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Such changes of potential near the poles are called electrotonic. Catelectrotonic changes around the cathode, and anelectrotonic changes around the anode are distinguished.

Catelectrotonic and anelectrotonic changes in membrane potential have no connection with the active response of an excitable formation to the current applied. They are of a purely physical nature, and are therefore commonly considered to be passive changes of potential in contrast to active changes, i. e. the active depolarization and hyperpolarization of the membrane occurring during excitation and caused by changes in its permeability to sodium and potassium ions.

THE KINETICS OF ION PERMEABILITY CHANGES OF A MEMBRANE DURING STIMULATION

A rise in membrane potential at the anode (passive hyperpolarization) is not accompanied with any change in the ion permeability of the membrane even when a strong current is applied. Therefore,

pic. 126. Changes in the sodium and potassium permeability of a membrane during depolarization by a direct current. Displacement of membrane potential (top) by 56 mV

when a d-c circuit is closed no excitation occurs at the anode. In contrast, a fall in membrane potential at the cathode (passive depolarization) first entails a brief increase in permeability to sodium ions and then a slow increase in permeability to potassium ions (Fig. 126).

The first signs of a slight increase in sodium permeability are noted with a current strength around 50 to 80 per cent of the threshold value. As the current increases in strength and approaches the threshold, sodium permeability rises further, and finally an action potential arises when the current reaches the threshold.

The increase in permeability to sodium ions during a threshold stimulation does not reach a maximum immediately. At the first instant depolarization of the membrane at the cathode causes a relatively slight increase in sodium permeability, but as positively charged sodium ions begin to enter the protoplasm under the influence of this change, depolarization of the membrane increases, which leads to another considerable rise in permeability to sodium and consequently to further depolarization, which in turn again increases sodium permeability, etc.

This ever-growing cyclic process is called regenerative depolarization, and can be represented schematically as follows:

The reason for the increase in sodium permeability during depolarization is still obscure. One of the most common views is that the pores through which sodium ions can diffuse into the cell are closed (“plugged”) in a state of rest by larger calcium ions, while the difference in potential across the membrane (resting potential) retains the latter in the pores. When a stimulus depolarizes the membrane, the calcium ions pass out of the pores, and so make way for sodium ions.

As it has already been pointed out, the increased sodium permeability lasts for only tenths of a millisecond. It then reduces and cannot be raised again by increasing depolarization, because of the development of a particularly active process in the membrane known as inactivation (p. 17), the nature of which is also obscure. There are reasons to suppose that inactivation is based on certain chemical changes in the structural elements of the membrane, possibly a reconstruction of its protein-lipoid complexes caused by prolonged depolarization. Evidence for that is the sharp reduction of the rate of inactivation with a fall in temperature, and also the fact that it may be retarded in myelinated nerve fibres by Ni", Co**, Cd*' ions blocking the sulph-hydryl groups of proteins.