Download 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
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
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
Phase Locking


Auditory nerve fibers fire preferentially during
particular phases of a waveform
Do not fire every time
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
Ascending Pathway
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
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
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)
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
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