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If electrodes are now put into the right and left halves of the vessel, a measuring instrument will show the potential difference by which the solution with the smaller concentration of ions of K_tSO₄, into which positively charged potassium ions mainly diffuse, is electrically positive to the solution with the greater concentration.

The potential difference (E) can be calculated by Nernst’s formula:

E = 58 log mV at 18°C.

There are many grounds for supposing that similar relationships exist in living nerve fibre, since the concentration of potassium ions in protoplasm is more than 30 times that in the external solution, while the organic (protein, etc.) anions of protoplasm do not in practice penetrate the membrane.

In a state of physiological rest the diffusion of positively charged potassium ions from protoplasm to the external fluid lends a positive charge to the outer surface of the membrane, and a negative charge to the inner one.

An important argument in favour of this view is the fact that the difference in potential across the membrane of a muscle fibre (about 90 mV), calculated by Nernst’s formula, has proved to be close to that obtained in experiments using an intracellular microelectrode.

It has also been established that an increased concentration of potassium ions in the external medium and, consequently, a reduction of the difference in the concentration of these ions on both sides of the membrane lead to a drop of the resting potential, and it has been noted that within a certain range of concentrations the changes show a clear quantitative coincidence with those calculated from the formula.

The most important direct evidence in favour of these conjectures, however, was obtained by Hodgkin and his co-workers (1962) in experiments with substituting saline solutions for protoplasm in the giant nerve fibres of squid. Protoplasm was cautiously squeezed out of a fibre about one millimetre in diameter, and the collapsed membrane filled with an artificial saline solution.