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In cases when the concentration of potassium ions in the solution was close to the intracellular, a potential difference approximating that of the resting potential of a normal fibre (50 to 80 mV) was established across the membrane. A reduction of potassium ion concentration in the internal solution led to a regular reduction or even to a distortion of the resting potential.

Experiments like that have shown that the concentration gradient of potassium ions is really the principal factor determining the value of the resting potential of a nerve fibre.

Apart from potassium ions, sodium ions diffusing into protoplasm from the extracellular fluid, where their concentration is high, also give rise to a resting potential. The diffusion is largely hampered by the low permeability of the membrane to sodium at rest. Nevertheless, in diffusing through the membrane sodium ions transfer their positive charges into the protoplasm, which somewhat reduces the value of the resting potential produced by the diffusion of potassium ions out of the cell, which is why the resting potential of most nerve cells and fibres is not 90 millivolts, as it would be expected if the potential were produced exclusively by potassium ions, but only between 60 and 70 millivolts.

Thus, the value of the resting potential of nerve fibres and cells is determined by the ratio between the positively charged potassium ions diffusing outward from the cell per unit time and the positively charged sodium ions diffusing in the opposite direction. The higher the ratio the greater is the resting potential, and vice versa.

ACTION POTENTIAL

If a sufficiently strong stimulus (for instance, an electric shock) is applied to part of a nerve or muscle fibre, it will give rise to excitation, the main manifestation of which is a rapid variation of the membrane potential, which is known as the action potential.

Action potential can be registered by two methods: by means of electrodes applied to the outer surface of a fibre (extracellular leads), and by means of a microelectrode introduced into the protoplasm (intracellular lead).

With an extracellular lead it can be observed that the surface of the excited portion of the fibre becomes electrically negative in relation to adjacent areas at rest for a very brief interval measured in thousandths of a second.

Physiologists for long believed that the action potential was merely the result of the transient disappearance of the potential difference existing across the membrane at rest. Accurate measurements by means of intracellular microelectrodes, however, have shown that the amplitude of the action potential exceeds the value of the resting potential by 30 to 50 millivolts, an excess due to the fact that the resting potential does not simply disappear with excitation, but that the potential difference is reversed, so that the outer surface of the membrane receives a charge which is electrically negative in relation to its inner side.