BME 5401/BME4409 Homework 2.
Due Sept 19 in class
PART A BOX your
answer
Consider a cell with the following ion concentrations (mM): [Na]
out
=
118, [Na]
in
=
15, [K]
out
=
9,
[K]
in
=
150. Compute:
1. Nernst potential for Na (E
Na
)
2. Nernst potential for K (E
K
)
Using the classic electrical circuit model for the passive Na and K membrane channels with R
Na
=
3.5 X 10
8
ohms and R
K
=
2.0 x 10
5
ohms and the Nernst potentials from Questions 1 and 2,
compute:
3. leakage current
4. membrane potential
PART B Box your answers
Download the HodgkinHuxley Simulator from:
http://www.cs.cmu.edu/~dst/HHsim/
.
(Downloading Matlab code will create folder "hhsim_31_source". Run Matlab with this as the
current directory).
Click on the Channels button in the main window to call up the Channels window. In the
Channels window, turn off all the channels except the first one (passive sodium), by clicking on
their respective buttons. Notice that the resting potential
Vr
displayed in the Membrane window
is now equal to the Nernst potential for sodium, 52.4 mV. A mouse click on the red line will show
you the exact value of the membrane voltage. The membrane voltage may not be exactly equal
to V
r,
but clicking the yellow
Nudge
button a few times will nudge things along until the voltage
reaches the theoretical asymptote. Or you can click
Run
, wait a while, and then click
Stop
.
1.
Calculate the effect of halving the external sodium concentration (still only sodium
channels on). Use the simulator to confirm your answer by looking at the value of passive
Vr
in the Membrane window. (You can type new values into any of the boxes in the
Membrane window, or use the
<
or
>
buttons to change the values.)
2.
Calculate the [Na] external concentration required to achieve
Vr =
55.5 mV. Confirm your
answer using the simulator.
3.
Suppose we double the temperature from 6.3 deg
C to 12.6 deg
C. What is the new value
of V
r
?
Explain why
Vr
doesn't double.
4.
Reset the simulator by typing "hhsim" again in the MatJab command window. Click the
"Stim1" button to deliver a depolarizing current and elicit an action potential. Describe the
actionpotential:
(a)
what's the peak voltage?
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 Fall '10
 Johannesvanoostrom
 Potassium channel, R Na, Stiml

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