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Resting membrane potential
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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.
The resting membrane potential (RMP) is the electrical potential difference across the plasma membrane of a cell when the cell is at rest and not undergoing any significant electrical activity. This potential difference is created by the unequal distribution of ions across the membrane, with positively charged ions (such as sodium and calcium) being more concentrated outside the cell and negatively charged ions (such as chloride and potassium) being more concentrated inside the cell.
Each ion has its own equilibrium potential, which is determined by the Nernst equation. It states that an ion's resting membrane potential (Vm) equals 61.5 times the log of the concentration of the ion outside the cell, divided by the concentration of the ion inside the cell, for an ion with a single charge like sodium, and Vm equals 30.75 times the log the concentration of the ion outside divided by the concentration of the ion inside for an ion with a double charge like calcium.
Vm = 61.5Log [ION]out[ION]in for single charged ions (E.g. Na+) Vm = 30.75Log [ION]out[ION]in for double charged ions (E.g. Ca2+) The cell's resting membrane potential will therefore be the summation of each individual ion's equilibrium potential.
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