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The Auditory Nervous System

The Auditory Nervous System. Classical Ascending Pathway. Review: Sensory Transduction. Basilar membrane, organ of Corti, & tectorial membrane Outer hair cells: “cochlear amplifiers” Displacement of stereocilia = depolarization Excitatory neurotransmitter released onto auditory nerve fibers.

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The Auditory Nervous System

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  1. The Auditory Nervous System Classical Ascending Pathway

  2. Review: Sensory Transduction • Basilar membrane, organ of Corti, & tectorial membrane • Outer hair cells: “cochlear amplifiers” • Displacement of stereocilia = depolarization • Excitatory neurotransmitter released onto auditory nerve fibers

  3. Outline • Properties of auditory nerve fibers • Preservation of temporal and spatial coding throughout auditory ascending pathway • Focus on determination of azimuth via ITDs and ILDs in the superior olivary complex

  4. Auditory Nerve Fibers • Receptive fields: each fiber tuned to a specific frequency • Frequency Tuning Curves • Characteristic Frequency (CF): maximal sensitivity • All CFs cover entire audible frequency range

  5. Phase Locking • Auditory nerve fibers fire preferentially during particular phases of a waveform • Do not fire every time

  6. Two-Tone Suppression • One tone lowers response to a second tone • Excitatory response areas surrounded by suppressive response areas • Micromechanical properties of the cochlea: Outer hair cells • Broad frequency range of natural sounds • Gain Control to prevent saturation

  7. Ascending Pathway

  8. Overview of Functions • Cochlear nucleus: parallel processing begins • Superior Olivary Complex (SOC): sensitive to ITDs and ILDs (azimuth) • Inferior Colliculus: convergence of lower brain stem centers • ICC: direct input from cochlear nucleus combines with input from the SOC • Medial Geniculate & Auditory Cortex

  9. Superior Olivary Complex • Medial Superior Olive (MSO): ITDs • Delay lines & coincidence detectors • Composed of neurons with low CFs • Input from spherical bushy cells: primary-like • Low jitter

  10. Giant Synaptic Terminals: Endbulbs & Calyces • Endbulbs of Held in ventral cochlear nucleus • Hundreds of synapses • Calyceal endings in medial nucleus of the trapezoid body (MNTB)

  11. Superior Olivary Complex • Lateral Superior Olive (LSO): ILDs (“sound shadow”) • Ipsilateral = excitatory (spherical bushy cells) • Contralateral = inhibitory (globular bushy cells & synapses with calyceal endings in MNTB) • Composed of neurons with high CFs

  12. Summary • Auditory nerve fibers preserve spatial and temporal coding of sensory input • ITDs for low frequency sounds and ILDs for high frequency sounds are processed in parallel • Next, focus on integration of information in the higher brain centers

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