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A Message to Teachers Part I: Modeling the Resting Potential

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A Message to Teachers Part I: Modeling the Resting Potential A Message to Teachers Thank you for using our tools to help your students visualize the molecular world! The lesson/activity guide that accompanies the Synapse Kit is intended to help you consider different ways in which you may use these materials. It is not the intent of the Center for BioMolecular Modeling to require you to work through the entire lesson from start to finish (although you may do so if you wish). We do encourage (and perhaps insist on) your modification of these lessons and activities to meet the learning objectives of your specific students or to accommodate the physical limitations of the environment in which you teach. Enjoy! Part I: Modeling the Resting Potential Introduction: All living cells, including nerve cells, maintain a difference in the concentration of ions across their membranes. These ions play an essential role in neuronal signaling. Ions are unequally distributed between the interior of the cell and the surrounding fluid. A small excess of negatively charged ions on the inside of the cell membrane and a small excess of positively charged ions on the outside of the cell membrane results in a negatively charged cell interior relative to the outside environment. This difference in electrical charge across the membrane can be measured with a voltmeter and ranges from -60 to -80 mV (millivolts) when the neuron is not sending a signal. The membrane potential at rest is referred to as the neuron’s resting potential. For the purpose of this lab activity, we will consider a neuron with a resting potential of -70 mV. A membrane that exhibits a membrane potential is said to be polarized. MSOE Center for BioMolecular Modeling Synapse Kit: Section 1-2 | 1 Set up the membrane of the axon of a neuron according to the diagram to the right. The permeability characteristics of a neuron’s plasma membrane are, in part, Outside determined by specific membrane transport channels. Channels in the nerve cell membrane that transport negatively charged ions are relatively few in number or are closed and not displayed in the model. Inside 1a. Label the membrane, the voltage- gated sodium channel, the potassium leak channel, the voltage-gated potassium channel and the sodium-potassium pump in the diagram. Outside Table 1. Average Ion Concentrations Inside and Outside of Mammalian Neurons Intracellular Concentration Extracellular Concentration Ion (mM) (mM) Potassium (K+) 140 5 Sodium (Na+) 15 150 Chloride (Cl-) 10 120 Large anions (A-), such as 100 Negligible some proteins, inside the cell Potassium ions (K+) represented by the square purple models and sodium ions (Na+) represented as round blue models are the key players in establishing the resting potential. According to the table above, in most neurons, the concentration of Na+ is higher outside the cell while the concentration of K+ is higher inside the cell. Distribute the sodium and potassium ions to illustrate this observation in the model you have constructed. MSOE Center for BioMolecular Modeling Synapse Kit: Section 1-2 | 2 1b. In a resting neuron, what positive ion is most abundant outside of the plasma membrane? 1c. The K+ gradient dictates that potassium ions will flow in what direction? 1d. What happens to the resting potential of the cell when K+ ions move? 1e. What direction does the concentration gradient dictate the Na+ ions flow? 1f. Carefully examine the sodium-potassium pump. What is the exchange ratio of Na+:K+ ions of the sodium potassium pump? (HINT: Focus on the shapes in the pump.) 1g. What molecule is necessary for the sodium-potassium pump to work against the chemical gradient? 1h. What factors impact establishing the resting membrane potential? MSOE Center for BioMolecular Modeling Synapse Kit: Section 1-2 | 3 Optional Activity: Comparing the Voltage-Gated Sodium Channel to the Voltage-Gated Potassium Channel 1i. How does the voltage-gated potassium channel distinguish sodium ions from potassium ions? 1j. What structural differences can you observe in the channel pore of these voltage-gated channels? Part II: Modeling the Action Potential Set up an axonal membrane as shown in the diagram below and to the left. Remember to set up your model so that there are more extracellular Na+ ions than intracellular Na+ ions and more intracellular K+ ions than extracellular K+ ions. You will also need to periodically refer to the graph shown below and to the right as you work your way through the modeling process: The Action Potential Changes in membrane potential in a local area of a neuron's membrane result from changes in membrane permeability. MSOE Center for BioMolecular Modeling Synapse Kit: Section 1-2 | 4 Next, you will simulate the mechanism that produces an action potential