How are the Potentials in the Body generated?

In the 18^th Century, Galvani demonstrated using microelectrodes that there is a potential difference between the outside and the inside of a cell, or a potential difference exists across the membrane of a cell. The inside of a cell has a negative charge and the outside has a positive charge, the potential difference being around 90mV. This discovery formed the basis of the explanation of the action of living tissue in terms of bioelectric potential.

It has been established that the body, which is composed of living tissues produces multiple signals with two internal sources namely muscles and nerves. Normal muscular contraction is associated with the migration of ions, which generates potential differences measurable with suitably placed electrodes. A typical example, the heart and the brain produces characteristic patterns of voltage variations which when recorded and analyzed are useful in both clinical practice and research.

Potential differences are also generated by the electrochemical changes accompanied with the conduction of signals along the nerves to or from the brain. These signals are of the order of a few microvolts that give rise to a complicated pattern of electrical activity when recorded. The ability of being able to record and analyze this electrical activity occurring in the living tissues made it possible to be able to diagnose certain diseases affecting the muscles and nerves.

Bioelectric potentials are generated at a cellular level and the source of these potentials is ionic in nature. A cell consists of an ionic conductor separated from the outside environment by a semipermeable membrane which acts as a selective ionic filter to the ions. This means that some ions can pass through the membrane freely whereas others can’t. All living matter is composed cells of different types.

Body fluids surround the cells of the body. These body fluids are ionic and provide a conducting medium for electric potentials. The principal ions involved with the phenomena of producing cell potentials are Sodium (Na⁺), Potassium (K⁺) and Chloride (Cl^–). The membrane of excitable cells readily permits the entry of K⁺ and Cl^– but impedes the flow of Na⁺ even through there may be a very high concentration gradient of Sodium across the cell membrane. This results in the concentration of Na⁺ ions more on the outside of the cell membrane than on the inside. Since Sodium is a positive ion, in its resting state, a cell will have a negative charge along the inner surface of its membrane and a positive charge along the outer portion. The unequal charge distribution is a result of a certain electrochemical reactions and processes occurring within the living cell and the potential measured is called the resting potential. The cell in such a condition is said to be polarized. A decrease in this resting membrane potential difference is called depolarization.

The resting potential is approximately -90mV with respect to the inside of the cell.

When the cell is excited or stimulated momentarily, the outside of the cell acquires a negative charge and the inside acquires a positive charge of +20mV and the cell is said to be depolarized as shown in the figure below:

After about 2 ms, the cell automatically comes back to its resting state. The waveform that is generated due to depolarization and repolarization when the cell is excited by an external stimulus is called action potential.

Waveform of an Action Potential

The curve showing the potential inside the cell with respect to time during resting potential, depolarization and repolarization is called waveform of the action potential. The cycle time of the waveform varies from cell to cell. In nerve and muscle cells, the repolarization is fast after depolarization and the cycle is completed in a short time (about 1.0 millisecond) whereas the cell of the heart repolarizes very slowly and the cycle takes more time (150-300 milliseconds). The waveform of action potential does not depend upon the intensity of the stimulus, however, the stimulus must be strong enough to activate the cell i.e. value should reach stimulus threshold.

After the completion of the cycle, there is a short time during which the cell cannot be excited. This period is called the absolute refractory period and it lasts about one millisecond. This happens because the energy associated with the action potential is developed from metabolic processes within the cell which takes time for completion. The cell can then be excited after the refractory period by a strong stimulus in a period of time called relative refractory period. This relative refractory period lasts longer compared to the absolute refractory period.

You can also read: Sources of Biomedical Signals in a Human Body

Propagation of Action Potential

Each excited cell generates an action potential and a current begins to flow due to this. The other cells in the neighbourhood of the excited cells also get excited due to ionic flow and the action potential begins to travel which is termed to as the propagation of the action potential. The rate at which it propagates is called propagation rate or conduction rate. The conduction rate depends upon:

• Type of cell
• Diameter of the muscle or nerve

Since the nerve action is fast, it is found that the conduction rate of action potential in the nerve is fast (about 2 to 140 m/s) while it is very slow in heart muscle (0.03 to 0.4 m/s). The bioelectric signals of clinical interest and research which are often recorded are produced by the coordinated activity of large groups of cells. In this type synchronized excitation of many cells, the charges tend to migrate through the body fluids towards the still unexcited cell areas. Such charge migration constitutes an electric current and hence sets up potential difference between various portions of the body, including its outside surface. Such potential differences can be picked up by placing conducting plates (Electrodes) at any two points on the surface of the body and measured with the help of a sensitive instrument. These potentials play an important role in diagnosis and therapy. The primary characteristics of typical bioelectric signals are shown below: