modeling-the-action-potential

Modeling the Action Potential Introduction: In this activity, you will set up a model to simulate how a neuron processes information. There are several facts that you should know before beginning the simulation.

1. The players in this simulation: a. Sodium ions (represented by red beans) are in higher concentration outside the neuron than inside the cytoplasm in a resting cell (that is, a neuron that has not been innervated). b. Potassium ions (represented by black-eyed beans) are in greater concentration inside the cell than outside the cell membrane. c. Large protein molecules that are trapped inside the cell and add to the net negative charge inside the neuron are represented by wads of paper. As a result of this buildup of negative charges, a voltage reading taken inside a cell would register at a -70 millivolts (mv).

2. There are three forces acting in a neuron at rest: charge differential, diffusional forces and electrical forces.

3. A neuron, however, has certain unique structural adaptations so that an impulse may be transmitted. There are proteins within the neuron cell membrane that acts as channels and gates. Each type of protein channel or gate recognizes only one type of ion.

Objectives of this simulation: a. Predict the direction of movement of Na+, K+ across a semi-permeable membrane. b. Describe the mechanism that allows a neuron to remain at “rest”. c. Simulate the electrical, chemical and concentration changes that occur in the neuron during an action potential.

Materials: Red, black-eyed beans, toothpicks, and wads of colored paper (5), large sheet of white paper, marker, post-it notes with either a + sign on it or a – sign on it, phospholipid bilayer models, protein channel models.

Directions:

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1. Each team of two students will arrange the materials according to the following diagram.

Spread out the different beans in either the cytoplasm of the ECM. First answer these two questions.

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Follow Mrs. R’s directions as you set up your nerve cell model. You will use your model cell to answer questions. The bean mover can move the beans and the timer can record results.

Look at the number of sodium ions inside and outside the cell. If the sodium channel were suddenly opened so that sodium ions could move across the cell membrane, a. in which direction would they tend to move based on the concentration gradient? b. in which direction would they tend to move based on their charge?

Neuron at rest:

Outside cell

Na

+

red 50

+

K white 15

Cytoplasm

+

K

+

white ?#

Na red ? #

Large anions

cytoplasm

(wads of colored paper) = anions; large negatively charged proteins trapped inside the cell

+ toothpick

= K+ channel protein

=

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phospholipid bilayer

2

+ toothpick = Na+ channel protein

3.Place a post-it note with the + in the side of the cell membrane that has a positive charge and the – post-it note on the negative side.

4. A sodium channel opens for about one millisecond. Each group’s timekeeper will time the opening of the sodium channel for 10 seconds, representing one millisecond. Before you begin timing, decide in which direction the sodium ions will move based on your answers to questions 1a and and 1b. The gatekeeper (bean mover) should open the channel. When your time keeper is ready, begin the 10 second time period and move the sodium ions across the channel ONE AT A TIME but as quickly as you can in the direction you think they would go until 10 seconds is up.

5. Close the sodium channel. Answer questions 6 – 8.

6. Look at the number of sodium ions on each side of the cell membrane now. Compare the number of sodium ions before and after the sodium channel opened for 10 (milli)seconds.

7. Is the internal medium of the cell more negative or more positive than it was before the sodium opened and closed? Explain.

8. Look at the numbers of white beans inside the cell in comparison to the ECM. If the K+ channel was suddenly open so that potassium ions could move acoss the cell membrane, a. in which direction would they tend to move based on concentration differences, and b. in which direction would they tend to move based on their charge differential?

9. A potassium channel opens for 4 milliseconds, Follow the same directions as in number 4 above, but now open the potassium channel for 4 seconds. Determine the direction of movementof your K+ ions.

10. Compare the number of K+ ions in the cytoplasm when the neuron was at rest to the number after the channel was open for 2 (milli)seconds.

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12. If this movement of sodium and potassium were to continue, what would happen to the charge differential, the concentration gradient and the electrical difference on both sides of the cell membrane? What would result?

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11. Is the cytoplasm more negative or positive than it was before you opened the potassium channel (but AFTER you closed the sodium channels)? Explain.

13. What mechanism has evolved to bring the neuron back to its resting potential? Assume the third channel is a Na/K pump. What role does it play? Demonstrate this with your red and white beans.

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14. In the axon membrane, a sodium channel inactivation occurs. This means that after the Na+ channels open and close, they cannot open again for a few milliseconds. What is this time period called?

15. Now that you have finished the simulation, we will look at an Action Potential step-by-step on the PowerPoint and then view a short video.