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To understand the reason for this phenomenon it is necessary to consider the changes in membrane potential caused by direct current.

PASSIVE CHANGES IN MEMBRANE POTENTIAL AT THE CATHODE AND THE ANODE

Passage of an electric current through nerve or muscle fibres primarily produces changes in the membrane charge.

In the region where the positive pole (anode) is applied to the surface of the tissue the positive charge on the outside of the membrane increases, i. e. hyperpolarization takes place, but when the negative pole (cathode) is applied to the surface, this positive charge is reduced, i. e. depolarization occurs (Fig. 124).

Fig. 124 shows that both when the current circuit is closed and opened changes in the membrane potential of the nerve fibre arise and cease gradually rather than instantaneously describing an exponential curve. That is because the surface membrane of a living cell has the properties of an electrical condenser (Chagovets).

Potential
f Cathode | Anodefig. 124. Depolarization of a membrane (a) at the cathode and hyperpolarization (t>) at the anode during the passage of a weak subthreshold current through a nerve fibre. The switching on and off of the current is indicated by the arrows I and f

। Outer surface

fig. 125. Diagram of a simple electric circuit reproducing the electrical properties of a membrane

Я — resistance; C — capacitance; E — electromotive force of membrane at rest. Average values of H, C, and E for a motor neurone arc given

M0~⁹F ft ^a-№^fohm t~^10mV I Inner surface

The plates of this “tissue condenser” are the outer and inner surfaces of the membrane, while the dielectric is the layer of lipoids, which possess considerable resistance. In view of the existence of pores in the membrane, through which ions can pass, the resistance of this layer is not infinite, as it is in an idea) condenser. The surface membrane of the cell is therefore usually likened to a condenser with a resistor connected in parallel, through which leakage of charges can occur (Fig. 125).

The rate at which the membrane potential changes when current is switched on and off depends on the capacitance (C) and resistance (R) of the membrane. The lower the product of these two values (RC), which is known as the membrane time constant, the quicker the rise of potential at a given current strength; conversely a lower rate of potential increase corresponds to a larger value of RC.

Changes in membrane potential not only occur directly at the points where the direct current cathode and anode are applied to the nerve fibre, but are also observed at a certain distance from the poles, though their value gradually diminishes the further the point is from cathode or anode.