Lecture 5

Question 1

In the cochlea, what are the resting potentials of the various parts?
What is the resting potential of the hair cells?
What is the driving force for fast transduction of energy?

Answer 1

Endolymph = +80mV (due to stria vascularis)
Perilymph = 0mV
Perilymph = +2-5mV
Inner Hair Cells = +40mV
Outer Hair cells = -70mV

Adding +80mV + -70mV = 150mV

Question 2

What is the process that occurs in the endolymph for transduction?

Answer 2

Process is to pump potassium (+) in and sodium is pumped out (-). This uses ATP and a lot of energy to do so.
From the capillary, the minerals (K+) and glucose,, oxygen go into the gradient. Potassium goes into the endolymph and out through the channels (gate), and returns to the blood or interstitial fluid in the stria vascularis - this creates a gradient that allows potassium to be pumped into the cell.
More potassium is going in than is going out - down its electrical concentration gradient - semipermeable channel, because is has the ability to conduct ions across it.

Question 3

What is the resting potential for IHC and OHC?

How are these relevant to the movement of potassium?

Answer 3

IHC = -40mV
OHC = -60--70mV

Inside the cell has the -60/-70mV resting state. Amino acids with negative charge stay in there, and ions flow in and out freely, leftovers are the negative ions. The K+ concentration gradient is filtering these, moving from high-low. The electrical potential pushes it in as well.
More potassium at the top - going down the gradient - chemical and electrical forcing it.
-40mV (more coming in, less coming out - IHC)

Question 4

What is the process generally for the sodium potassium pump?

Answer 4

BM moves up and down (sinusoidally), causing movement in the stereocilia of HC, HC move causing tip links to stretch, opening ion channels.
Potassium moves down its concentration gradient and goes into the cell. This sets up the concentration gradient inside the cell - passively leaks into endolymph (+).
Positive potassium ions inside the hair cell (was -70mV, now putting in + charged ions) raising it up causing depolarization.
The amount of depolarization depends on how far bent the stereocilia are.
This creates receptive potentials.
Once the cells go back to rest, hyperpolarization occurs.

Same mechanism for IHC and OHC.

Question 5

In hair cell stimulation, what are the frequencies of the action potentials?

Answer 5

Nerve fibre depolarizes = higher frequency of action potential
Nerve fibre hyperpolarizes = lower frequency of action potential.

Question 6

In the compartments of the cochlea, why is the endolymph positive?

Answer 6

Reissner’s membrane (between perilymph and endolymph) is slightly leaky to calcium. Inside hair cel is -70mV.

Question 7

Why is calcium necessary for transduction?

Answer 7

Calcium is a catalyst for the system. The protein structure then changes its conformation. Need calcium in the cell, otherwise tip links do not open very well.

Question 8

When hair cells are bent in different directions, what does this do for transduction?

Answer 8

When hairs are bent towards the tallest stereocilium, the cell’s voltage is increase, more NT is released and auditory nerves connected to HC increase firing rate (depolarization).
When hairs are bent away from the tallest stereocilium, the cell’s voltage is decreased (hyperpolarized). Less NT is released and auditory nerves connected to HC decrease their firing rate.

Question 9

What occurs in the hyperpolarization stage?

Answer 9

Reduction in positive ions inside the cell, less NT is release - action potentials down in frequency. Hairs bent away from the tallest stereocilium.