Neurophysiology
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Neurons
Gross Anatomy
The Central Auditory Nervous System
Frequency and Intensity encoding
Central Auditory Processing
Binaural Processing
Central Control (Descending Systems)
Cortical Processing
The Neuron
• Dendrites receive synaptic stimulation (neurotrans.)
• Action Potential generated in soma near axon
• AP conducted along axon from Node to Node
(saltatory conduction)
• AP produces release of neurotransmitter at terminal
boutons
An Action Potential (or Spike)
Two Descriptors for Neurons
• Afferent (sensory)-- carrying signals
toward the brain
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• Efferent (motor) -- carrying signals
from brain to periphery
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Afferent & Efferent Neurons
4 Types of Cochlear Neurons
• INNER HAIR CELLS
> Multiple (10 to 20) Afferent synapses
> (Efferents synapse on afferent dendrites)
• OUTER HAIR CELLS:
> Large Efferent synapses engulf base of cell
> Small (& not very active) Afferent synapses
IHC Innervation Pattern
OHC Innervation
Pattern
Inner hair cells
• Synapse at the base
with up to 20 afferent
neurons
• “Divergence”
• Efferents synapse on
afferent dendrites
under IHCs
IHC activation alters firing rate
Afferent neurons have their cell
bodies in the Spiral Ganglion (4)
Central Nervous System
Structures
• Nucleus = a group of nerve cell bodies
• Fiber Tract = a group of axons
Major Components of the Central
Auditory Nervous System (CANS)
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VIIIth cranial nerve
Cochlear Nucleus
Superior Olivary Complex
Lateral Lemniscus
Inferior Colliculus
Medial Geniculate Body
Primary Auditory Cortex
Brainstem
Mid-brain
Thalamus
Temporal Lobe
Mid-Saggital View of Brain
4th Ventricle
Corpus
Callosum
Cerebellum
Thalamus
Pons
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MedGen Body
Inf Coll
Lat Lemn
SOC
Coch Nuc
VIIIth CN
Neural Web-Site
http://rprcsgi.rprc.washington.edu/
neuronames/hierarchy.html
Section Thru Brainstem Shows
Cochlear Nucleus
The Superior Olivary Complex
Connections To the Superior
Olivary Complex
Superior Olivary Processing
Supports Localization
• Lateral SO-- Interaural Intensity Differences
• Medial SO-- Interaural Time Differences
(These are the two primary acoustic cues for
localizing sounds)
Dorsal (back) Side of Brainstem
• Thalamus
(medial geniculate)
• Inferior Colliculus
• 4th Ventricle
• Area of Pons
Inferior Colliculus
Thalamus in Purple
Auditory Radiations Connect
• Medial Geniculate
Body (in purple)
to
• Primary Auditory
Cortex (in blue)
Lateral-Superior view of brain
Primary Auditory Cortex (AI):
superior surface of the temporal lobe
Brain Photos Web-Sites
http://rpiwww.mdacc.tmc.edu:80/se/anatomy
/brain/
http://www.ets.uidaho.edu/med532/start.htm
Neurophysiological Measures
• Gross Evoked Potentials-- Voltage changes
in response to auditory stimulation recorded
from the scalp
• Single-Unit Measures-- Voltage (or other)
changes recorded within a neuron
Auditory Evoked Potentials
• Recorded in different time intervals
(“epochs”) following a sound
• Earlier epochs come from lower in the
system
• Later epochs come from higher in the
system
Examples of AEP Epochs
• Electrocochleography-- within 5 milliseconds
• Auditory Brainstem Response-- thru 10 ms
• Middle Latency Response-- thru 75 ms
• Auditory Late Response-- thru 200 ms
Auditory Brainstem Response
IV
Amplitude (V)
III
I
V
II
Amp V
Wave V Latency
Time (ms)
0
10
GENERATORS of ABR WAVES
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III
IV
V
= Distal VIIIth nerve
= Medial VIIIth nerve
= Cochlear Nucleus
= Superior Olivary Complex
= Lateral Lemniscus &
Inferior Colliculus
Single-Unit Measures
• Post-Stimulus Time Histogram-- Shows
firing rate changes over time
• Period or Interval Histograms-- Show
phase-locking of neural firing
Tuning Curves
• Iso-Rate Function -- Shape similar to what
we’ve already described (Fig 6.12 b)
• Iso-level Function -- Shows spike rate as a
function of frequency-- peak at a single
frequency (Fig 6.12a)
Two-Tone Suppression
• The response to one tone can be reduced or
eliminated by introducing a second tone
near the neuron’s CF.
• (Fig 6.16)
• Second tone can be either one which
normally would excite the neuron or not
Two-tone Suppression
Regular
Tuning
Curve
Frequency Coding
• The Place Code-- each neuron has a
characteristic frequency
• Periodicity Pitch-- neurons phase-lock to
stimuli
Intensity Coding
• Firing rate increases in single neurons
• Spread of activation to a wider range of
neurons-- “Density of Discharges”
• Latency of Firing (shorter delay at higher
levels)
Efferent (Descending) Control
• Cochlear Efferents come from Superior
Olivary Complex --The Olivo-Cochlear
Bundle (OCB)
• Uncrossed OCB-- synapses on dendrites
under inner hair cells
• Crossed OCB-- synapses on outer hair cells
• Both use inhibitory neurotransmitters
Uncrossed OCB-- synapses
on dendrites under inner
hair cells
Crossed OCB-synapses on
outer hair cells
Efferent Control (cont’d)
• The Acoustic Reflex
• Auditory Cortex and Thalamus also send
descending fibers to auditory brainstem
locations
The Acoustic Reflex
Afferent:
Efferent:
• VIIIth nerve
• VIIth nerve nucleus
• Cochlear Nucleus
• VIIth nerve
• Superior Olivary
Complex
• Stapedius muscle
Primary Auditory Cortex (AI):
superior surface of the temporal lobe
6 Cortical Layers
• Thalamic inputs >IV
• project to pyramidal cells
in layer III
• Divergence from III
– within AI
– other cortical areas
– contra AI
• V and VI >>thalamus &IC
Cortical Neurons
• Tonotopically and Spatiotopically organized
• Highly Adaptable
• Sensitive to CHANGES in Frequency and
Intensity
– Coding virtual pitch
– demodulating complex signals (e.g. speech)
Cortical Processing
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Pattern Recognition
Duration Discrimination
Localization of Sounds
Selective Attention
Cerebral Dominance/Laterality
• Language Processing in the left hemisphere.
(Remember the right ear has the strongest
connections to the left hemisphere)
• Most people show a right-ear advantage in
processing linguistic stimuli
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Neurophysiology