Membrane potential - Answers to problems

Hi!
I am glad that many (if not most)of you have tried to solve the problems. One common error is that while calculating the change in RMP, you have calculated (Old RMP - New RMP) instead of (New RMP - Old RMP). As a result, many of you have concluded that increase in ECF K+ makes the RMP more negative, which uis of course wrong. Here are some suggestions for solving the problems...

Problem No. 1
RMP = -90 mV

Problem No. 2
Part 1
Normal Vrest= -90mV
If [K+]o = 9mMol/L; [Na+]o = 160mMol/L
Then RMP = -75mV (Use Goldman-Hodkin-Katz Equation)
Change in RMP from normal = -75 - (-90)= 15mV
ie Vm becomes LESS negative

Increase in ECF [K+] makes RMP LESS negative
Brings it closer to firing (Threshold level)
Increases excitability
Increases risk of abnormal heart beats - arrhythmias

Part 2
If [Na+]o is corrected to normal level
RMP = -76mV
Change in Vrest from previous = -76 - (-75)= -1mV
In resting membrane , PK+>>PNa+
Therefore, changes in [K+] affect RMP but not changes in [Na+]

Increase in ECF [Na+] does not affect RMP much, but it will affect the amplitude of the action potential - greater the Na+ concentration gradient (out-to-in) - bigger the action potential spike.