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The invention of the galvanometer and other instruments for measuring electricity in the twenties of the 19th century enabled physiologists to make accurate measurements of the currents generated in living tissues by means of special physical devices.

Using a galvanometer, Matteucci (1838) first showed that the muscle exterior was electrically positive to interior, and that the potential difference of the state of rest declined sharply during excitation. Matteucci also carried out what is known as the secondary contraction experiment: when a contracting muscle was brought into contact with the nerve of another nerve-muscle preparation, the muscle of the latter also contracted. The phenomenon is explained as follows: during excitation the action potentials are strong enough to stimulate the nerve in contact with the first muscle, so leading to contraction of the second.

The theory of electrical phenomena in living tissues was more fully developed by Du Bois-Reymond in the forties and fifties of the last century. His special contribution was the irreproachable technique of his experiments. Using a galvanometer, an induction apparatus and non-polarizing electrodes improved and adapted by him to the needs of physiology, Du -Bois-Reymond presented indisputable evidence of the presence of electrical potentials in living tissues both at rest and during excitation. Techniques for registering biopotentials have been continuously improved; since then, in 1880*s, for example, the telephone was used for electrophysiological studies by Vvedensky, a capillary electrometer by Lippmann, and at the turn of the century, the string galvanometer by Einthoven.

Today, thanks to the development of electronics physiologists have at their disposal very much better measuring instruments of low inertia (loop oscillographs) or with practically no inertia at all (cathode-ray tubes). The necessary degree of amplification of biocurrents is ensured by a-c and d-c electronic amplifiers. Microphysio-logical methods of study have also been developed that enable potentials to be taken from individual nerve and muscle cells and from nerve fibres. In this connection the giant nerve fibres (axons) of squid are particularly important as they may be up to one millimetre in diameter, which makes it possible to introduce thin electrodes into them, to perfuse them with solutions of various composition, and to use tracers to study the ion permeability of the excitable membrane. Modern concepts of the way biopotentials arise are largely based on data from experiments on such axons.

RESTING POTENTIAL