You have learned that the establishment of the resting potential and the propagation of the action potential is based on the electrochemical properties of the neuron in a complex interaction among various charged ions and molecules and membrane bound proteins. In the early 1950 two scientists, Allen Hodgkin and Andrew Huxley conducted ground-breaking research that produced a mathematical model to explain the events that take place during the action potential. They won the Nobel Prize for their work.

Introduction: Getting to know the simulation.

When you first open this simulation you will see a box divided into two compartments representing the interior of the neuron (lower compartment) and the extracellular environment (upper compartment). The cell membrane between the compartments contains three gated channels, blue for chlorine, red for sodium, and green for potassium. The corresponding ions have the same colors. Once you start the program, the membrane voltage (membrane potential) will be graphed to the right in the upper portion of the screen and the membrane currents for each ion will be graphed below.
    To begin, click on "Start" and then "Excite." Run the program until you see the "Excite Again" option appear. At this point you can run the program over again from the beginning. You can "Pause" and "Continue" the program at any time. Try using the program a few times to familiarize yourself with its features before proceeding to the actual Experiment.

The Experiment

1. Click on "Start" but before proceeding take note of the distribution of the Na, K, and Cl ions on both sides of the cell membrane.
2. Now click "Excite" and let the program run until the first gated channel opens. At this point click on "Pause." Record which gate has opened and in what direction the corresponding ions are moving. Also not which of the ion currents has changed the most at this point. Finally record the membrane voltage that the cell has reached at this point and the approximate time.
3. Click "Continue" and run the program until the next gated channel opens or closes. Pause and record the same type of data as in the previous step.
4. Continue running the program in this fashion until the program ends.

Interpretations & Conclusions: As you conduct this experiment keep the following in mind.

• As you run this program and record your data, keep in mind that the Na and K are positive ions and Cl is negative.
• Correlate the movement of these charged ions with the changes in membrane potential in terms of the direction of the change (positive or negative) and the rate of change (slope of the curve).
• Appreciate the role of diffusion and of membrane bound proteins in these events.

1. Which of the three gated channels opens first? Why does this result in the membrane potential moving quickly in the positive direction?
2. Explain how this positive movement of the membrane potential is reversed. Discuss the role of the ions involved and the timing of their involvement.
3. From our discussion in class, describe the structures found in the axonal membrane that are not incorporated into this simulation and the role they play in the action potential.

Introduction: Getting to know the simulation.

When open the program you will see a control box with a number features that can be used to manipulate the action potentials. Along the top going from left to right is the "Start/Reset" button, three controls for changing the current of the ions (we won't use these), and a "time" control that sets the duration of the experiment. Along the bottom are the controls for two stimuli, S & S2 that you will use to manipulate the time at which the stimuli are applied to the neuron, the strength of the stimulus voltage, and the duration of the stimulus. The options on the right hand side select what properties of the action potential will be graphed. We will only be using "voltage."

The Experiment, Stimulus Threshold: When you open the program, the default setting has the neuron stimulated twice producing two action potentials. For now you will study a single action potential. Therefore, change the time of S2 from 18 to 50 msec in order to put it beyond the 40 msec time frame of the program. You may need ot click on "Replot."

1. Click on "Start" and note the general shape, duration, and height of the action potential in response to the stimulus (at 5 msec with a strength of -120 mV, and a duration of 0.1 msec).
2. Now decrease the strength of the stimulus from -120 mV to -110 mV, click on "Replot" and note any changes in the shape, duration, and height of the action potential.
3. Continue reducing the strength of the stimulus by increments of -10 mV, recording your observations at each step. At some point the action potential will fail to develop. At this point begin increase the stimulus in increments of 1 mV until the action potential reappears. This is the "threshold" stimulus, the minimum strength of a stimulation needed to generate the action potential.

The Experiment, Stimulus Frequency: Reset the program. Change the timing of S2 to 30 msec, replot, and record the usual data. Now you will see a yellow horizontal line that measures the time from the end of the first action potential to the begining of the second (DI). There is also a smaller, purple horizontal line measuring the duration of the second action potential (APD). Change the timing of S2 from 30 to 25, 20, 18, 17, 16, and 15 msec recording your data including the DI and corresponding APD at each step. At some point you will reach the minimum time between stimuli that will generate repeated action potentials.

Questions: Answer these questions and send them to me as an email attachment in MS Word or RTF format.

1. What is the threshold stimulus strength for this neuron? Is there an analogous threshold for stimulus duration? Explain
2. What is the minimum time between stimuli that still generate separate action potentials? What events are taking place in the neuron that account for this? Hint: check out your textbook.
3. Suggest what would happen if you stimulated a neuron just below its threshold strength but for longer durations? Run this experiment and report your results.
4. Graph the relationship between DI and APD and suggest a reason for these changes in terms of the events taking place in the neuron.

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