AssessmentsResting membrane potential
Resting membrane potential
USMLE® Step 1 style questions USMLE
An investigator is studying the equilibrium potentials of a neuronal cell with a resting membrane potential of -70 mV (VM). The equilibrium potentials are shown below:
ENa+ +65 mV
ECl- -85 mV
ECa2+ +120 mV
EK+ -85 mV
Which of the following changes occur if the ion permeability to sodium across the cell membrane is increased?
Content Reviewers:Rishi Desai, MD, MPH
Contributors:Tanner Marshall, MS
Each cell in the human body is wrapped in a membrane that separates the inner environment and outer environment, and positively and negatively charged ions aren’t equally distributed on both sides of the membrane.
Fundamentally, it’s these differences in concentration and charge as well as permeability across the membrane that establishes the cell’s resting membrane potential.
Generally speaking there is a higher concentration of Na+ or sodium, Cl- or chloride, and Ca2+ or calcium on the outside of a cell, and a higher concentration of (K+) or potassium and (A-), which is just what we just write for negatively charged anions, on the inside of a cell.
These anions include a variety of amino acids and proteins that are produced by the cell.
Let’s start with the sodium-potassium pump which uses ATP to move three sodium ions out of the cell for every 2 potassium ions that it moves into the cell, this is the workhorse of the cell and it helps establish the concentration gradient for potassium and sodium.
Let’s focus on potassium, which has a concentration of 150 mMol/L on the inside of the cell and about 5 mMol/L on the outside of the cell.
With so much potassium within the cell relative to outside the cell, there will be fairly strong concentration gradient moving potassium ions out of the cell.
Although these ions can’t simply diffuse through the phospholipid bilayer membrane, it turns out that potassium can get across the membrane using potassium leak channels and inward rectifier channels that are scattered throughout the membrane.
So using those channels, the concentration gradient pushes potassium out of the cell, and that potassium brings with it some positive charge and leaves behind unpaired anions which carry negative charge because they aren’t able to go through the leak channels.
Over time as more potassium ions leave the cell, a negative charge builds up within the cell and this starts to attract positively charged potassium ions back into the cell, and this is called the electrostatic gradient.
This electrostatic gradient is established with the movement of relatively few ions, so it doesn’t upset the overall concentration gradient that was already established.
For potassium, the exact point when the potassium moving out of the cell due to the concentration gradient equals the potassium moving back into the cell due to the electrostatic gradient is called the equilibrium potential or nernst potential for potassium, and it’s about -92 mV.
In other words, -92 mV is the electric potential for attracting potassium into the cell that is needed to balance the concentration gradient that is pushing potassium out of the cell.
So the equilibrium potential of an ion is dependent on two things: the concentration gradient for the ion and the cell being permeable to that ion.
If we’re only dealing with a single ion, then the equilibrium potential for the ion equals the resting membrane potential for the cell.