Intro | Anvil | Ear Canal | Semicircular Canals | Cochlea | Eardrum | Hammer | Auditory Nerve | Stirrup
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Movement of the last ossicle, the stirrup, transforms the oscillations of the middle ear into pressure waves within the fluid that fills the inner ear, or cochlea. The Cochlea was given the Greek name for spiral-shaped snails due to its resemblance to these creatures. The auditory receptor cells, called hair cells, lie embedded within the basilar membrane. This membrane divides the spiraled cochlea into upper and lower chambers. Movement of the fluid within the cochlea causes stimulation of the hair cells. Although the entire membrane vibrates in reponse to the pressure waves (frequency theory), the point along the membrane where the wave peaks represents the frequency of the stimulus (place theory).
Details regarding the intricate structure of the inner ear, provide a better understanding of how this structure transduces sound waves, encoding them for auditory perception. Within the cochlea lies the Organ of Corti. This structure is composed of the basilar membrane, the hair cell receptors, and the tectorial membrane. The tectorial membrane (from tectum meaning roof) lies over the hair cells; it serves as a shelf against which the cilia of hair cells brush upon movement. Sound waves cause the basilar membrane to move relative to the tectorial membrane. The cilia of the hair cells bend when contact is made to the tectorial membrane and the hair cell discharges.
The human cochlea contains approximately 3500 inner hair cells (those lying on the inside of the cochlear coil) and 12,000 outer hair cells (those lying on the outside of the cochlear coil). Hair cells form synapses with bipolar neurons whose axons form the auditory nerve. Tips of the outer hair cells are attached directly to the tectorial membrane. When the inner hair cells move they make contact with the overhanging tectorial membrane. This contact initiates depolarization.
The hair cells are organized along the basilar membrane according to their frequency response. This arrangement is called tonotopic organization, and is similar to the systematic mapping of the receptive fields representing the visual world. High frequency sounds produce their maximum displacement of the cochlear basilar membrane at the oval window, the entry point of the inner ear. At this location, the basilar membrane is thin (approximately 0.15mm thick) and stiff. The further into the spiral formation of the cochlea, the greater the displacement of the membrane by lower frequency sounds, with 100 Hz sounds causing the greatest displacement. At this distal apex of the cochlea, the basilar membrane is at its widest (approximately 0.5mm) and flaccid. Confirmation of this tonotopic organization of the basilar membrane comes from the effectiveness of cochlear implants. These devices are used for restoration of hearing in people whose deafness is secondary to hair cell damage. A cochlear implant consists of an external microphone, a miniaturized electronic signal processor, and an internal flexible array of microelectrodes. Each electrode in the implant array stimulates a different part of the membrane. The signal process in the external device analyzes the sound detected by the microphone and sends appropriate signals to the appropriate regions of the basilar membrane. The fact that this device works as well as it does, is strong confirmation of the importance of place coding.