• The second lab exercise simulates an action potential (AP) traveling in an axon which is abnormal due to the disease multiple sclerosis. This axon has lost its myelin wrapping over its left half.

Simulation of the Action Potential

bare axon myelinated axon

• In the first lab exercise, you will observe how the action potential depends on ENa, Ek, and temperature.

First simulation: the stationary action potential

• The text of this tutorial will be displayed on the right. Scroll down to this button and click it.

• Click ”Tutorials" on the home page. Look in the Basic Patch column (on the left) and click the “Na Action Potential” tutorial.

These two control panels will come up (a third is beneath them--ignore it). Don't move these panels--more are coming!

Panel and Graph Manager RunControl

Click "Stimulus Control“ to insert an electrode in the axon.

This menu will come up.

This blue ball shows that you have put an electrode in the axon to inject some positive current and trigger the AP. Now you can ignore this menu.

Click “Voltage vs Time Plot".

This mV vs time graph will come up. You can resize it.

Click "Reset & Run" in the RunControl panel to inject a pulse of current and trigger the AP.

Note on the graph that ENa is represented by the blue line and Ek by the brown line.

Click on “Patch Parameters" in the P&G Manager panel to bring up the menu for doing experiments.

The parameters menu allows you to change the ion concentrations and observe the effect on the AP.

Change the concentration of Na and K in the blood (extracellular) using this menu.

How to change a default value:• Click the up or down arrows

or• Enter a new value in the white space (**make sure your cursor is positioned in the white space).

• Click this box. A red mark will appear indicating the parameter is changed. You can go back to the default by clicking it off.

Now change [Na]out (see Q sheet) and rerun the simulation.

Answer question 1

Note this readout of ENa and EK.

Go back to the original Na concentrations. Now change [K]out and rerun the simulation.

Answer question 2

Change the temperature in the RunControl menu. Try 2, 10, and 20 degrees. What happens to the duration of the AP?

Answer Q. 3.

When you are finished with the “stationary” AP simulation, click Quit in the Panel and Graph manager. (Don’t click the upper right corner X.)

Next you will observe a propagated AP in the “Partial Demyelination” tutorial.

• Scroll down as you did before to this button and click it.

Second simulation: an axon affected by Multiple Sclerosis

• Go to the Axons pull-down menu and select Partial Demyelination. The text of this tutorial will be displayed.

These two panels will come up again, now specialized for the Partial Demyelination simulation.

Panel and Graph Manager RunControl

Get ready to observe the AP traveling from the bare axon, where it travels slowly,

into the myelinated region, where it travels rapidly.

Click on Voltage vs. Space in the P&G Manager.

This graph will come up. Note that mV will be plotted versus µm (space, not time). The x axis is the axon.

resting potential = -65mV

Now get ready to "insert" the stimulating electrode into the axon so that you can inject current to trigger an AP.

current

Click "Stimulus 'Trode in Bare Axon".

This menu will come up.

This line represents the axon

This blue ball shows that the axon will be stimulated at its left end.

Click "Reset & Run" in the RunControl panel to inject a pulse of current and trigger the AP.

The AP should travel along the axon from left to right on the graph. To slow it down, use slider in panel, or pause it:

Pausing the AP: Press "Reset" then "Continue for." This will cause the AP to travel for the time indicated, pause, and travel again when you click. You can set the time interval yourself in the white space.

Important! Controlling the rate of the AP

Click “reset”

Set this time interval to 0.1

Now when you click “Continue for”, the AP will advance in 0.1 ms intervals each time you click.

Stop the AP as instructed on your Work Sheet and answer Questions 4 through 9.

• Now get ready to take the electrode out of the left end of the axon and "insert" it into the right end.

• Follow the next instructions--**Don’t just move the blue ball !!!!

Put your cursor right on the word "close" and click it to close the Stimulus Control panel. (The upper right box doesn't close these panels).

Closing the panel removes the electrode.

Get ready to observe the AP traveling the other direction:

You will now put the electrode in the right-hand end in order to start the AP in the myelinated portion.

Click on "Stimulus Trode in Node [4]".

This will insert a stimulating electrode in the right-hand end.

Return to the P&G Manager.

A new Stimulus Control panel will come up. The blue ball (the electrode) is now at the right-hand end.

Nodes:

0 1 2 3 4

current

What happens?

Click "Reset & Run" in the RunControl panel to trigger the AP.

Answer Question 10.

Next: How can you make the AP propagate into the demyelinated portion of the axon?

Multiple sclerosis patients find that temperature has a strong effect on their condition.

But do they feel better when they are cooler or warmer?

Experiment with changing the temperature.

Change the temperature in the RunControl menu and rerun the simulation.

Answer Questions 11 - 13.

Before you go on to the next section, make sure the temperature is restored to its original value by unchecking its box.

Experiment with drugs that affect ion channels.

Design two drugs with separate actions on the channels in the membrane.

Click on "Bare Axon Parameters" in the P&G Manager panel.

The parameters menu that will appear allows you to change the properties of the bare axon.

On the Bare Axon Parameters menu, change a value for the "density" of the Na and K channels.

If you " block" channels with a drug, they are not available to open, and thus are "not there." Thus to apply this drug you decrease the channel density.

Alternatively a drug might increase channel density.

Answer Questions 14-16.