Membrane potential is the voltage across the plasma membrane. The resting membrane potential is between -60 mV and -80 mV. There are steps in forming the resting membrane potential. First, the known concentration of K+ is 140 mM (millimolar) inside the cell and 5 mM outside of the cell and the Na+ concentration is 150 mM outside of the cell and 15 mM inside the cell. Cl- concentration is 120 mM outside the neuron and 10 mM inside the cell. Such concentrations of chlorine, potassium, and sodium always remain constant because there is sodium potassium in the plasma membrane of the neurons. Also, ATP is used to transport ions against their concentration gradients to maintain the constant concentrations. For instance, ATP helps sodium to go out of the cell and potassium enter the cell. Since the exchange of ions occur at the concentration gradient, it is important to understand that the concentration of gradients of sodium and potassium across plasma membrane represent a chemical type of potential energy. In addition, ion channels in the selective permeability contribute to the electric potential of neurons. For example, there are K+ channels and N+ channels. There are also ions that move through the membrane through these ion channels to generate a potential across the membrane. Consequently, the diffusion of K+ through the potassium channels is very important for forming the resting potential because the outward flow of potassium results in the negative membrane potential of between -60 mV and -80mV. Excess negative charge inside the cell exerts an attractive force that stops the add’l flow of potassium ions outside of the cell. As a result, there is the creation of equilibrium potential which is the membrane voltage for a moving ion when it is at equilibrium. Nernst created an equation to calculate the resting potential of an ion:
E(ion)= 62mV(log [ion]outside/[ion]inside)
The resting potential for potassium is -90 mV. And the resting potential for sodium is 62 mV.