Hey this a question take from the second USMLEW practice exam. Not sure I am understanding the concept.

A researcher studies membrane potentials for isolated neurons. He reports two action potentials for the same neuron. The first depolarizes the membrane to +40mV, while the second reaches +10mV. Which of the following manipulations by the researcher best explains the lower amplitude of the 2nd action potential?

A.) Decreased sodium concentration outside the cell
B.) Dec Sodium concentration inside the cell
C.) Decreased K inside the cell
D.) Decreased K outside the cell
E.) Increase Calcium outside the cell

Generally, when you have depolarization, Na goes into the cell and that would move the action potential from -70mV to a higher number above +80. If the AP goes from +40 to +10, shouldn't that mean that less Na is entering the cell and there would be an increase in extracellular sodium? The answer for this was A.

Hi there,
They are talking about two different action potentials. In the first situation the cell is depolarized to +40 mV (from the resting MP of -90mV) and in the second Action potential, it is depolarized to +10 mV (from -90 mV). So in the second situation there was less Na outside to move inside and depolarize it.
It is the inward movement of Na that causes depolarization, so C & D are out. And Ca causes plateau in cardiac muscles. Here they are talking about neurons, so E is out. A & B remain ...and dec Na inside will cause more Na influx due to inc gradient.

(By reading your explanation , i assume you are thinking in the opposite way...the dec in Na conc outside is the cause and 10 mv depolarization is the effect. They are asking about the cause for the effect on depolarization, not the other way around... I hope I am making some sense )
edited by on 1/23/2014
edited by on 1/23/2014
edited by on 1/23/2014