Resting Membrane Potential & Nernst Equation

Need help preparing for the Biology section of the MCAT? MedSchoolCoach expert, Ken Tao, will teach everything you need to know about Resting Membrane Potential & Nernst Equation of Nerve Cells/Neurons. Watch this video to get all the MCAT study tips you need to do well on this section of the exam!
The resting membrane potential is the membrane potential of a cell at rest. In terms of a neuron, when it is not firing any action potentials, it is at rest. It neither sends output out nor receives input from presynaptic neurons. Furthermore, the “resting membrane potential” or “membrane potential” is not the electrical potential at one point. Instead, these terms refer to the comparison of the electrical potential inside the whole cell versus outside the cell.
Sodium-Potassium Pump
For cells to achieve a resting membrane potential, their membranes contain specialized proteins that pump ions into and out of the cell. One such pump is known as the sodium-potassium pump, or the Na+K+ ATPase, which moves three sodium cations out of the cell and two potassium cations into the cell. This process uses active transport and results in the creation of an electrochemical gradient.
The Electrochemical Gradient
The formation of an electrochemical gradient across a cell membrane can drive many biological processes throughout the body, such as nerve conduction, muscle contraction, and secretion of hormones. It is essential to understand how both electrical and chemical gradients form to understand how these processes function.
The electrical gradient, in particular, develops as a result of a net loss or gain of charge across the cell membrane. The Na+K+ ATPase pumps three cations, or positively charged ions, out of the cell in exchange for two anions, or negatively charged ions. This process creates a net loss of positive charge from the cell. Remember, this only creates an electrically charged gradient across the membrane; it does not explain why the resting membrane potential is negative.
Chemical gradients develop from the concentration gradient of ions inside and outside the cell. The Na+K+ ATPase helps create this gradient by pumping out three sodium ions in exchange for two potassium ions. The cell loses sodium ions, and the concentration of sodium ions is higher on the outside of the cell than the inside. Likewise, as the cell gains potassium ions, the potassium ion concentration will be higher on the inside of the cell and lower on the outside.
Other pumps, aside from the Na+K+ ATPase, help create an electrochemical gradient across the cell membrane by transporting other ions such as chloride and calcium. The names of the proteins and transporters responsible for generating these gradients are beyond the scope of the MCAT exam. However, it is vital to understand the relative concentrations of each ion across the cell membrane. For chloride, its ion concentration is higher on the outside of the cell and lower on the inside. Calcium, too, has a higher concentration on the outside of the cell. However, it is unique in that its concentration on the inside of the cell is extremely low. This fact gives calcium a considerable concentration gradient compared to the other ions.
The Nernst Equation
Knowing the ion concentrations inside and outside of the cell helps determine the membrane potential of an ion in which there is no net movement of that ion. This value is known as the equilibrium potential, and it is calculated using the Nernst equation:
In the Nernst equation, Eion is defined as the membrane potential at which there is no net movement of a particular ion, and it takes into account both the electrical gradient and chemical gradient of that ion. The value of R is the gas constant, and the value of F is Faraday’s constant. The z variable is the charge of the ion of interest. In this way, the z for sodium is +1, chloride is -1, and calcium is +2. The value of T is the temperature.
The Equilibrium Potential
To make things clear, the equilibrium potential and resting membrane potential both refer to the potential at which there is no net movement of ions. The difference is the resting membrane potential accounts for all of the ions of the cell. The equilibrium potential, on the other hand, refers to a single ion and the potential at which there is no net movement of that ion alone. Using the Nernst equation, it is possible to determine the equilibrium potential of sodium, potassium, chloride, and calcium.
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[@bbbehra8841]

Awesome explanation! I never understood this throughout my undergrad classes

[@MedSchoolCoachMCATPrep]

Glad it was helpful!

[@ajowka1]

Excellent and clear explanation.

[@christuohino3960]

awesome explanation

[@arenamaleque9259]

How did you calculate 60mV?

[@aleenasaud1533]

i love you king