Two Halves:Vestibular--transduces motion and pull of gravityCochlear--transduces sound energy

(Both use Hair Cells)

INNER EAR

Subdivision into spaces containing endolymph (blue), and spaces containing perilymph (red)

The Endolymphatic

SacTermination of

vestibular aquaductOutside of temporal

bone; next to dura mater lining of the brain

Thought to maintain endolymphatic volume/pressure

Cochlea is Divided into 3 “Scala”

Scala Vestibuli Reissner’s Membrane

Scala Media Basilar Membrane

Scala Tympani

Helicotrema - the opening between 2 outer Scala

Fluids filling the Inner Ear

Perilymph- in S. Vestibuli and S. Tympani High Sodium / Low Potassium concentrations Low Voltage (0 to +5 mV)

Endolymph- in S. Media High Potassium / Low Sodium concentrations High Positive Voltage (80 mV)

Cross-Section of the Cochlea

Third Turn

Second Turn

First Turn

A Cross Section Shows the 3 Scala

Within S. Media is the Organ of Corti

I = Inner Hair Cells P = Pillar Cells

O = Outer Hair Cells D = Deiter’s Cells

IHCs, OHCs And Their Stereocilia

OHCs (at top) 3, 4 or 5 rows Approx 12,000 cells 10 to 90 microns V- or W-shaped ranks of stereocilia

50 to 150 stereocilia per cell IHC (at bottom)

1 or 2 rows Approx 3,500 cells 35 microns straight line ranks of stereocilia 50 to 70 stereocilia per cell

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-chemicalOuter Hair Cells have both forward and

reverse transduction--

Mechanical Electro-chemical

Mechanical Electro-chemical

Frequency Analysis - the Traveling Wave

Bekesy studied cochleae from cadavers, developed the Traveling Wave theory

1. Response always begins at the base2. Amplitude grows as it travels apically3. Reaches a peak at a point determined by

frequency of the sound4. Vibration then dies out rapidly

Bekesy’s Theory describes Passive Mechanics

Based on work in “dead” cochleaeHighly 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 shapesharp tip (shows best sensitivity at one freq)steep high frequency tailshallow low frequency tail

Tuning Curves

Passive Only

Active + Passive

More on Tuning & Tuning Curves:

Seen for basilar membrane, hair cells, nerve cells

Frequency of “tip” is called the CHARACTERISTIC FREQUENCY

OHC Length and CF

High Freqs Low Freqs

Tectorial Membrane

Hair Cell Activation

Involves Ion Flow into cellThrough channels in the stereocilia

Bending stereocilia causes # of open channels to change.

Toward Modiolus = Fewer channels openAway 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 linksBending 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 cellCalcium flows into cell

Voltage shifts to a less negative value

More neurotransmitter is released

Synapse Basics

Pre-Synaptic cell contains vesicles

Gap between cells is Synaptic Cleft

Post synaptic cell may show darkened area adjacent to membrane

AfferentAfferent & 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 cellsSynapse 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)

An Action Potential (or Spike)

IHC activation alters firing rate

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 Potentials:

Resting Potentials: voltages which exist without external stimulation

e.g., Endolymphatic Potential,

Cell Membrane PotentialStimulus-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 potentialLasts for duration of stimulus.

Compound Action Potential (CAP)

Summation of APs in large number of VIIIth nerve neurons

following onset (and offset) of stimulus

Ele

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The

SP

/AP

Rat

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