in the axon of the neuron. Follow the steps below to model this action potential. Step 1 - Resting state: The voltage-gated sodium and potassium channels are closed. Set your voltmeter at -70 mV. Step 2 - Depolarization: A stimulus opens the voltage-gated sodium channels and Na+ follows its concentration gradient into the neuron. The influx of Na+ causes a depolarization across the cell membrane. An action potential will be triggered if the depolarization reaches threshold (often between -40 and -55 mV). Set the voltmeter at threshold. For the purposes of this activity, we will consider the threshold potential to be -50 mV. Step 3 - Rising phase of the action potential: Depolarization opens most of the voltage-gated sodium channels, while the voltage-gated potassium channels remain closed. Open the voltage-gated sodium channels and move a few more Na+ into the neuron. Depolarization continues until the inside of the membrane is positive with respect to the outside (usually +30 mV). Set the voltmeter at the peak value of the action potential. When the patch of neuron membrane is generating an action potential and its voltage-gated sodium channels are open, the neuron cannot respond to another stimulus no matter how strong. Step 4 - Falling phase of the action potential: Voltage-gated sodium channels become inactivated, blocking Na+ inflow. Voltage-gated potassium channels slowly open permitting K+ to follow its concentration gradient out of the cell causing the voltage across the membrane to fall. Position the voltage-gated sodium channels to their closed and inactive position. Open the voltage-gated potassium channel and move the K+ ions out of the neuron. Demonstrate a fall in voltage due to this ion movement on the voltmeter. Only a stonger than normal stimulus can reopen the voltage-gated sodium channels at this time. MSOE Center for BioMolecular Modeling Synapse Kit: Section 1-2 | 5 Step 5 - Hyperpolarization: The voltage-gated sodium channels begin to reset back to their original position. Move the voltage-gated sodium channels to their closed but able to be activated position. The voltage-gated potassium channels are still open causing the voltage to undershoot the resting potential. Set the voltmeter to dip below - 70 mV. Step 6 - Repolarization: Repolarization restores resting electrical conditions but does NOT restore resting ionic conditions. The ion redistribution is accomplished by the sodium-potassium pump. Step 7 - Re-establish the ion distribution: 7.1 7.2 & 7.3 7.4 7.5 & 7.6 7.7 7.1: Bind three three intracellular sodium ions to the appropriate spots in the protein. 7.2: Bring the ATP in close proximity to the pump. MSOE Center for BioMolecular Modeling Synapse Kit: Section 1-2 | 6 7.3: Sodium ion binding stimulates phosphorylation of the pump protein by ATP. In other words, a phosphate group is added to the sodium-potassium pump from the ATP molecule. (You will not be able to demonstrate this step with the model). 7.4: Phosphorylation causes a change in the shape of the protein. You can demonstrate this by “swinging” the sides of the protein so that it opens to the outside of the cell. 7.5: The shape change reduces the protein’s binding affinity for sodium ions and increases the binding affinity for potassium ions. Remove the sodium ions from the protein and deposit them outside the cell and bind two potassium ions to the appropriate spots in the protein. 7.6: Potassium ion binding triggers the release of the phosphate group from the protein. (Again, you will not be able to demonstrate this step with the model). 7.7: Loss of the phosphate group results in the restoration of the protein’s original shape which then releases the potassium ions. Swing the sides of the protein back so that they open to the inside of the cell and deposit the potassium ions. 7.8: Repeat this process one more time. 2b. What value is being used for the resting potential in your model neuron? 2c. What “factors” may cause a change in the voltage established across the membrane this neuron? 2d. Of what significance is a rise in membrane potential to -55mV to the neuron? Describe what protein and how it is affected by this change in voltage. MSOE Center for BioMolecular Modeling Synapse Kit: Section 1-2 | 7 2e. What is meant by depolarization? 2f. Color the area on the graph to the right where depolarization has occurred. 2g. Sketch the axon membrane that has been depolarized. Be sure to include all channels and ions in their proper locations and positions. 2h. What events occur at the peak of the action potential? 2i. Why is it important that another action potential cannot be generated during the rising phase of an action potential? MSOE Center for BioMolecular Modeling Synapse Kit: Section 1-2 | 8 2j. Why does the voltage across the neuronal membrane fall? 2k. Color the area on the graph to the right where repolarization has occurred.
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