PLEASE PRINT OUT THESE PAGES FIRST AND THEN KEEP THE PRINTED TEXT BESIDE
YOU AS A GUIDE WHEN YOU LOAD AND RUN "NEURON". THIS WILL SAVE YOU HAVING
TO JUGGLE BETWEEN NEURON AND NETSCAPE WINDOWS ONCE YOU HAVE THE SIMULATION
LOADED.
To begin working with this chapter you should have downloaded and installed
Neuron, as described in Chapter 1.
-
For this simulation, you will need to load the same
simulation as you used for Exercise 2 that shows the behavior
of a patch of membrane. This simulation allows you to observe currents
flowing through the voltage-dependent sodium and potassium channels as
well as to change the values of the various conductances and Nernst potentials.
-
This time, though, once loaded, click on "Vclamp" (NOT "IClamp")
in the bottom middle window to bring up the stimulation parameters
for voltage clamp conditions.
-
Click on "Init & Run" in the bottom right-hand box to test the program.
A voltage clamp simulation will be run. If you look at the Graph of membrane
potential, you will observe that the action potential, seen in the current
clamp simulations, has been replaced by a square upward "step" of voltage
from -65 mV at the very beginning of the trace to +10mV. This step was
commanded by the voltage clamp commands detailed in Box E. Click
here to read a description of the various Windows.
-
If you want to refresh your memory about the meaning of voltage clamp recording,
refer to the section in Chapter 3
- use the Back command in your Browser to return to this page.
-
Test the hypotheses below, draw appropriate graphs (yes, you
have to decide this time what to plot against what to illustrate your report)
and answer the related questions: If you want an example
of how to write up your report, refer to
Chapter 4 - use the Back command in your Browser to return to this
page.
Hypotheses to be tested and related observations to be made
-
Test the hypothesis that the current through the voltage-dependent sodium
channels reverses when membrane potential exceeds the Nernst potential
for sodium ions.
-
Describe the dependence of the maximal sodium conductance on the testing
level of membrane potential
-
Test the hypothesis that prior depolarization inactivates voltage-gated
sodium channels (Hint 1: Looking at the behavior of the sodium channels
is much easier if you set the potassium conductance to zero. Hint 2 : You
can change the level and duration of the conditioning level in the Stimulus
Parameter Window, a long duration of the conditioning level at a slightly
depolarized level may be useful for showing inactivation).
-
Test the hypothesis that the potassium channels DO NOT inactivate following
prior depolarization.
When you have loaded the simulation you should see Windows arranged
as below. Click on the following letters: A B
C D E F
G for a description of the function of each Window.
Return to Instructions
A. Graph of the membrane potential of the patch of neuronal membrane
(mV) vs. time (ms). Very soon after the beginning of the trace, the membrane
potential is stepped by the voltage clamp circuit from a "Conditioning
Level" of -65 mV to a "Testing Level" of +10 mV (see Window E)
Return to letters
B. Graph of the total current (nA) passing through the voltage clamp
circuit into the cell. Equal to zero under current clamp conditions! The
current has three components.
1. A
spike of outward capacitive current arising from the movement of charge
across the membrane when the membrane potential is suddenly shifted from
-65 mV to +10 mV. - This current is not physiologically meaningful
- it appears because of the suddenness of the commanded voltage step (remember
that im=Cm(dVm/dt).
2. Current flowing into the neuron through voltage-gated sodium
channels
3. Current flowing out of the neuron through voltage-gated potassium
channels
Return to letters
C. The sodium current (green) and potassium current (blue) per unit
membrane area (in nA/cm2) passing through the voltage-dependent
sodium and potassium channels. Inward current is down (NEGATIVE), outward
current is up (POSITIVE). Together, these currents, when summed, contribute
to the total current trace in Window B.
Return to letters
D. The sodium and potassium conductance arising from the activation
of the voltage-dependent channels (in mho/cm2).
Return to letters
E. Voltage clamp stimulation parameters - the amplitude etc. of the
current pulse delivered through the current electrode to excite the action
potential. Membrane potential is stepped by the voltage clamp circuit from
a "Conditioning Level" to a "Testing Level" (see Window E)
Return to letters
F. The parameters of the sodium and potation conductances:
gnabar_hh = maximal value of potassium conductance (proportional
to the density of sodium channels)
gkbar_hh = maximal value of potassium conductance (proportional
to the density of sodium channels)
ena - Nernst potential for sodium
ek - Nernst potential for potassium
Return to letters
G. The Run/Control Box - click on Init & Run to run a simulation
Return to letters
