Читать «Human Physiology. В двух томах. Том 2» онлайн - страница 9

After-potentials are also linked with changes in membrane permeability to sodium and potassium ions. It is supposed, for example, that the positive after-potential is due to the fact that membrane permeability to potassium ions remains heightened for a time after termination of the action potential as compared with the initial level. On the other hand, an increase in the flow of potassium ions from the protoplasm leads to an increase in membrane potential, i.e. to an after-hyperpolarization of the membrane.

Negative after-potential is apparently due to membrane permeability to sodium ions remaining increased for a time after termination of the action potential as compared with the initial level.

The sodium theory of the origin of the action potential was advanced by Hodgkin, Katz and Huxley in 1952. An important argument in favour of it is the direct relationship between the amplitude of the action potential and the concentration of sodium ions in the external solution.

Experiments on giant nerve fibres perfused with artificial saline solutions have yielded important new evidence supporting these conjectures. It has been found that when a saline solution rich in potassium ions is substituted for the axoplasm the fibre membrane retains not only its normal resting potential but also its capacity over a long period to generate hundreds of thousands of action potentials of normal amplitude.

If, however, the potassium ions in the intracellular solution are partially replaced by sodium ions, so reducing the gradient of sodium concentration between the external medium and the internal solution, action potentials are sharply reduced, and with complete substitution of sodium for potassium, the fibre completely loses its capacity to generate action potentials.

These experiments leave no room for doubt that the surface membrane is in fact the site where the potential difference occurs both at rest and during excitation. It has become cledr that the difference in concentration of sodium and potassium ions inside and outside the fibre is the source of the electromotive force giving rise to the resting and action potentials, while the action potential is caused by changes in the ion permeability of the cell membrane.

THE LAWS OF STIMULATION

Any agent that sharply increases the sodium permeability of a membrane stimulates excitable tissue.

Nerve and muscle fibres can be stimulated by electric current, mechanical action (pinching, striking, or cutting), sudden chilling or heating, or by various acids, alkalis, and concentrated salt solutions, etc.