Cochlear Functions

• Transduction- Converting acoustical-mechanical energy into electro-chemical energy.

• Frequency Analysis-Breaking sound up into its component frequencies

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

Hair Cell Activation

• Involves Ion Flow into cell

• Through channels in the stereocilia

• Bending stereocilia causes # of open channels to change.

• Toward Modiolus = Fewer channels open

• Away from Modiolus = More open

Ion Channels are opened by “TIP LINKS”

• Tip Links connect tip of shorter stereocilia to the side of a stereocilium in the next taller row

• Bending toward taller rows pulls tip links

• Bending toward shorter rows relaxes tip links

Tip Links

Resting (or Membrane) Potentials

• Inner Hair Cell = - 45 mV

• Outer Hair Cell = - 70 mV

Stereocilia bent toward tallest row

• Potassium flows into cell• Calcium flows into cell

• Voltage shifts to a less negative value

• More neurotransmitter is released

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

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 Increases Thru a 30 dB Range

0

10

20

30

40

50

60

70

80

90

0 5 10 15 20 25 30 35 40 45 50 55 60

Stimulus Level (dB SPL)

Spik

e R

ate

(AP

s/se

c)

Spike Rate

Cochlear Mechanics

• Passive: Bekesy’s Traveling Wave Theory

• Active: Outer Hair Cell Motion enhances stimulation of inner hair cells

The Traveling Wave

BASE

APEX

Bekesy’s Theory describes Passive Mechanics

• Based on work in “dead” cochleae

• Highly damped -- not sharply tuned

• Active Undamping occurs in live and healthy cochleae

• Like pumping on a swing--adds amplitude

The Active Component Adds to Bekesy’s Traveling Wave

The Active Component

• Improves Sensitivity for soft sounds

• Improves frequency resolution

Frequency Tuning Curves Show these Effects

= plots of response threshold as a function of frequency

They have a characteristic shape

• sharp tip (shows best sensitivity at one freq)

• steep high frequency tail

• shallow low frequency tail

Tuning Curves

Passive Only

Active + Passive

More on Tuning & Tuning Curves:

• Frequency of “tip” is called the CHARACTERISTIC FREQUENCY

• Can be seen for: basilar membrane,

hair cells, nerve cells

Tonotopic Organization

• Mapping of Characteristic Frequency by place in the auditory nervous system

• In the Auditory portion of cranial nerve VIII

the map is spiral-shaped

with highest CF neurons on the outside,

lowest CF neurons at the center.

Head-Related Effects

• Head-Baffle--the mere presence of your head alters the sound field.

• Head Shadow - loss of energy at far ear for frequencies above approx 2000 Hz

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