Question

Write a paragraph of 3-5 sentences describing 1) whether the statement is correct or incorrect and 2) the reasoning for your answer. Your answer should demonstrate your understanding of the physiological concepts underlying the statement and be professionally written.

Assume a cell with the following properties:

[Na]_out = 120 mM; [Na]_in = 12 mM

[K]_out = 5 mM; [K]_in = 125 mM

Membrane is permeable only to Na⁺ and K⁺

If the plasma membrane of the cell becomes more permeable to Na⁺, both influx of Na⁺ into the cell and the Na⁺ equilibrium potential will increase.

Answer 1

The statement is correct as outside the cell, the main positively charged ion is sodium (Na+ ) with a small amount of potassium (K+ ) and chloride (Cl− ) ions. In the resting membrane, none of the cations and anions in the solutions on either side of the membrane are at electrochemical equilibrium. Consequently, they are diffusing across the membrane with different diffusion rates and in different directions at all times in the resting membrane. The only time an ion won't diffuse is when

(1) it is at electrochemical equilibrium or

(2) the membrane is not permeable to it at all.

The resting membrane is a polarized membrane. That is, unlike charges are separated at the membrane with the inside negative and the outside positive.

[Na]out = 120 mM; [Na]in = 12 mM

[K]out = 5 mM; [K]in = 125 mM

When the membrane is stimulated by an electronic stimulator in the laboratory, its resting membrane potential begins to decrease. That is, it becomes less negative and hence less polarized. If it is depolarized to a critical level known as the excitation threshold, an action potential will be produced at the point of stimulation. Once the membrane potential is depolarized to the excitation threshold, its Na⁺ channels (routes through which Na⁺ ions cross the membrane) suddenly open and a tremendous increase in Na⁺ conductance g",,+ occurs with Na⁺ ions now free to diffuse down both their chemical and electrical gradients. This is called sodium activation or influx of Na+into the cell. Here conductance g is the electrical analog of permeability P. Thus, there is a sudden and marked increase in sodium permeability on the part of the membrane when the excitation threshold is reached.

As the positively charged Na⁺ ions suddenly diffuse inward sufficient positively charged Na⁺ is removed from the immediate membrane exterior surface and transferred to the immediate membrane interior surface to totally eliminate the internal negativity and replace it with positivity. Measurement now would record a reversed potential showing the interior now positive with respect to the exterior.