular canal
5. The pressure wave is transmitted through the vestibular canal to the tympanic canal, and ultimately causes the basilar membrane to vibrate
6. The tectorial membrane stimulates the hair cells of the organ of Corti, which send impulses to the brain via the cochlear nerve
7. Sounds of different frequencies resonate and stimulate the basilar membrane in different ways and in different areas
8. Loudness is based on a greater number of hair cells being stimulated
9. The human ear typically responds to sounds between 20 and 20,000 cycles per second (Hz), which is much more sensitive than the human eye

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