Membrane potential

Membrane rest potential is an electrical potential (reserve) formed between the outer surface of the cell membrane and the inner side of the plasma membrane. The inner side of the membrane with respect to the outer surface always has a negative charge. For cells of each species, the rest potential is almost constant. Thus, in warm-blooded cells in skeletal muscle fibers, it is 90 mV, for myocardial cells - 80, for nerve cells - 60-70. Membrane potential is present in all living cells.

In accordance with modern theory, the considered electric reserve is formed as a result of active and passive movement of ions.

Passive motion occurs along the gradient of concentration, it does not require energy expenditure. The cell membrane at rest has a greater permeability for potassium ions. In the cytoplasm of the nerve and muscle cells of their (potassium ions) is present thirty to fifty times more than in the intercellular fluid. In the cytoplasm, the ions are in free form and diffuse, in accordance with the concentration gradient, into the extracellular fluid through the membrane. In the intercellular fluid, they are retained by intracellular anions on the outer surface of the membrane.

In the intracellular space contains mainly anions of pyruvic, acetic, aspartic and other organic acids. Inorganic acids are contained in a relatively small amount. Through the membrane anions can not penetrate. They stay in the cage. Anions are located on the inside of the membrane.

In connection with the fact that the anions have a negative charge, while the cations have a positive charge, the outer surface of the membrane has a positive charge and the internal one has a negative charge.

In the extracellular fluid, sodium ions are eight to ten times more abundant than in the cell. Their permeability is negligible. However, due to the penetration of sodium ions, the membrane potential is reduced to some extent. In this case, diffusion of chloride ions into the cell takes place. The content of these ions is fifteen to thirty times higher in extracellular fluids. Due to their penetration, the membrane potential increases somewhat. In addition, there is a special molecular mechanism in the membrane. It ensures active promotion of potassium and sodium ions in the direction of increased concentration. Thus, ionic asymmetry is maintained.

Active movement of ions is the result of the functioning of the potassium-sodium "pump" (pump). The active movement of sodium ions from the cell is due to the penetration of potassium ions into the cell. In the conjugate pump transport is carried out by carriers, which, in turn, are transported by metabolic energy during the decomposition of ATP. Due to the hydrolysis energy, ATP 2 molecules of potassium ion penetrate into the cell, and 3 sodium ions are transported outward.

In a state of rest, up to twenty percent of cellular energy resources are expended in the fibers of the musculature to ensure the functioning of ionic pumps .

The action of the enzyme adenosine triphosphatase is the splitting of ATP. Poisoning of nerve fibers with cyanides, monoiodoacetate, dinitrophenol and other substances, including those stopping the synthesis and glycolysis of ATP, provokes its (ATP) decrease in the cytoplasm and the termination of the functioning of the "pump".

The membrane is permeable also for chloride ions (especially in muscle fibers). In cells of high permeability, potassium and chlorine ions equally form membrane rest. In other cells, the contribution of the latter to the indicated process is negligible.