Cochlear Physiology II

Shearing force

bending of the stereocilia

Upward displacement of BM

stereocilia bending outwards (towards the kinocilum)

Depolarization

excitation

Downward displacement of BM

Stereocilia bending inwards (away from kinocilum)

Hyperpolarization

Inhibition 

T or F Hair cell at rest is normally polarized

T

Endocochlear potential

endolymph +80 mV

Intra-cellular potential of hair cells

IHCs: -40 to -50 mV

OHCs: -70 mV

Depolarization process

Stereocilia bend towards the kinoclium

Gated channel opens

K+ enters cell making it less negative

Excitation

IHC: release neurotransmitter that excites the auditory nerve fiber

OHC: electrically induced motion that causes OHCs to contract

Hyperpolarization process

Stereocilia bend towards the shortest one (away from kinocilum)

Channel closes

Inhibition

IHCs: decreased frequency of firing of aud nerve fibers

OHCs: electrically induced motion that causes OHCs to expand

Cochlear microphonic

Alternating current

Predominately generated by the OHCs

Occurs only during the presentation of acoustic stimuli

Reflects the intensity and frequency components of the sounds input

Summating potential

Direct current

Sum of the potential of the hair cells

Predominately generated by the IHCs

Shift in baseline potential when stimulus is present  

Compound action potential

Short alternating current

Produced by spiral ganglion neurons

The sum of the action potentials of many individual auditory neurons that are firing nearly simultaneously within the bundle of auditory nerves

IHC Aud biological transducer

Shearing force caused by the pressure waves of sound displaces the stereocilia

Ion channels open

K+ ions flow into the cell, causing depolarization

IHCs release glutamate which reaches the nerve fibers and transmit signals to the brain

The role of prestin

Motor protein in the OHCs that changes the length of the cell in response to transmembrane voltage

Depolarization: contract

Hyperpolarization: expand

Cochlear amplifier

OHC

Provides better frequency selectivity

OHC depolarization produces an OHC contraction, pulling the reticular lamina and BM closer together. The upward pull of the BM add energy and amplify the traveling wave

Tuning curve

Gives frequency and intensity information

Tells you frequency specificity of the BM

Becomes less accurate when there is OHC loss

Steps for auditory transduction

1. Vibrating object creates an acoustic sound

2. outer ear: pinna and EAC provide filtering and gain of sound

3. Middle ear: provides impedance match and further gain before the input enters the fluid-filled cochlea

4. Inner ear: transverse movement of the BM causes shearing motion of the tectorial membrane

5. Inner ear: displacement of stereocilia opens MET channels, resulting a graded electrical potential across the body of the hair cell (IHC release neurotransmitter, OHC contraction motion amplifies low level sounds and increase frequency selectivity)

6. graded potential exceeds threshold of auditory nerve fiber and creates an action potential

7. auditory nerve bundles encode the information and carry it to the higher nuclei