Resting Potential

Every cell has a membrane that is charged. It is developed due to the ions present inside and outside the cell. The membrane's functions are to protect the cell, maintain a membrane charge, and many other functions. The membrane is permeable to specific ions, such as potassium and sodium, which help in maintaining membrane charge.

Knowing these facts, resting potential can be defined as, "A ground value of transmembrane voltage maintained in plant and animal cells."

A potential is generated in a cell when the potassium ions are separated from the intracellular anions, which are immobile. A cell membrane is more permeable to potassium ions, therefore, they flow from the cytosol to the extracellular matrix.

This movement of potassium ions continues till there is a build up of negative charge on the inner side of the membrane. This process takes place once a concentration gradient of potassium is set up by ion transporters or ion pumps. This potential is mostly determined by the concentration of the ions on either sides of the cell.

Nervous system consists of excitable cells called neurons. Like all cell membranes, a neuron has charged ions on either sides. When a nerve cell is not stimulated, there is a net positive charge on the outside and the inside has a net negative charge. The resting potential of a neuron is around -70mV due to unequal distribution of charge across the membrane.

The process of diffusion usually ensures equal distribution of ions in a medium. However, when considering cellular membranes, this process does not play a major role, as compared to the specific ion channels and ion pumps, that allow the maintenance of this potential.

This unequal distribution of ions is necessary, when you consider the change in charge that is generated when an electrical stimulus passes through. It keeps a neuron prepared for the propagation of a nerve impulse, by the generation of an action potential.

This is generated only when a neural impulse has to pass through. It is generated by the opening and closing of sodium and potassium channels, present on the nerve cell membrane. Here are the changes that take place in a neuron during the passage of a nerve impulse.

In this stage the neuron is at rest, with an excess of potassium ions on the inside and excess of sodium ions on the outside. The potential inside the cell is maintained at -70mV. The ion channels are closed at this stage.

An external stimulus causes the opening of sodium ion channels. Thus, the sodium ions begin to move inside the cell causing an increase in the positive charge. The potential must reach -55mV. This is the threshold value.

Once the membrane attains the threshold value, an action potential is generated. The sodium channels open completely to depolarize the membrane. This rapid depolarization reaches +30 mV. This is also known as graded potential.

At this stage, the outer side of the cell membrane is more negative than as compared to the inside. The change in the resting potential opens the sodium channels on the adjacent side as well. This allows the passage of the nerve impulse in the form of a wave. This is the depolarization stage.

Once the action potential starts declining, the potassium channels begin to open. As a result the potassium ions from the inside begin to move outside and restore the negative charge in the inner part of the membrane. Due to this, the value goes below the resting potential. This is a stimulus for the potassium channels to close.

At this stage, the sodium potassium pump restores the original concentration of the ions to restore the potential. These pumps make use of energy, that is ATP, for this purpose.

If there was an equilibrium at 0 rather than -70mV, then reaching the threshold value would have been impossible.

3 ions contribute to the membrane charge of a cell. This equilibrium value is calculated using the Goldman's equation-

E_m = (P_K⁺/P_tot)E_K⁺ + (P_Na⁺/P_tot)E_Na⁺ + (P_Cl^-/P_tot)E_Cl^-

Here, Em is the membrane charge, P is the relative permeability of the respective ion, E is the equilibrium value of the respective ion and Ptot is the total permeability of all the ions.

It is important to remember that the resting potential cannot be the cells equilibrium, as it depends on the expenditure of energy. While calculating, it is necessary to consider the parameters of Goldman's equation, but relative permeability and conductance is also to be considered.

• Smooth muscle cells: -50mV
• Skeletal muscle cells: -95mV
• Astroglial cells: -80mV

Resting potential is thus a very important phenomenon in nervous transmission, muscle contraction, and functioning of organs. Without this, the functions of various organs would be impaired.