Week 11 Discussion PDF

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Exercise 1: Nernst/Goldman Simulator -Go to the website http://www.nernstgoldman.physiology.arizona.edu/ and either open the simulator in your browser (“click launch now”) or download the application Part 1: Single Ions -Go to the “Nernst @37” tab and select “potassium.” Scroll the external and internal concentrations of potassium ion up and down. (Note that the sliders for internal and external concentrations have different scales—that’s just a quirk of the software.) 1. What happens to the membrane potential when the concentrations of K+ inside and out are equal? Why?

2. What happens to the membrane potential when the concentration of K+ is 10 X higher inside the cell than outside? Explain this by the Nernst equation.

3. What is the Nernst equilibrium potential for K+ with the typical intracellular and extracellular neuronal concentrations of K+ shown in the table below? Adjust the concentration sliders to be as close as you can get to the values in the table, they don’t have to be exact. K+ equilibrium potential =

4. Increase the extracellular concentration of K+ stepwise as shown in the results of the experiment in the Figure below. Start with an inside K+ concentration of 124 mM/outside 3 mM, and increase the outside concentrations near to those shown in the Figure below (outside concentrations 3 to 120 mM K+). Write down the membrane potentials that result from each outside K+ concentration. Do the membrane potentials with greater outside K+ concentration follow the trend as in the Figure on the next page?

Figure shows the membrane potentials recorded from a neuron at different outside concentrations of K+ from 3 to 120 mM.

Part 2: multiple ions Go to the “Goldman @37” tab. 1. What is the difference between the Goldman and Nernst equations?

2. In the Goldman tab, keeping the ion concentrations at the default setting, change the permeability (e.g., PK+ for potassium (K+)) of each ion. a. What happens to the membrane potential when you increase the permeability of sodium? Of potassium? Of chloride?

b. Write each observation down for each ion and explain why the membrane potential changed the way it did.

3. Finally, starting with all intra- and extracellular ion concentrations of the Table below and permeabilities at zero, create a neuron that has a resting membrane potential of exactly -45 mV. You may change any permeability parameter you wish. Write down the ion concentrations and the permeability settings you chose.

Your settings for: PK+ ___________________ [K+]out _______________ [K+] in________________ PNa+ ___________________ [Na+]out _______________ [Na+] in________________ PCl- ___________________ [Cl-]out _______________ [Cl-] in________________ 4. How does the Goldman equation explain that despite the K+ dominance of the membrane potential a normal neuron’s resting potential is never at the Nernst Equilibrium potential for K+?...