Auditory Transduction (2002)

(orchestra tuning) (conductor taps podium) The sense of hearing is accomplished by a process known as "auditory transduction. " The ear converts sound waves in the air into electrical impulses, which can be interpreted by the brain. As sound enters the ear, it passes through the external auditory canal, where it meets the tympanic membrane.

The tympanic membrane then vibrates in response to the sound. (orchestral music plays) Sounds of a lower pitch, or "frequency", produce a slower rate of vibration. (low-pitched sound plays) And sounds of a lower volume, or "amplitude", produce a less dramatic vibration.

(quiet, low pitched sound plays) Higher frequency sounds produce faster vibrations. (high-pitched sound plays) The tympanic membrane is cone-shaped, and articulates with a chain of three bones, called the "auditory ossicles". They consist of the malleus, .

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. . the incus, .

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. . and the stapes.

The movements of the tympanic membrane vibrate the ossicles. . .

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passing on the information of frequency and amplitude. The three bones pivot together on an axis, shown here in red. (rhythmic, repetitive sound plays) The pivotal axis is due to a series of ligaments which hold the bones in place within the middle ear cavity.

The anterior malleal ligament. . .

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and the posterior incudal ligament are of particular importance for the pivotal axis. Two structures which normally obscure this view of the middle ear have been removed. They are: the chordae tympani nerve.

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. . and the tendon of the tensor tympani muscle.

Through the ossicles, the vibrations of the tympanic membrane are transferred to the footplate of the stapes. (music plays) The stapes moves with a piston-like action, which sends vibrations into a structure called the "bony labyrinth". The labyrinth is filled with a fluid called "perilymph".

If it were a completely closed and inflexible system. . .

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the movement of the stapes would be unable to displace the perilymph. . .

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and therefore, unable to send vibrations into the bony structure. Due to the flexibility of a small membrane called the "round window",. .

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. the stapes' movement can displace the perilymph, allowing vibrations to enter the labyrinth. (music plays) The corridor leading to the round window is found in the spiral portion of the bony labyrinth.

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. . known as the "cochlea".

Vibrations produced by the stapes are drawn into the spiral system. . .

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and return to meet the round window. The portion of the spiral passage in which vibrations ascend to the apex of the cochlea. .

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. is called the "scala vestibuli". The descending portion of the passage is called the "scala tympani".

A third structure, called the "cochlear duct", is situated between the scala vestibuli and the scala tympani. The cochlear duct is filled with a fluid called "endolymph". .

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. and when viewed in cross-section, the membranes separating the two fluid filled systems are visible. They are: "Reissner's membrane".

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. . and the "basilar membrane".

The membranes are flexible, and move in response to the vibrations traveling up the scala vestibuli. The movements of the membranes then send vibrations back down to the scala tympani. A specialized structure, called the "organ of Corti".

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. . is situated on the basilar membrane.

As the basilar membrane vibrates, . . .

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, the organ of Corti is stimulated,. . .

which sends nerve impulses to the brain via the cochlear nerve. The actual nerve impulses are generated by specialized cells within the organ of Corti. .

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. called "hair cells. " The hair cells are closely covered by a structure called the "tectorial membrane".

As the basilar membrane vibrates. . .

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the tiny clusters of hairs are bent against the tectorial membrane. . .

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triggering the hair cells to fire. The entire basilar membrane does not vibrate simultaneously. Instead, specific areas along the basilar membrane move variably.

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. . in response to different frequencies of sound.

Lower frequencies vibrate the basilar membrane closer to the apex of the cochlea. . .

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whereas higher frequencies produce vibrations closer to the base. This arrangement is known as "tonotopic organization". (music plays) Together, this sequence of events is responsible for our acoustic perception of the world around us.