turbances in the surroundings may lead to a large increase of permeability, and hence to marked stimulation.[1]
On this hypothesis we can also understand why the state of excitation is transmitted from one region of the irritable element to another. It is highly probable that the effect of a local stimulation is propagated over the surface of the muscle-cell or nerve-fiber because of the electrical variation which the permeability-change at the excited region itself produces. This electrical variation affects the adjacent regions of the membrane, and alters their permeability, with corresponding electrical effects, and so the effect spreads. The explanation of the conduction-process in a nerve or other irritable tissue is on this view identical with that of the stimulation-process. There is, in fact, good evidence that the region in a state of excitation simply excites the adjoining regions electrically by means of its action-current, and that the effect is transmitted in essentially this manner.
It is possible to change the polarization of the membrane, and hence its permeability, in other ways than by passing an electrical current. Or we may alter the permeability directly, by acting on the cell by chemical substances, or by suddenly changing the temperature, or by mechanical action. When such treatment produces a sufficient increase of permeability, we may suppose that all ions become free to pass the membrane, and that a polarization-change then occurs, with consequent stimulation which, like other forms of stimulation, is self-propagating. On such a view the ordinary forms of mechanical and chemical stimuli are at bottom electrical in their nature. Such stimuli act by directly altering the permeability of the membrane and hence its electrical polarization.
On the other hand, the properties of the membrane may be so modified under certain conditions that it fails to respond to changes of polarization by changes in its permeability. This occurs, for instance, in narcosis. I have found that narcotics, in the concentrations at which they anesthetize the musculature of Arenicola larvæ, also check or prevent the permeability-increasing action of isotonic sodium chlo-
- ↑ The assumption of a permeability-increase at the time of stimulation 1 is the only hypothesis, so far as I know, that accounts at once for the two characteristic and invariable accompaniments of stimulation, (1) the negative electrical variation, and (2) the temporary loss of irritability (refractory period) during the electrical variation. The time-relations of these two outwardly diverse phenomena coincide, as Tait has shown, and both are to be regarded as expressions or consequences of the same change, namely, a temporary increase in the permeability of the limiting membranes. This increase involves a temporary loss of the semi-permeability which is essential to the maintenance of the normal polarization of the membrane, and also—according to Nernst's theory—essential to electrical stimulation. I therefore regard the existence of a refractory period as furnishing strong support to the general theory of stimulation and conduction outlined above.
