Effect of Electrotonus on Accommodation in Nerve

Abstract

Hill 1 has proposed an approach to the response of excitable tissues involving two processes, one a rise of the “local potential” and the other a change of threshold called “accommodation”, the rates of which are represented by the time constants “κ” and “λ” respectively. Blair 2 has pointed out some theoretical inadequacies arising from investigations of the effects of electrotonus on rheobase and chronaxie (theoretically .693k), but in the absence of similar studies on λ, the extent of such limitations is not clear. Consequently, the present investigation of λ was undertaken.

The technic described by Solandt 3 employing exponentially rising currents was used to determine the λ of the sciatic nerves of Rana pipiens. The same nonpolarizable electrodes, 2 cm apart, were employed to produce a 2-second electrotonus and to apply the exponential currents. Special precautions were taken to minimize residual and progressive effects. Most experiments were performed at 20°C.

The chief results obtained are summarized in the accompanying figure. The ordinate represents the relative change in λ (i.e., the ratio of λ during electrotonus, λ_e, to λ of the normal nerve, λ_n) and in rheobase (i.e., the rheobase during electrotonus, V_e, divided by its normal value, V_n), while the abscissa is the intensity of electrotonus (E/V_n) in rheobases. It can be seen from the continuous curve, which represents the modifications in λ, that λ is increased by anelectrotonus and decreased by catelectrotonus. In none of the 168 measurements of λ during electrotonus was an exception to this found. It is obvious, too, that λ is a continuous function of the electrotonic intensity, a function which is similar to that of rheobase represented by the broken curve.