In neurons at rest, the membrane potential is called the resting membrane potential. If a neuron were permeable only to potassium, its resting membrane potential would be –90mV, the equilibrium potential for potassium. However, resting neurons are also slightly permeable to sodium, and the electrochemical gradient for sodium causes it to move into the cell. The resting membrane potential results from the movements of both sodium and potassium ions. The positively charged sodium ions that have entered the neuron make the membrane potential more positive than -90mV, which is the equilibrium potential for potassium. For many neurons the resting membrane potential is close to –70mV. When the resting membrane potential is not equal to the potassium equilibrium potential, the forces acting on the potassium are no longer equal and opposite. At –70mV, the chemical force pushing potassium out of the cell is greater than the electrical force pulling potassium back into the neuron, but only a little bit. The force on potassium is small, but the neuron is very permeable to potassium. As a result, a small amount of potassium moves continuously out of the neuron. At –70mV, the force on sodium is very large, but the neuron is only slightly permeable to sodium. As a result, a small amount of sodium moves continuously into the neuron. At –70mV, the resting membrane potential, potassium leaks out of the neuron, and sodium leaks into the neuron. Just as a boat that begins to leak will eventually sink, a leaking neuron will eventually fail to function. If ions continue to leak, the neuron will be unable to communicate. If the ions leaks continue, the concentration gradients for sodium and potassium will decrease. As the concentration gradients decrease, the membrane potential moves toward zero. When there are no longer any chemical or electrical forces to move ions across the membrane, the neuron cannot send or receive the electrical signals it needs to communicate. The captain can keep her boat afloat by bailing water out as fast as it enters. Neurons can prevent the potassium and sodium gradients from running down by transporting potassium back into the cell and sodium back out of the cell. Of course the neuron doesn’t use buckets to move ions. A membrane enzyme called the sodium-potassium pump actively transports ions to compensate for the sodium and potassium leaks. This pump uses the energy of ATP to move sodium and potassium against their electrochemical gradients. Three sodium ions are pumped out of the neuron for every two potassium ions that are pumped in. The pump compensates for the sodium and potassium leaks, keeping the resting membrane potential at –70mV. It is important to remember that the sodium-potassium pump does not create the membrane potential. Its job is to maintain it. Now let’s observe the overall movement of ions in a resting neuron. Here’s a summary of what we’ve covered: • The concentrations of sodium and chloride are high outside cells in the extracellular fluid, and the concentrations of potassium and organic anions are high inside cells. • The permeability of a cell for ions depends on the number and type of ion channels in the cell membrane. • The electrical and chemical forces for a particular ion combine to become a single force, the electrochemical gradient, which causes the movement of that ion across the cell membrane. • In nonexcitable cells, the membrane potential depends only on potassium. Potassium comes to equilibrium when the membrane potential for the cell is –90mV. • The resting membrane potential in neurons depends on the distribution of sodium as well as potassium across the cell membrane. Resting membrane potentials in neurons are commonly around –70mV. • The sodium-potassium pump is essential for maintaining the resting membrane potential in neurons.