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In a state of rest there is a difference in potential of the order of * 60-90 millivolts between the outer surface of a cell and its protoplasm, the cell surface being electrically positive with respect to the protoplasmr This potential difference is commonly called the resting membrane potential. It can only be measured accurately by means of microelectrodes designed for intracellular recording.

A microeledrode is a micropipette, i.e. a thin capillary drawn from glass tubing with diameter at its tip around 0.5 micron. The micropipette is filled with a saline solution (3 M KC1) into which a metal electrode is dipped and connected to a measuring instrument, e.g. an oscillograph with a d-c amplifier. The electrode is positioned above the object to be examined, e.g. a muscle, and introduced into a cell by means of a micromanipulator, an instrument with a micrometer screw. Another electrode of normal size is applied to the muscle surface, or is placed in the Ringer’s solution in which the object under examination is immersed (Fig. 113).

As soon as the microelectrode pierces the cell membrane, the oscillograph beam is immediately deflected downward from its initial position and becomes stabilized at a new level (see Fig. 113), thus revealing a potential difference between the exterior and the interior of the cell.

With successful introduction the cell membrane firmly envelops the tip of the microeiectrode and the cell retains its functional capacity for several hours without sign of damage.

The potential difference across the cell membrane can also be observed without resort to microelectrodes, it being sufficient to make a transverse incision on a nerve or a muscle and apply recording electrodes: one to the site of incision, the

fic. 113. Measuring the membrane potential of a muscle fibre (A) by means of an intracellular microelectrode (diagram)

M — microelectrode; I — neutral electrode. The beam on the oscillograph screen (B) indicates that the potential difference between M and 1 is zero before the membrane (A) Is punctured with the microelectrode. At the moment of puncture (shown by the arrow) a potential difference is discovered, showing that the inner side of the membrane is electrically negative in relation to Its outer surface

other to the intact surface. An electrical measuring instrument will then show a flow of current (resting current) between the two portions of tissue, the intact one being electrically positive in relation to the site of the incision. This method, however, does not permit measurement of the complete difference in potential across the membrane because the fluid bathing the tissue from the surface and contained in the intercellular spaces short-circuits (shunts) the recording system. The measured potential difference between the damaged and intact portions of tissue therefore usually does not exceed 30 to 50 millivolts.

ORIGIN OF RESTING POTENTIAL

Various theories have been put forward to explain the nature of resting potential. Current views stem from the work of Chago-vets in 1896, when he was still a medical student, in which he advanced the idea of the ionic nature of the bioelectrical processes and tried to explain the origin of the potentials by means of Arrhenius' theory of electrolytic dissociation. In 1902, Bernstein developed the membrane-ion theory which was later modified and substantiated experimentally by Hodgkin and Huxley (1952) and is now widely accepted. According to this theory, bioelectrical potentials are caused by unequal concentrations of potassium, sodium, and chlorine ions within the cell and outside it, and by the variable permeability of the surface of the membrane to them.