Physiology of Hearing

Question 1

The tympanic membrane and the chain of ossicles serve as

Answer 1

An impedance-matching device.
Air medium to fluid medium:
The ear must detect sound waves traveling in air, but the neural transduction mechanism depends on movement in the fluid-filled cochlea, where acoustic impedance is much higher than that of air.

Question 2

Impedance matching in the ear depends on

Answer 2

(1) the ratio of the surface area of the large tympanic membrane to that of the smaller oval window
(2) the mechanical advantage of the lever system formed by the ossicle chain. This impedance matching is sufficient to increase the efficiency of energy transfer by nearly 30 dB in the range of hearing from 300 to 3500 Hz.

Question 3

Perilymph fills

Answer 3

The scala vestibuli and the scala tympani.
Similar to CSF
Low K+ = 2 mM
High Na+ = 140 mV
Potential = 0mV

Question 4

Endolymph fills

Answer 4

The scala media.
High K+ = 145 mM
Low Na+ = 2 mM
Potential = +80mV with repect to perilymph

Question 5

Potential gradient between hair cell and endolymph

Answer 5

+80 mV - (-40 mV) = 120 mV electrochemical gradient
This large electrochemical gradient is important because both the hair cell and the endolymph contain a lot of potassium, so with just potassium alone, there is
not really much of a chemical gradient to drive depolarization. However, because there is still a big charge difference between the two, K+ will be driven to flow from the endolymph into the hair cell, causing it to depolarize.

Question 6

Making of endolymph

Answer 6

Made from the perilymph across the spiral ligament and stria vascularis:
* Na/2Cl/K channel -> K+ is secreted across the spiral ligament and stria vascularis into endolymph in scala media. Na and Cl goes back into perilymph

Question 7

Mechanosensitive channels in the hair cells

Answer 7

TRP - Transient Receptor Potential

Depolarization at -10 mV

Question 8

Mechanism of action

Answer 8

1. Stapes will move outward. This causes the pressure in the scala vestibuli to decrease
   In scala tympani, the pressure will increase
2. Basilar membrane moves upward, causing organ of corti to move towards the tectorial membrane
3. Stereocilia of the outer hair cells are connected to the tectorial membrane, the movement of the tectorial membrane will cause the stereocilia to bend against the longest one.
   Channels will open (TPR) - causing K+ influx, depolarization
4. Contraction of outer hair cells: PRESTIN
   I. Depolarization causes contraction of prestin motor protein
   II. Causes contraction of outer hair cells
   III. Outer hair cells will act as an electromechanical transducer - convert electrical signal into mechanical signal
5. Contraction of the outer hair cell will enhance the upward movement of the basilar membrane = amplifier
6. Endolymph, because of the movement of the basilar membrane, flows out of the inner sulcus
   - This will cause the inner hair cells to be stimulated
   - Stereocilia will bend against the longest one
7. K+ influx, Depolarization of the inner hair cells
8. Opening of VD Calcium Channels: exocytosis of glutamate
9. Afferent neurons are activated and generate A.P