Audiologist. Masters Degree in Audiology or AuD Registered with CASLPO, Ontario College Assess hearing in any age. Prescribe hearing aids. Assess and provide therapy for balance disorders, tinnitus and

other pathologies related to the ear and balance system.

Pinna: modify sounds, improves localization abilities, improves speech reception.

Ear Canal: protective function, improves speech reception.

Resonant Frequency of the outer ear-improves speech reception

Ossicular Chain: impedance matching, 27:1 area difference

Movement of ossicles in response to sound is measured in millionth of a nanometer

Physics of the Middle Ear: Mass, Stiffness and Resistance.

Gain and Resonance:14 dB gain, 2700 Hz resonant frequency.

Stapedial Reflex-function?

Impedance matching

Resonant frequency

Cochlea: a small snail-like organ, embedded in petrous part of the temporal bone

Transduction, mechanical to electrical.

30 mm long, 3 coils Fluid filled, encased in bone. Oval and Round Window

“With over a million essential moving parts, the auditory receptor organ, or cochlea, is the most complex mechanical apparatus in the human body “

Hudspeth AJ. The cellular basis of hearing: the biophysics of hair cells.

Science 1985; 230: 745-752

Cells on the BM are arranged tonotopically High frequency responsive cells – base Low frequency responsive cells – apex.

Pressure change induced by sound at oval window creates a pressure wave

Travelling wave moves from base to apex Wave travels 100’s of times slower on BM than

in air Several ms to travel length of cochlea Peak occurs where BM resonates with stimulus

IHC◦ 3500◦ 40 stereocilia per cell◦ each IHC innervates

20 neurons◦ primarily afferent

OHC◦ 14000-16000?◦ 140 stereocilia per

cell◦ each OHC receives

efferent info from 1800 neurons

Contains nerve cells for transduction and support structures

3 rows of OHC 1 row of IHC Receptor cells have a

tonotopic arrangement-high frequency at base, low frequency at apex

2 primary functions◦amplify sounds below

~50 dB HL◦sharpen the peak of

the travelling wavethe OHC modify the

response of the IHC:◦the configuration of

the stimuli on the BM◦cortical input to the

OHCActive Mechanism

K+ poorNa+ rich

K+ richNa+ poor

Active Transport

•Endolymph

•Deflection opens K+

•Depolarization opens Ca+ channel

•Faster than 2nd messenger transduction.

Lewis & Hudspeth, 1983

The IHC are the auditory receptor cells

Deflection of the IHC results in direct mechanical to electrical transduction

40-80 gated ion channels per cilia

IHC deflection determines response

Response occurs at <1º deflection

Loud sounds cause deflection <1 stereocilia width

High spont rate- 18-250 spikes/sec 60 % of neurons Low threshold, (high baseline rate changes are easily detected)

Medium spont rate- 1/2-18 spikes/sec. 25% of neurons

Low spont rate- <1/2 spikes/sec 15% of neurons

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Frequency specific pattern of motion on BM = tuning function.

Series of band pass filters

Using light microscope and stroboscopic illumination, von Bekesy observed BM motion in cadavers (1928, 1942).

Travelling wave and pattern of vibration. Observations predicted a wide bandwidth

for frequency specificity But…psychoacoustically observed

frequency specificity was much better than predicted.

Others suggested an ‘active mechanism’ at work for fine tuning.

In vivo laser inferometry = sharper filter function.

Von Bekesy cadaveric ears high intensity (140 dB)

stimuli

The BM is a non-linear systemBasilar membrane I/Otarget point = 8 kHz

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OHC are easily damaged by excessive noise (mechanical damage) or by chemical and disease processes.

Damage to the OHC means that the cochlea cannot transmit an efficient signal to the brain.

The hair cells for high pitches get the most “traffic” and are usually damaged first.

Typical pattern of a young adult with normal hearing.

Hearing loss occurs at different pitches.

A person can (and often does) have good hearing in the low pitches and poor hearing in the high pitches.

Cochlear ‘carpet’ is wearing out!

Axons from the spiral ganglion synapse with the superior olivary nuclei at the level of the medulla.

This is the primary ipsilateral synapse.

Most nerve fibres continue contralaterally to the Inferior Colliculus, to the MGN in the Thalumus.

The fine frequency discrimination of the cochlea depends on a healthy organism

Damage to OHC quickly results in loss of sensitivity and discrimination

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Cx26 codes for a gap junction protein called connexin 26.

Gap junctions are plasma membrane channels that allow the movement of small molecules and ions between adjacent cells.

Gap junctions of the inner ear may play a role in maintaining potassium homeostasis, which is important for inner-ear function and, thus, hearing.

Mutations in Cx26 have been indicated in the disruption potassium circulation resulting in deafness.

Connexin 26 is expressed in the stria vascularis, spiral ligament, and between the supporting cells in the cochlea

Recycling of potassium that is used by the hair cells to generate an action.

Prevalence of hearing loss: 1-3/1000

50% of these are genetic of which 70% can be attributed to Cx26

Carrier rate is as high as 1/36