Resting Potential

In neurons, as in other cells, ions are unequally distributed between the interior of cells and the surrounding fluid, resulting in a negatively charged environment in the cell relative to the outside. For a resting neuron, the resting potential is about -70 millivolts (mV).

Involved Membrane Channels

Potassium ions (K⁺) and sodium ions (Na⁺) play an essential role in the formation of the resting potential. During a resting potential, the concentration of K⁺ is higher inside the cell, while the concentration of Na⁺ is higher outside. This concentration gradient is well-established by important membrane channels.

Sodium-potassium pump: maintain the Na⁺ and K⁺ gradients; uses the energy of ATP hydrolysis to actively transport Na⁺ out of the cell and K⁺ into the cell, in a ratio of three to two, respectively. The pump acts slowly, producing a small net charge, so a neuron relies on ion channels to create a drastic gradient.

Potassium channel: also called the potassium leak channels, these channels permanently open and are crucial to the establishing of a resting potential. A neuron has many potassium channels, so the large efflux (outflow) of K⁺results in a negatively charged environment inside the cell.

Sodium channel: also called the sodium leak channels, these channels are also permanently open, just like the potassium channels. However, there is way less sodium channels than there are potassium channels, so Na⁺ cannot readily pass through the membrane, resulting in a positively charged environment outside the cell.

Electrochemical Equilibrium

Two vectors determine the state of equilibrium of a neuron-the concentration force and the electrical force. When both vectors are in balance, the neuron is said to be in equilibrium. Considering an electrochemical equilibrium requires imagination and logic; when dealing with questions of cells whose ions are not in equilibrium, keep in mind that you should deal each ion separately, balancing one force before considering the other.

When a neuron reaches equilibrium, you can try using the Nernst equation to calculate the equilibrium potential of individual ions.

                            [ion]outside   
Eion = 62 mV( log ――――――――  )

                       [ion]inside

For instance, plugging the K⁺ concentration (extracellular concentration: 5 mM, intracellular concentration: 140 mM) into the Nernst equation reveals that the equilibrium potential for K⁺ is -90 mV, and Na⁺ (extracellular concentration: 150 mM, intracellular concentration: 15 mM) is +62 mV.

Because neither K⁺ nor Na⁺ is at equilibrium in a resting neuron, there is a net flow of each ion across the membrane. As long as the resting potential remains, the K⁺ and Na⁺ current, as well as the

ion concentrations, will hold steady, until an action potential is induced. And that will be another story to begin with.

Membrane channels and pumps generate the resting potential.

Reference:

Campbell, et al. Biology: A Global Approach. 11th ed., Pearson, 2017.