Lecture 6 Flashcards
What are cochlear potentials and how to we measure them?
Take animal, open up temporal lobe and put microelectrode into the scala media, the other goes into the scala tympani.
Need voltage and ground (mV) to measure potential difference
Can also put electrode in bone
Cochlear Potentials: what are the
summating potentials?
From a slow potential that happens because the HC are depolarizing/hyperpolarizing repeatedly.
Direct current potential - constant shift in baseline voltage. Shift in either direction possible
Cochlear Potentials: what are the cochlear microphonic potentials?
Steady state frequency following a response; follows the sound/oscillation of stereocilia hyperpolarizing and depolarizing, response at that frequency.
Stereocilia move at the frequency put in.
Alternating current; follows stimulus cycle-by-cycle both in frequency and relative level for moderate levels.
Source = OHC
Is a sinusoidal response - direct reflection of sound input (BM displacement, showing travelling wave)
Cochlear Potentials: what are the compound action potentials?
Collection of action potentials in the nerve fibres firing simultaneously.
Action potentials are bound to frequency.
Synchrony (not all the time) likely from neurons near base of cochlea and early in stimulus - neurons serving more apical areas stimulated at different times.
Sound on - BM deflects, NT dumped into nerve fibres, all bind to receptors, open up sodium channels = nerve fibres firing almost at onset of stimulus.
How is it that the stereocilia are mechano-sensitive?
Endolymph in scala media is K+ rich, and is +80mV re the perilymph in the scala tympani and scala vestibuli. Time dependent modulation of membrane potential is caused by opening of ion channels on stereocilia.
Bundle deflection through viscous fluid stretches the tip links which increases channel opening probability.
Intra-cellular potential changes cause release of NT and firing in afferent neurons servicing the IHC.
Why is recorded CM primarily due to OHCs?
3:1 (OHC: IHC). Stereocilia stick out the top, and tallest tips of stereocilia of OHC are embedded in tectorial membrane.
OHC change the mechanical properties of the tectorial membrane to sharpen the frequency response.
Recording from endocochlear space as K+ comes in, space becomes more negative, stereocilia tilt back, K+ coming back through system and goes back to being positive (+80, +60, +80, +60).
Oscillation following BM movement because stereocilia as following that as well.
Why can IHC cover a wider range of amplitudes of sound?
Each IHC has about 10 auditory nerve fibres attached to it. Fibres have different diameters, spontaneous rates and different thresholds.
There is a row of IHC along entire BM - each connects to small population of nerve fibres. BM analysis sound into spectral components.
How does measurement of the cochlear/8th nerve potentials work?
Why do this?
Sound put in the ear, measure from cochlea.
Transtympanic - needle through TM, sits on promonotory on cochlea.
Potentials created by electricity of fluid, electrode picks it up. Sound transducer.
Extratympanic - sits on TM, picking up electrical potential on cochlea and on TM.
Do this to find the function of the OHC/IHC
What is the difference in travelling waves when you have OHC vs. no OHC?
What are the two main results?**
SEE COCHLEAR POTENTIALS slides - 20.
In a live cochlea, there are peaks and troughs. Taking small soft sounds and adding energy into BM at right time, get the large deflection - double bounce (x50)
= Gain increases frequency selectivity (representation of pitch). Gain over limited range of frequencies can sharpen frequency selectivity of the membrane
= Gain at low levels results in compression - increased sensitivity to a tone at characteristic frequency (low-level tone produces larger response)
Passive cochlea (no OHC) don’t see this
On the tympanic membrane what is the active region?
Where the OHC are active and responding to sound, adding energy.
They stiffen up the BM (at more basal section) and cause the BM to deflect more. The greatest deflection is the characteristic frequency, vibration of the BM in response to tone serves to sharpen the tuning of the BM.
(Effect restricted to small region of BM around active OHCs, who respond only to stimuli near characteristic frequency of their place).
What is the result of the active mechanism for OHCs?
OHCs more active for low-level sounds than for higher-level sounds.
- Increases sensitivity
- Leads to compressive loudness growth
- Increases frequency selectivity
- Produces otoacoustic emissions
How is the shearing force created for OHC?
When contracted (moving up and down, causing stereocilia to move), it has to do with pivot points where organ of corti has stereocilia embedded, and pivot point for tectorial membrane - responds with delay.
