Bioelectricity Health Article

Definition

Bioelectricity refers to electrical potentials and currents occurring within or produced by living organisms. It results from the conversion of chemical energy into electrical energy. Bioelectric potentials are generated by a number of different biological processes, and are used by cells to govern metabolism, to conduct impulses along nerve fibers, and to regulate muscular contraction. In most organisms bioelectric potentials vary in strength from one to several hundred millivolts. The most important difference between bioelectric currents in living organisms and the type of electric current used to produce light, heat, or power is that a bioelectrical current is a flow of ions (atoms or molecules carrying an electric charge), while standard electricity is a movement of electrons.

Historical background

Prior to the eighteenth century, European physicians and philosophers generally believed that nervous impulses were conducted to the brain via an organic fluid of some kind. The experiments of two Italians, the physician Luigi Galvani and the physicist Alessandro Volta, demonstrated that the true explanation of nervous conduction is bioelectricity. Impulses within the nervous system are carried by electricity generated directly by organic tissue.

In the nineteenth century, such researchers as Emil du Bois-Reymond invented and refined instruments that were capable of measuring the very small electrical potentials and currents generated by living tissue. One of du Bois-Reymond's students, a German scientist named Julius Bernstein, is generally credited with the hypothesis that nerve and muscle fibers are normally polarized, with positive ions on the outside and negative ions on the inside; and that the current that can be measured results from the reversal of this polarization. In the early part of the twentieth century, several British scientists identified the chemical substances involved in the transfer of information between the nerves and muscles.

Cell membrane potential

Bioelectricity begins with the fact that all animal cells have electrical properties derived from the ability of the cell membrane to maintain unequal charges inside and outside the cell. The cell membrane is semipermeable, which means that it forms a selective barrier to ions, which are electrically charged atoms or atom groups. The semipermeability of the cell membrane allows the cell to maintain concentrations of ions in the cytosol (the fluid portion of cell cytoplasm) that differ from those in the fluid outside the cell. Potassium and chloride ions can diffuse through the membrane relatively easily, while sodium ions cannot diffuse into the cell at all.

Because of the semipermeability of the cell membrane, the concentration of sodium in the fluid outside the cell is higher than in the cytosol; the concentration of potassium is higher inside the cell than outside, and the concentration of chloride is higher outside the cell than inside. There are thus two forms of energy stored across the cell membrane—a chemical force (the differences in ion concentration) and an electrical force. This bioelectric potential across the cell membrane is called the resting potential. In most cells the resting potential is about 50 millivolts.

Diffusion

The most important ions in bioelectrical phenomena are sodium (Na⁺), potassium (K⁺), calcium (Ca₂⁺), and chloride (Cl^-). The first three types of ions carry a positive charge while the chloride ion carries a negative charge.

Ions can move across the cell membrane in two ways. First, they can move through pores called ion channels. Most ion channels are specific to a particular ion or group of ions. In addition, most ion channels are gated, which means that they require a stimulus to open them. Because ions move passively through the channels, the only direction they can travel via channels is from areas of high concentration to areas of low concentration. This movement from areas of higher to areas of lower concentration is called diffusion.