Modeling the Action Potential

Objective:

To model physically or with diagrams the role of ion concentrations, ion channels and ion pumps in the maintenance of the membrane potential and the generation of the action potential, at the level of 85% proficiency for each student.

In order to achieve this objective, you will need to be able to:

Predict the movement of Na⁺ and K⁺ across a permeable membrane.
Describe the mechanism that allows a neuron to remain at rest.
Simulate the electrical and chemical changes that occur in the cell during an action potential.

Materials:

Group Supplies

· 1/2 pounds of Black-eyed peas peas = Na⁺ (Light beans are positive charged ions)

· 1/2 pounds of Baby lima beans = K⁺ (Light beans are positive charged ions)

· 1/2 pounds of Black beans = Cl^- (Dark beans are negative charged ions)

· 2 Post-it-Notes

· 3 Tooth picks

· 6 pie pans per each group of 3 students

· 5 sheets of paper

· 1 sheet posterboard (or butcher paper) about 100 x 90 cm

· 1 marking pen

· 1 metric ruler

Methods and Results:

1. Mark the poster board down the center, along its long dimension, with a line that is interrupted in several places for 2 to 3 cm. These interruptions will represent the ion gates. Place toothpicks along the line where the interruptions have been made to "close" the gates; the toothpicks will be pivoted to "open" the gates. Mark one side of the line "Inside the cell," and the other "Outside the cell" (see Fig. 1).

Figure 1. Illustration of student model. (a) Nerve cell "at rest" with all channels closed. (b) Nerve cell with sodium channel having just opened.

Fill the pans with peas or beans as follows. You can estimate the proportions by just looking at them. (Note: In the directions below, the word "many" means enough peas or beans to fill the pan; the word "few" means just enough peas or beans to cover the bottom surface of the pan.)

* Black-eyed peas (sodium ions): many in one pan, few in another
* Baby lima beans (potassium ions): many in one pan, few in another
* Black beans (chloride ions): many in one pan, few in another.

Wad five pieces of construction paper into tight balls. These wadded-up balls will represent negatively charged proteins that cannot pass through the cell membrane.
Arrange the pans and wads of paper on the poster board as shown in Figure 1.
Place a Post-it note marked "negative" on the inside of the cell membrane of the model and a note marked "positive" on the outside of the membrane.
Look at the numbers of black-eyed peas representing the sodium ions in the pans inside and outside the cell. If the sodium channel were suddenly opened so that sodium ions (peas) could move across the cell membrane:

Aa. Which direction would they tend to move based on their concentration: into the cell or out of the cell? Explain.

Ab. Which direction would they tend to move based on their charge: into the cell or out of the cell? Explain. (Remember, the inside of the cell is negative with respect to the outside at rest.)

A sodium channel opens for about one millisecond (one 1000th of a second). Time the opening of the sodium channel for 10 seconds, representing one millisecond. Before he/she begins timing, decide which direction the sodium ions will move based on your answers to Questions Aa and Ab. When you begin timing, open the sodium channel by moving the appropriate toothpick. Then take the peas out of the pan and drag them through the sodium channel one at a time in the direction you think they will go until the time has elapsed. (Remember, the sodium ions cannot pass through other channels nor through the membrane where there is no channel.) (Note: Leave the sodium ions where they are now as you answer Questions B - F, but close the sodium channel. If you have used Post-it notes to mark "negative" and "positive," remove these notes now.)

B. Look at the numbers of sodium ions on each side of the cell membrane now. Compared to the number in each pan at rest, are there: a. More sodium ions inside the cell now than there were before opening the sodium channels, or b. Fewer sodium ions inside the cell now than there were before?

C. Based on your answer to Question B, do you think the internal medium of the cell is: a. More negative than it was before opening the sodium channels, or b. More positive than it was before? Explain.

