Practice question for the Nervous System

[mpelter]

Hi everyone!

Hope you all are having a great week. Here is a practice question for the Nervous system:

One of the most dangerous jobs at the Acme pH Electrode Company is using a syringe needle to fill the pH electrodes with a concentrated KCl solution. Responding to a 911 call one day, the police and later the coroner arrived to find the electrode-filling technician dead on the floor, with a half-full syringe of KCl laying next to him, a suspicious red mark over a vein in his wrist, and a surprised look on his face. Convinced that the technician accidentally injected himself with a high concentration of KCl, the coroner announced that the cause of death was cardiac arrest, even before he performed the autopsy.

 

Why should injection of KCl cause sudden death in this way?

 

I know it’s a bit long, but sometimes the questions can be this long or longer. Pick out the necessary information from the paragraph. Write it down or draw a diagram if it helps.

Remember, it’s important to keep track of the flow of the ions. I also think that the Goldman-Hodgkin- Katz website that Kristen found is very helpful. Go check it out!

 

I’ll post the answer to this on Sunday. In the meantime, tell me your thoughts on this question.

 

Let me know what you all think!

Megan

[mpelter]

Someone sent me this response:

 

"With an injection of concentrated KCl, there will be an unequal balance of charges across the ECF and ICF. The ECF as an entity itself will still be neutral, but there are now more positive and negative charges in the ECF when compared to the ICF. I think the K+ will try to flow into ICF via leakage K+ channels, but this won't happen fast enough. There will be an unequal balance of charges across the cellular membrane, and like Dr. Herrera said in class, if there's an unequal distribution of charges, then lightning bolts will occur. So in a sense, the technician was electrocuted by the concentrated KCl, which caused cardiac arrest."

 

I'll post the answer on Sunday with my own commentary. In the meanwhile fee free to email me with more responses/questions!

Have a great week!

Megan  

[mpelter]

Hi all,

 

Here is another response:

 

"This is what I think thus far.

Even though there is equal number of cations and anions injected, this will still cause an excess of positive charge in the extracelluar fluid compared to normal conditions. An increase in extracellular KCl would disrupt the membrane potential by making it more positive than when its at rest. This more positive membrane potential will open "some" voltage gated sodium channels, and then they would close before further depolarization and enter the refractory stage. The lower number of available sodium channels to be opened would cause the membrane potential to lose its ability to reach threshold, and thus not stimulate the all or nothing response which would lead to cardiac arrest."

 

David

 

Thanks David! It's great to hear from you!

I'll be posting the answer soon.

Megan

[mpelter]

Hi Everyone,

 

The answer to this question is:

 

"A large increase in extracellular K+ concentration would strongly depolarize most cells in the body since, for most cells, the plasma membrane is permeable to K+. Most cells would be disrupted by the depolarization, but the immediately fatal effect would come from the depolarization of cardiac muscle cells. Depolarization of cardiac muscle cells would inactivate their voltage-gated Na+ (and Ca2+) channels, blocking generation of action potentials and contraction."

 

Both replies were great! I have just a couple of comments about the responses. While you both did an excellent job in describing the events of the action potential (and what doesn't happen when something goes wrong), you could make your answer even better by using key words. Instead of saying  that the membrane potential became more positive, say that the membrane has depolarized. Also, be sure to think the entire answer through and explain why. Why does the cell become depolarized when there is an increase in extracellular K+? Why does this sudden depolarization cause the cardiac muscle cells to keep contracting, causing a cardiac arrest?

 

Hope this helps! If any one has any questions, please feel free to email me.

Good luck on your exam!

Megan

[mpelter]

Hi everyone,

 

A couple of people have emailed me with questions regarding the inactivation of the voltage-gated Na+ channels. It is true that the Na+ channels will open during depolarization. However, remember that you have a large amount of KCl in the ECF. Its highly unlikely that that will leave very quickly. So you are going to have (at least for some time), a constant depolarization. Also remember that there is a refractory period during which the sodium channels are inactivated. They require the membrane potential to return to normal to start working again, right? With constant depolarization (due to the high extracellular KCl), they are stuck within this phase and are unable to undergo an action potential generation. The cardiac muscle cannot relax (or continue to beat), and the poor technician has cardiac arrest.

 

Great questions everyone! Please feel free to email me with more questions/responses.

Good luck to you all!

Megan

[Kristen Chen]

Hi Guys! 

Like Megan said, if you have any last minute, pressing questions please feel free to email any/all of us (ie. cc us all) with your questions and we'll do our best to answer them in a timely manner.  Also post on the discussion forum! 

Our emails are: 

 Sara <meg...@usc.edu>,

 Megan <mpe...@usc.edu>

 Kristen <chen...@usc.edu>

 Danny <dann...@usc.edu>

I know Megan and I are up for questions, and Sara and Danny are great resources as well (not sure what their schedules are like right now) but it's worth a shot asking us!    

 

Good luck everyone! You guys are gonna rock it! :)  

Kristen

[TerminalSanctuary]

Hi everyone, 

I'll be available for questions most of the night.  I've been a little inactive due to hectic week but I can take questions tonight.  So feel free to send any of us any questions!

Good luck!! :)

-Danny

[mpelter]

Hi Everyone!

 

I got a lot of great responses for this question! The answer is it would cause an EPSP.  My reasoning is as follows: 

 

Now we have chloride with an equilibrium potential that is more positive (between -55 mV and -70 mV). Normally, the chloride ions flow inward. However now  the ions will be flowing outwards instead of inwards, right? In order to reach this now more positive equilibrium potential, the negative charge has to flow out, causing the cell to depolarize. So increasing the permeability of the chloride would then cause an EPSP.

 

Best of Luck! Keep up the good work!

Megan