Imagine a cell that is permeable to K + and Na + . Based on the intracellular an
ID: 146351 • Letter: I
Question
Imagine a cell that is permeable to K+ and Na+. Based on the intracellular and extracellular concentrations of these ions, you have calculated EK to be -50 mV and ENa to be +20 mV.
Part A) What conditions would have to be met in order for Vm to be -50 mV?
Part B) If PK and PNa were equal, what would Vm be? Explain.
Part C) At time 0, this cell is much more permeable to Na+ than to K+. After 10 milliseconds, the permeabilities change such that the cell becomes much more permeable to K+ than to Na+. Describe what happens to Vm during this time period and explain why this happens.
Part D) Describe the ionic currents that occur after the 10-msec timepoint in the scenario described in part c) above. Which ions move? In which direction do they move? Why?
Imagine a cell that is permeable to K+ and Nat. Based on the intracellular and extracellular concentrations of these ions, you have calculated Ek to be -50 mV and ENa to be +20 mV. Part A What conditions would have to be met in order for Vm to be-50 mV? Part B If Pk and PNa were equal, what would Vm be? Explain. Part C At time 0, this cell is much more permeable to Nat than to K+. After 10 milliseconds, the permeabilities change such that the cell becomes much more permeable to Kt than to Nat. Describe what happens to Vm during this time period and explain why this happens. Part D Describe the ionic currents that occur after the 10-msec timepoint in the scenario described in part c) above. Which ions move? In which direction do they move? Why?Explanation / Answer
A: To reach the Ek= -50mV, the cell membrane should be negative from inside. This is achieved by hyperpolarization of the membrane through entry of K ions from the axon interior to extracellular fluid. The voltage gated K channels should remain open even after repolarization to achieve hyperpolarization.
B: Since the neuron membranes are more permeable to K ions, the resting membrane potential is near the equalibrium potential of K ions. If the permeability for K and Na ions would be equal; the Vm would be more positive and would lie somewhere in between -50 and + 20mV
C: At time 0, this cell is much more permeable to Na+ than to K+. This would make the Vm value to become more positive (near +20 mV) as the Na ions would enter the cell i.e. depolarization. As the permeability changes again, the K ions would leave the cell which is now more positive. The exit of K ions would restore the Vm near the equlibrium potential of the K ions i.e. repolarization and hyperpolarization.
D: After 10 milliseconds, K ions would exit the cell since the neuron's interior is more positive with respect to the extra cellular fluid. So K ions would leave the cell to restore the negative value of the membrane potential.