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Lesson 1. The Calculated Action Potential

The purpose of this first module is to introduce the structure of these simulations. The action potential can be triggered by the "F5" key and is plotted in the upper right corner of the computer screen; the size of the stimulus is shown in a separate, lower plot. Additional information is shown in blue boxes; right-click to open these boxes. The "Objectives" box outlines the information that you should learn in the module. The "Structure" box describes the contents of the screen. A "Help" box is also present.

The Calculated Action Potential

Lesson 2. The Threshold of the Action Potential

In this module, you will have the opportunity to change the amplitude and duration of the stimulus in order to determine how strong a pulse is required to reach threshold. The values are changed by either dragging the slider or right-clicking on the white window and entering the number directly. Note that there are built-in limits to the magnitude of these variables.

Plot the results on your worksheet.

The Threshold Of The Action Potential

Lesson 3. Currents of the Action Potential.

In this module, you can observe the currents that comprise an action potential. Green plots the total current which is inward as the action potential grows, and is positive at the falling phase of the action potential. Sodium and potassium currents are also plotted. You can alter the amplitude of the stimulus to see the consequence of moving closer to threshold and farther away.

Determine the relationship between each current and the various phases of the action potential. Answer the questions on the worksheet.

Currents Of The Action Potential

Lesson 4. The Initiation of the Action Potential

The currents that generate the foot of the action potential are displayed on a greatly expanded scale in this module. The stimulus can be changed to extend or shorten the prodromal phase. Observe the amplitude and time course of each current and answer the questions on the worksheet.

The Initiation Of The Action Potential

Lesson 5. The Sodium Current

As you have seen, sodium currents are central to the initiation of the action potential. In this module, the sodium conductance and driving force are plotted along with the sodium current and the action potential. You should examine the curves carefully to determine the cause for each phase of the sodium current.

The Sodium Current

Lesson 6. The Relative and Absolute Refractory Periods.

A second action potential can be very difficult to elicit, depending on how close it is to the initial action potential. This module explicitly tests the excitability of the nerve after an action potential. You can alter the time of second pulse, and the size of the pulse.

Plot the threshold of the second pulse as a function of time on the worksheet.

The Relative And Absolute Refractory Periods

Lesson 7. Accommodation.

For the same reason that there is an absolute refractory period, nerves lose excitability entirely when held at a membrane potential more positive that approximately -50 mV. The reason is illustrated in this module.

Accommodation

Lesson 8. Pacing a Train of Action Potentials

The simplest way to generate a string of action potentials is to pass a small, constant inward current. The process is illustrated in this module.

Pacing A Train Of Action Potentials

Lesson 9. Inhibitory Currents of the Action Potential.

As you have learned in histology, inhibitory currents diminish the excitability of nerves. In the simplest case, the reversal potential is near rest; at other times it can be more negative than rest. You can test these two cases in this module.

Inhibitory Currents Of The Action Potential

This lesson has sought to explain the workings of nerve excitability in one of the simplest cells, the unmyelinated nerve axon. Many other cells have far more complicated action potentials, but all behave according to the general principles described here.

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