HAIR CELL FUNCTION & PURPOSE

• The Organ of Corti It rests on top of the basilar membrane. It sets inside the scala media (cochlear duct) and contains the inner and outer hair cells and of course the tectorial membrane.

HAIR CELL FUNCTION & PURPOSE

•The Organ of Corti•This is where the electrical “language” is transmitted between the brain and received sound stimulation.

                                                     

Note how embedded

OHCs actually pull tectorial membrane

down

Outer Hair Cells: The Active Cochlear Mechanism

HAIR CELL FUNCTION & PURPOSE

•Outer Hair Cells• There are approximately 12,000 outer hair cells. They have the appearance of test tubes and mostly receive efferent information from the brain.

HAIR CELL FUNCTION & PURPOSE

• Outer Hair Cells• Not only do they react to information received from the brain but they also receive “chemical messages” from inside the cochlea which tell them to either elongate or shrink (like pistons).

HAIR CELL FUNCTION & PURPOSE

•Outer Hair Cells• The net effect of their mechanical action is to change the mechanical properties of the basilar membrane at specific spots.

BaseApex

Basilar Membrane

Outer Hair Cells Sharpen the Peak!They are the “muscles” of the cochleaThey usually get damaged first

Lows Highs

HAIR CELL FUNCTION & PURPOSE

• Outer Hair Cells• They play a very significant role in the reception of soft sounds.•Without outer hair cells, we would have approximately forty to sixty decibels of sensorineural hearing loss.

Here’s a “Passive” Traveling Wave

BaseApex

Basilar Membrane

A wave without outer hair cells

Fig 1-4, Venema, T. Compression for Clinicians 2nd edition, Thomson Delmar Learning 2006

Outer Hair Cell Contributions to the Traveling Wave

1. Amplify2. Sharpen

BasilarMembrane Displacement

BasilarMembrane

TravelingWave

Envelope ofTraveling Wave

OHCsSharpenPeak

Fig 1-5, Venema, T. Compression for Clinicians 2nd edition, Thomson Delmar Learning 2006

HAIR CELL FUNCTION & PURPOSE

• In summary, outer hair cells have a twofold purpose:

1. They amplify soft sounds (below forty to sixty decibels).

2. They “fine-tune” the frequency resolution of the basilar membrane.

HAIR CELL FUNCTION & PURPOSE

• Inner Hair Cells• There are approximately 3,000 inner hair cells which have a pear or flask shape. They mostly send afferent information to the brain via the eighth cranial nerve.

HAIR CELL FUNCTION & PURPOSE

• Inner hair cells•More specifically, the 3,000 hair cells stimulate approximately 30,000 eighth cranial nerve fibers. This occurs from each cochlea.

Hair Cells: A Closer Look

Inner hair cells

Outer hair cells

Picture from Australian HearingHear & Now, issue 4, 1998

HAIR CELL FUNCTION & PURPOSE

•Hearing instruments cannot sharpen the peaks of the traveling wave. They will only increase the amplitude (size) of the wave. •With more outer hair cells missing less resolution may be received by the brain—especially when noise is introduced.

Normal Inner

& Outer Hair Cells

Fig 1-7, Venema, T. Compression for Clinicians 2nd edition, Thomson Delmar Learning 2006

Damaged

Hair Cells

(mostlyouter)

Fig 1-8, Venema, T. Compression for Clinicians 2nd edition, Thomson Delmar Learning 2006

HAIR CELL FUNCTION & PURPOSE

•We will discuss more next week regarding reduced hair cell function and hearing instrument performance.