Download Inner Hair Cells

CHAPTER 8
Peripheral Auditory Nervous System
and Haircells
IHC Synapses
OHC Synapses
• Efferents have greater surface area than
afferents
Transduction• Inner Hair Cells are the true sensory
transducers, converting motion of
stereocilia into neurotransmitter release.
Mechanical Electro-chemical
• Outer Hair Cells have both forward and
reverse transduction-Mechanical  Electro-chemical
Mechanical Electro-chemical
Sensory Physiology
• The basic unit of the nervous system
is the Neuron or nerve cell
• Neurons undergo voltage changes– SPONTANEOUSLY
– IN RESPONSE TO STIMULATION
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
Two Descriptors for Neurons
• Afferent (sensory)-- carrying signals
toward the brain
• Efferent (motor) -- carrying signals
from brain to periphery
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
Neural Activity
• Post-synaptic Potentials-- Local, Variable
changes in voltage near synapse
• Action Potentials-- Conducted through axon, “all
or none,” “spike”
• For image of AP’s traveling down an axon:
http://bio.winona.msus.edu/berg/ANIMTNS/actpot.htm
An Action Potential (or Spike)
Action Potentials
• Are generated spontaneously
– At a slow rate by some neurons
– At a faster rate by some neurons
• And occur more frequently with
STIMULATION
• Spike rate increases through a range of
about 30 dB
Spike Rate (APs/sec)
Spike Rate Increases Thru a 30
dB Range
90
80
70
60
50
40
30
20
10
0
Spike Rate
0
5 10 15 20 25 30 35 40 45 50 55 60
Stimulus Level (dB SPL)
Signs of Peripheral Activation
• Otoacoustic Emissions (OAEs)
• Cochlear Potentials
Otoacoustic Emissions
• Low-level sounds produced by the cochlea
and recordable in the external ear canal.
•
•
•
•
Spontaneous
Click-evoked
Distortion Product
Stimulus Frequency
Recording OAEs
Spectrum of Sound in Ear Canal
OAEs: Measures of Cochlear Health
Cochlear Potentials:
• Resting Potentials: voltages which exist
without external stimulation
e.g., Endolymphatic Potential,
Cell Membrane Potential
• Stimulus-Related Potentials: voltages
occurring in response to sounds
We’ll talk about 3 of these from the cochlea
Cochlear Microphonic
• Least valuable from a clinical standpoint.
• Is an alternating current (AC) response that
mirrors the waveform of low to moderately
intense sound stimuli
• Appears to arise from outer hair cells in the
basal-most turn of the cochlea
Summating Potential (SP)
• Is a direct current or
DC potential
• Lasts for duration
of stimulus.
Compound Action Potential
(CAP)
• Summation of APs in large number of
VIIIth nerve neurons
• following onset (and offset) of stimulus
IHC activation alters firing rate
Afferent neurons have their cell
bodies in the Spiral Ganglion (4)