Summary
Highlights
Membrane potential is an electrical voltage across the membrane of every cell, caused by differences in charge in two separated areas. This separation is maintained by a semi-permeable membrane allowing only specific charged particles (ions) to pass through. All cells have a cell membrane and thus a membrane potential, but it is particularly important for sensory, nerve, and muscle cells, where it is also known as the resting potential. These excitable cells transmit signals by changing their resting potential.
The membrane separates two fluid compartments with different ion concentrations. The membrane is semi-permeable, meaning its permeability varies for different ions. Key ions involved are negative chloride and positive potassium ions inside the cell, and positive sodium and negative chloride ions outside. The membrane is most permeable to potassium ions due to specific ion channels. We'll use potassium ions as an example: their concentration is much higher inside the cell, creating a concentration gradient. To equalize this, potassium ions diffuse from inside to outside the cell. Since these ions are positively charged, this leads to a charge separation, making the outside more positive and the inside more negative. This electrical gradient opposes the diffusion of potassium ions, eventually leading to an equilibrium. The membrane potential is the result of the equilibrium potentials of all contributing ions.
You can calculate the membrane potential using two main formulas: the Nernst equation and the Goldman equation. The Nernst equation allows you to calculate the equilibrium potential of individual ions. The formula is E = - (RT/zF) * ln([Ion]_in / [Ion]_out), where R is the general gas constant, T is temperature, z is the charge, and F is Faraday's constant. At room temperature (25°C), this can be simplified. For example, to calculate the Nernst potential for potassium ions, with an intracellular concentration of 155 millimoles per liter and an extracellular concentration of 5 millimoles per liter, the potential is approximately -88 millivolts.
The Nernst equation can be extended to the Goldman equation to calculate the full membrane potential by factoring in the different permeabilities of the cell membrane for various ions. The formula accounts for the permeability (P) of potassium, chloride, and sodium ions. For potassium, P is 1; for chloride, 0.45; and for sodium, 0.04. When calculating, cation concentrations inside the cell are placed under the fraction bar, and anion concentrations inside the cell are placed above it. This calculation yields a value of -71 millivolts, which is close to the measured resting potential of about -70 millivolts in nerve cells.
Changes in membrane potential are crucial for signal transduction in nerve cells, for example, in the form of an action potential.