How is membrane potential determined

Membrane potentials result from a separation of positive and negative charges ions across the membrane, similar to the plates within a battery that separate positive and negative charges. Membrane potentials in cells are determined primarily by three factors: 1 the concentration of ions on the inside and outside of the cell; 2 the permeability of the cell membrane to those ions i.

There are also negatively charged proteins within the cell to which the cell membrane is impermeable. This leads to a separation of charges across the membrane and therefore a potential difference across the membrane.

At the peak of the action potential in a cardiac cell e. Therefore, while the resting potential is far removed from the E Na , the peak of the action potential approaches E Na. The concentration of sodium in the extracellular solution is about 10 times higher than the intracellular solution, so there is a concentration gradient driving sodium into the cell. Additionally, at rest, the inside of the neuron is more negative than the outside, so there is also an electrical gradient driving sodium into the cell.

As sodium moves into the cell, though, these gradients change in driving strength. Eventually, the concentration gradient driving sodium into the neuron and the electrical gradient driving sodium out of the neuron balance with equal and opposite strengths, and sodium is at equilibrium. The gradients acting on the ion will always drive the ion towards equilibrium. The equilibrium potential of an ion is calculated using the Nernst equation:.

The constant 61 is calculated using values such as the universal gas constant and temperature of mammalian cells. Therefore, to reach equilibrium, sodium will need to enter the cell, bringing in positive charge. Table 3. Intra- and extracellular concentration and equilibrium potential values for a typical neuron at rest for sodium, potassium, and chloride. Animation 3. At rest, both the concentration and electrical gradients for sodium point into the cell.

As a result, sodium flows in. As sodium enters, the membrane potential of the cell decreases and becomes more positive. Therefore, potassium can diffuse through the membrane but sodium cannot. Initially there is no potential difference across the membrane because the two solutions are electrically neutral; i. There is also a concentration gradient favouring sodium diffusion in the opposite direction but the membrane is not permeable to sodium.

Accordingly, after a few potassium ions have moved out of the cell, the cell will have an excess of negative charge, whereas the outside solution will have an excess of positive charge; i. They being positive are attracted by the negative charge on the intracellular side of the membrane and are repulsed by the positive charge on the extracellular side of the membrane. As long as the force due to the concentration gradient driving potassium ions outside the cell is greater than the electrical force driving it in the opposite direction there will be net outside movement of potassium ions; the cell will become more and more negative until the electric force opposing the exit of potassium ions outside of the cell equals the force due to the concentration gradient favouring its exit.

At the equilibrium potential there is no net movement of the ion because the opposing forces acting on it are exactly balanced. Your browser does not support script RMP Laboratory. All cells under resting conditions have an electrical potential difference across the plasma membrane such that the inside of the cell is negatively charged with respect to the outside.