Analyzing MACs in our neurons: Formulating a hypothesis and a follow-up experiment

Task

Your Assignment: 
The big question is: What type of ion channels are the MACs in our neurons?  Are they selective and  permeable for  only  one  ion  species,  or  are  they  unselectively  permeable  for  several different cation and/or anion species?  If they are indeed selectively permeable for only one  specific ion species which one is the most likely candidate? 

How can you tell this by analyzing the provided information (data from our experiments)? Your job is to  
I) formulate a hypothesis describing what type of ion channel we can expect our MACs to be,  and reasonably explain why and how you come to this conclusion (10 points), and  

II) think about, describe, explain, and discuss a possible strategy for a follow?up experiment that we  could  do  to verify your  hypothesis (4  points).  For  part II keep in mind what  factors determine an ion’s equilibrium potentials and how we could use this to test your hypothesis. 

In addition, answer the following questions that may help to complete I) and II): 

III) Looking at the results (Figure 1, Table 2) describe what forces drive the ion flow across the  membrane through open MACs at ? 100 mV, ?50 mV, 0 mV, +50 mV, and +100 mV.  How does  changes in VM determine the direction of ion flow across the membrane, i.e., the membrane  current we measured (see Figure 1 and Table 2)? (4 points)

IV)  Imagine  doing  this  experiment  without  voltage?clamp,  i.e.,  have  the  cell  at  its  natural membrane resting potential of VM = ?55.7 mV when starting stimulation and ions crossing the membrane  through open MACs carrying a membrane current.  How would  the  flow of ions across the membrane (and the current carried by those ions) affect the membrane potential VM under  non?voltage?clamp  conditions?    Will  VM  change  or  not,  and  if  it  changes  will  VM become  more  negative  (hyperpolarize)  or  less  negative  (depolarize)?  

Explain your answer! (4 points)
V) Will the results of this experiments change if it is conducted at 10 °C? What parameters will change, how does this affect the observed current, and would the change require you to review your hypothesis under I) and follow?up experiment II)? (3 points)

 

Sounds though?  Think about what you learned in class and use it to interpret information can be read out of Figure 1 and Table 2.  Ask yourself  the  following questions before writing up your assignment.  Think before you write, and proofread not only for spelling and grammar, but also for logical flow in your argumentation. 
? What forces drive the movement of ions across the cell membrane through open ion channels (here MACs) and determine the direction of ion movement at different VM? 
? What ion  species  (Na+, K+, Ca2+, Mg2+, or Cl?) can carry  the observed current across  the cell membrane at the given VM, i.e., move in the required direction across the membrane at the different VMs to result in the observed current IR? 
? Why  did  we  care  about  calculating  and  knowing  equilibrium  potentials  prior  to  our experiments? 
? How can we prevent ions from flowing through open ion channels or speed?up their flow in experiments like this one? 
? What will happen if we change  the extracellular concentration of one ion species? Will  this change the resulting membrane current carried by this ion species and the resulting data plot? 

MSVU BIOL/PSYC2260    Copyrighted Material for Class Use Only! No Unauthorized Distribution or Publication! 
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