Asked in Human Anatomy and Physiology
What is the effect of hyperkalemia on cell membrane potential?
Hyperkalemia is an increase in extracellular K. Driving force of an ion depends on two factors, voltage and concentration gradient. For K voltage gradient is pushing K into the cell but the concentration gradient is driving K out of the cell. However, the total driving force for K is out of the cell because the concentration gradient is that strong. When there is an increase in K on the outside, the driving force for K decreases.
The equilibrium potential for K is -95mV. This means if K was freely permeable to the cell's membrane, it would reach equilibrium at -95mV. Another way to look at this is that efflux of K is the same as influx of K and the cell's new resting membrane potential would increase from a normal value of -70mV to -95mV. Note that I said it would increase even though the value became more negative. This is because the change in membrane potential has increased.
Since the driving force of K has decreased, the equilibrium potential has also decreased. From a value of -95mV it is decreased to let's just say -80mV. Since a normal resting membrane potential is regularly -70mV, the decrease in equilibrium potential of K has decreased this resting membrane potential to say -60mV now. This is a depolarization of the cell.
If this process happens quickly, it will depolarize the cell to the threshold value and you will have an action potential. However, if the hyperkalemia is severe, the cell will stay depolarized because the K equilibrium has decreased to a point where the cell cannot hyperpolarize back to threshold or resting membrane potential.
If this process happens slowly, the inactivation gates of the sodium voltage-gated channels will automatically shut and the cell cannot depolarize even if it reaches threshold values. It must hyperpolarize back to resting membrane potential and the inactivation gates of the sodium voltage-gated channel will reopen.