7: The Auditory System

In lecture 7 we learned about the auditory system, i.e. how can we hear. So, as commonly known the tiniest bones in our body lie in the ear: the ossicles. From the ossicles, the tiniest one is stapes. Other ossicles are malleus and incus. The ossicles amplify the sound waves coming to the tympanic membrane (eardrum), that are originally collected by the auricle pinna of the outer ear and then channeled into the ear canal, where they end to the tympanic membrane. 

Even though it is commonly said, that the ossicles amplify the sound waves, the main amplification mechanism comes from the size difference between the eardrum and the oval window that connects the middle ear to the inner ear. Through this oval window, the amplified sound waves travel into the cochlea, which is a fluid-filled hearing organ. The Eustachian tube that opens up into the middle ear, equalizes the pressure between outside the ear and within the middle ear.

After the sound waves are in the cochlea, as the stapes moves in and out, the perilymph starts to flow, which then again generates a traveling wave in the basilar membrane. Depending on the frequency of the sound, the traveling waves differ: high-frequency sounds dissipate near the base of the basilar membrane, whereas low-frequency sounds dissipate in the apex of the basilar membrane. One very interesting thing we learned, related to these frequencies, is tonotopy. It is the systematic organization of sound frequencies within the auditory system. The tonotopic maps can be found the basilar membrane, and each auditory relay nuclei, medial geniculate nucleus (MGN) and auditory cortex.

Finally, the auditory receptor cells are located in the organ of Corti. They convert mechanical energy (bending of cilia) into a change in membrane polarization (neural signal). So e.g. when sound causes the basilar membrane to deflect upward, the reticular lamina moves up and cause the stereocilia to bend, which then generates a receptor potential. From the ear (from spiral ganglion), the neural signal travel via to the auditory cortex via numerous pathways, but the primary pathway is through ventral cochleae nucleus, superior olive, inferior colliculus, MGN and as last, auditory cortex.

Alexandra & Alisa

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