Physiology · MU Sofia

86. Auditory system. Sound transduction. Central processing of the auditory information. Encoding of sound frequency, loudness and localization

Covers how sound waves are converted into electrical signals in the Organ of Corti, following the steps from basilar membrane vibration through stereocilia bending, ion channel opening, the cochlear microphonic potential and glutamate release. It then traces the relays from the cochlear nuclei through the superior olivary complex, inferior colliculus and medial geniculate nucleus to the auditory cortex, and ends with how frequency, loudness and sound direction are encoded, including interaural time and intensity cues.

Inside: 5 sections · 4 labelled pictures · 7 flashcards · 6 min

Topic 85 of 104 in the MU Sofia syllabus for Physiology (Dental Medicine, Year 2). The free AceMed note covers it in 5 sections, about 810 words (6 min of reading), with 7 flashcards.

In this note

  1. Auditory system. Sound transduction. Central processing of the auditory information. Encoding of sound frequency, loudness and localization.
  2. Central Processing of Auditory Information
  3. Tonotopic Organization:
  4. Encoding of Sound Frequency, Loudness, and Localization
  5. Recap Questions

Auditory system. Sound transduction. Central processing of the auditory information. Encoding of sound frequency, loudness and localization.

Auditory transduction is the process of converting sound waves, which are mechanical vibrations in the air, into electrical signals that can be transmitted to the brain. This process occurs in the Organ of Corti, located within the cochlea of the inner ear.

Here's how sound transduction works:

  1. 1. Vibration of the Organ of Corti: Sound waves entering the ear cause the tympanic membrane to vibrate, which sets the ossicles in motion. The stapes footplate pushes on the oval window, generating pressure waves in the perilymph of the scala vestibuli. These pressure waves propagate through the cochlea, causing the basilar membrane to vibrate.
  1. 2. Bending of Hair Cell Stereocilia: The Organ of Corti contains sensory hair cells, which have hair-like projections called stereocilia on their apical surfaces. The stereocilia are embedded in the tectorial membrane, a gelatinous structure that sits atop the hair cells. When the basilar membrane vibrates, the stereocilia are deflected against the tectorial membrane.
  2. 3. Opening of Mechanosensitive Ion Channels: The bending of the stereocilia causes mechanosensitive ion channels in the hair cell membranes to open. The endolymph, the fluid bathing the stereocilia, has a high potassium concentration (K+). When the channels open, K+ ions flow into the hair cells, leading to depolarization.
  3. 4. Cochlear Microphonic Potential: The oscillating receptor potential in the hair cells, caused by the alternating depolarization and hyperpolarization, is known as the cochlear microphonic potential.
  4. 5. Glutamate Release and Activation of Cochlear Nerve Fibers: The depolarization of the hair cells opens voltage-gated calcium channels (Ca2+), allowing Ca2+ influx into the presynaptic terminals. This triggers the release of the neurotransmitter

glutamate, which binds to receptors on the afferent cochlear nerve fibers. The activation of these fibers generates action potentials that transmit the auditory information to the brain.

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