Practice Exam Question for Nervous System/Goldman Hodgkin Katz

[Kristen Chen]

Hi Everyone! :) 

So I know today's lecture material was probably pretty challenging and can get confusing so I thought I would provide you with a couple of tools! 

1) The first is a website where you can play around with the Goldman Hodgkin Katz equation/Nernst equation and manipulate ion concentrations and permeabilities.  Check it out here! 

2) The second is an old Herrera practice exam question that I think could help you guys better understand the material: 

A typical neuron is dissected from an animal's nervous system and placed in a recording setup.  The neuron is impaled with electrodes that allow the experimenter to set the membrane potential to any desired level.  The normal resting potential is -70 mV, E (Na) = +50 mV, E (K)= -90 mV, and E (Cl)= -80 mV.  At rest, the cell membrane is 50 times more permeable to K+ than to Na+ or Cl-.  In her first experiment, the scientist hyperpolarizes the membrane potential to -100 mV and then immediately uses a drug that opens K+ channels.  Which one of the following would be the largest current that would flow under these conditions.  

a) Inward current will flow, carried by K+ ions moving into the cell. 

b) Inward current will flow, carried by Na+ ions moving into the cell. 

c) Outward current will flow, carried by Cl- ions moving into the cell. 

d) All three ions (Na+, K+, and Cl-) will carry current across the membrane, but the direction each ion will move cannot be determined from the information given in the passage.  

HINT: Don't forget the take home message of this lecture!: If the permeability for a particular ion increases, the membrane potential will change, tending to move toward the equilibrium potential for that ion.    

I will post up the answer as soon as I get a few responses, but I know that those of you with access right now to google groups still cannot post, so think about this question, and answer it when you can! :)  Also, please go through all the answer choices and explain why you thought they were right or wrong. 

Good luck guys! :) 

~Kristen  

[Kristen Chen]

Since many of you can't post, if you want to email me your responses, I can post them so we can get this discussion started!  

A student response I got: 

"I think the answer is "a" because the current (or driving force) is -10. For cations, a negative current is carried by the cation moving into the cell. In this case, K+."

[Kristen Chen]

"I also think the answer is a. because the opened potassium channels will want potassium to enter the cell in order to bring the membrane potential back to -90 mV, the potassium equilibrium potential. So that is an inward current (since positive charges are entering the cell) of potassium ions."

Sarah

On Wednesday, January 30, 2013 9:31:00 PM UTC-8, Kristen Chen wrote:

[Kristen Chen]

Thank you for the practice question! I believe the answer is A because first K is the most permeable ion in this membrane. Second, the driving force is -100 + 90 = -10 so there is a inward force of the cation K.

Helen

On Wednesday, January 30, 2013 9:31:00 PM UTC-8, Kristen Chen wrote:

[Kristen Chen]

Hi Kristen!

The answer is A. Because the permeability of the membrane increased to potassium, the membrane potential will move towards E(K), so from -100 mV to -90 mV. This requires inward flow of current/positive charge, and K+ can move across because the K+ channels are open. B and C are wrong because the membrane is not very permeable to Na+ or Cl-. D is wrong because it can be determined from the information given. 

Sheena

On Wednesday, January 30, 2013 9:31:00 PM UTC-8, Kristen Chen wrote:

[Kristen Chen]

Great job, guys! The answer was A! :)

You guys were right on with the driving force!  Since the scientists hyperpolarized the cell, the cell is more negative than it would normally like to be, even compared to the Ek which is already very low.  It is therefore going to attract an inward current of K+ ions through the opened K+ channels.  B would have been accurate if they had opened the Na+, but they didn't so that wouldn't make sense.  And finally, if more Cl- entered the cell, the cell would just get more negative which is opposite of what we want to happen.      

On Wednesday, January 30, 2013 9:31:00 PM UTC-8, Kristen Chen wrote: