SLHS 302 Exam 2: Hair cells and cochlear potential

< 1 nanometer

Softest sounds we can hear moves the tips of hair cells by

depolarize

cell becomes more positive because positive ions flow in and negative ions flow out

hyperpolarize

cell becomes more negative because negative ions flow in (positive ions flow out)

Electrical potential

the amount of energy needed to move a charge from one place to another

Potential energy

energy held by an object because of its position relative to other objects

Electric current

a flow of electrical charge capable of carrying energy from one place to another measured in Amps

voltage

a measure of the electrical force or pressure that causes current flow and typically supplied by a batter measured in Volts

Alternating current (AC)

electric current reverses its direction of flow several times each second and used to refer to alternating voltage responses

Direct current (DC)

electric current flows in one direction only and often used to refer to steady voltages

Endocochlear potential

+80 mV

Scala vestibuli

+5 mV; perilymph

scala media

+80 mV, endolymph

Stria vascularis

the battery that helps maintain the endocochlear potential; supplies potassium, nutureitnts and oxygen to cochlear duct

OHCs potential

-60 mV

IHCs potential

-40 mV

scala tympani

0 mV, perilymph

cochlear potentials

Hair cells and auditory nerve create biochemical-electrical potentials

cochlear potentials

Relies on concentrations and flow of potassium and sodium

Hair cell receptor potential

negative potential within hair cells (relies mainly on the flow of potassium)

AC potentials

change as a function of the vibrating tissue in the cochlea

Cochlear potentials

Motions and interactions of cochlear structures modulate electric potentials

Cochlear potentials

Low frequencies; low-pass filtering effects of hair-cell membrane

Single cell potentials

voltage inside a single cell

~-60 mV

DC potential in absence of stimulation

Gross potentials

combined electrical activity from many individual cells (summed currents)

Gross potentials

Can be measured clinically

Summating potential

combined DC shift created from different potentials (multiple IHCs and OHCs) that interact with one another

Cochlear microphonic (CM)

combined AC response from multiple OHCs, which follows the acoustic stimulus fairly well

Compound action potential (CAP)

neural response generated from the sum of action potentials from many individual auditory neurons that are firing in synchrony

Compound action potential (CAP)

Dominated by basal fibers (fire synchronously)

Compound action potential (CAP)

Occurs at onset and offset because many neurons fire together

Compound action potential (CAP)

Reflects grossly the neural output of the cochlear as information is being sent along the auditory nerve bundle

Compound action potential (CAP)

Can be used to access cochlear function in people

Otoacoustic emissions (OAEs)

use a microphone in the ear canal to record sounds that are different when what you put in (or absence of sound)

OAEs

sounds produced by the ear

OAEs

Non-invasive measure of cochlear function (widely used for newborn hearing screening)

OAEs

Related to outer hair cell function

OAEs

Produced in the cochlear by OHCs

OAEs

OHCs provide mechanical energy to the basilar membrane at low sound levels

OAEs

acoustic

Types of OAEs

Spontaneous, stimulus-frequency, distorotion-product transient evoked,

Spontaneous OAEs (SOAEs)

Input: no sound

SOAEs

Emission: energy at particular frequencies

SOAEs

Benefit: presence suggests no gross cochlear pathology

SOAEs

Disadvantage: absence does not indicate much about health of cochlea

SOAEs

Observed in most healthy ears up to 18 years

SOAEs

Absence does not mean impaired hearing(not good for diagnostics)

SOAEs

Quiet sounds coming out of ears when no sound goes in

Stimulus-frequency OAEs

Input: long-duration tone

Stimulus-frequency OAEs

Emission: energy at same frequency

Stimulus-frequency OAEs

Benefit: place specific on BM

Stimulus-frequency OAEs

Disadvantage: hard to separate emission from stimulus (not used clinically yet)

Transient evoked OAEs

Input: click

Transient evoked OAEs

Emission: energy at many frequencies

Transient evoked OAEs

Disadvantage: not place specific on BM

Transient evoked OAEs

Benefit: easy to separate emission from stimulus in time

Distortion-product OAEs (DPOAEs)

Input: two long-duration tones (f1 > f2)

Distortion-product OAEs (DPOAEs)

Emission: energy at new frequency (2f1-f2)

Distortion-product OAEs (DPOAEs)

Benefit: easy to separate emission from stimulus in frequency

Distortion-product OAEs (DPOAEs)

Disadvantage: several sources

Distortion-product OAEs (DPOAEs)

Only emissions above the “noise floor” can be seen in analysis

Distortion-product OAEs (DPOAEs)

Background noise limits what sound can be recorded

Distortion-product OAEs (DPOAEs)

Most commonly used in clinic

Distortion-product OAEs (DPOAEs)

Indicate the entire peripheral auditory system is functioning

Distortion-product OAEs (DPOAEs)

None of these (and no damage to outer and middle ear) suggest hair cell or cochlea damage

Transtympanic ECochG

invasive; involves passing a needle through the ear drum to rest on exterior of the cochlea

Extratympanic ECochG

approaches can be done in clinic; foam plug covered in gold foil or electrode resting on ear drum

Synapse

point at which an electrochemical potential passes from one cell to another

~10-20

synapses per IHC

Normal HCs

excellent sensitivity and sharp tuning

Damaged OHCs

loss of sensitivity and broadened tuning

Damaged IHCs

loss of sensitivity without broadened tuning

Consequences of OHC loss

cochlear amplifier cannot work for frequencies corresponding to that place of damage

Consequences of OHC loss

up to 60 dB loss of hearing sensitivity

Loss of frequency selectivity

sounds with specific frequencies are muffled or distorted even after volume is turned up to overcome hearing loss

Consequences of Inner Hair Cell Loss

absence of sensitivity at that place (“dead region”)

IHC loss

Severe to profound hearing loss: any loss greater than 60 dB 

Distortion or abnormal tonal perception

amplifying a region with damaged or dead IHCs associated

Damage to IHC synapses

neural loss is first and hair cell loss is second with noise exposure and aging

Cochlear Synaptopathy

Single noise overdose (or aging) can cause immediate functional disconnection of auditory nerve fibers from inner hair cells which remain intact

Cochlear Synaptopathy

Permanent loss occurs more slowly

Cochlear Synaptopathy

Noise may or may not cause a temporary hearing loss

damage to the stria vascularis

common with age-related hearing loss

presbycusis

age-related hearing loss

damage to the stria vascularis

Reduce the transduction battery that powers both IHC and OHC function

damage to the stria vascularis

Causes progressive IHC and OHC dysfunction and loss of hearing sensitivity