D. Look at the numbers of lima beans representing the potassium ions in the pans inside and outside the cell. If the potassium channel were suddenly open so that potassium ions (beans) could move across the cell membrane: a. Which direction would they tend to move based on their concentration: into the cell or out of the cell? Explain. b. Which direction would they tend to move based on their charge: into the cell or out of the cell? Explain.

Relate your findings to the following terms:

* Membrane potential

A potassium channel opens for one to three milliseconds. Time the opening of the potassium channel for 30 seconds, representing three millisecond. Before he/she begins timing, decide which direction the potassium ions will move based on your answers to Questions Da and Db. When you begin timing, open the potassium channel. Take the beans out of the pan and drag them through the potassium channel one at a time in the direction you think they will go until the time has elapsed. (Remember, the potassium ions cannot pass through other channels nor through the membrane where there is no channel.)

E. Look at the numbers of potassium ions on each side of the cell membrane now. Compared to the number in each pan at rest, are there: a) More potassium ions inside the cell now than there were before opening the potassium channels, or b) Fewer potassium ions inside the cell now than there were before?

F. Based on your answer to Question E, do you think the internal medium of the cell is: a. More negative than it was before you had opened the potassium channel (but after you had opened the sodium channel and moved the peas), or b. More positive than it was before you had opened the potassium channel (but after you had opened the sodium channel and moved the peas)? Explain.
Sodium channels are opened in response to electrical signals, such as more positive ions in the internal medium of the cell. Communication along an axon is due to the sequential opening and closing of channels. Get together with two or more other groups and line up your models. Starting with one model, repeat step 7 and 8. As the internal medium of one model becomes more positively charged it will cause the internal medium of the next model to become more positively charged. This will cause the sodium channels in the next model to open and steps 7 and 8 are repeated for that model.

Relate your findings to the following terms:

* Threshold
* Action potential.

In the nerve cell axon, sodium channel inactivation occurs. This means that after the sodium gates open and close, they cannot open again for a few milliseconds. This time is called the refractory period. ("Refractory" means hard, durable, or immovable; the sodium channels cannot move to open.) If you were to line up a number of your models to represent a longer stretch of nerve cell axon, what effect would this have on the action potential? Does it make a difference if you start at one end of the long line, or in the middle?
When an action potential reaches the end of the axon of the nerve cell, it will then pass on information to the next cell. Calcium channels at the end of the axon are opened in response to electrical signals, such as more positive ions in the internal medium of the axon. There are more calcium ions outside of the axons than inside the axons. Thus, when calcium channels open, calcium moves into the internal medium of the cell. This will cause the release of a neurotransmitter.
Neurotransmitters can open a wide variety of channels in adjacent neurons.

Discussion:

Compare a graph of the action potential with what you just learned using the model. Refer to Graph 1 for a sample graph.

Graph 1. Sample graph of action potential.

After a lot of action potentials have occurred, will enough peas/beans/ions move to change significantly the concentration gradients you set up initially when your model was at rest? Would the sodium and potassium ions continue to move as they did during the action potential you simulated?
In order to continue to function properly, the cell must now somehow get back to its resting state. Do you have any ideas as to how the cell might do this?

Relate your thoughts to the following terms:

*Sodium-potassium-ATP pump.
*Membrane potential
*Threshold
*Action potential.
When an action potential reaches the end of the axon of the nerve cell, it will then pass on information to the next cell. Do you have any ideas as to how the cell might do this? Refer to figure 3. Relate your thoughts to the following terms:

* Axon terminal
* Neurotransmitter-gated channel
* Excitatory postsynaptic potential (EPSP)
* Inhibitory postsynaptic potential (IPSP).

Figure 3. The passing of information from one neuron to another.

Can you think of situations in which it would be important for the postsynaptic cell to be stimulated (EPSP)? inhibited (IPSP)? What kinds of problems might occur if it were not possible to control whether postsynaptic neurons were inhibited or stimulated? For example, what if postsynaptic neurons were always stimulated?