The Auditory Nervous System Classical Ascending Pathway

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

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

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

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

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

Ascending Pathway

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

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

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

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

Summary Auditory nerve fibers preserve spatial and temporal coding of sensory input 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 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 Next, focus on integration of information in the higher brain centers