New measurements of the capacity and the resistance of the myelin sheath and the nodal membrane of the isolated frog nerve fiber.

I T HAS BEEN SHOWN that the myelin sheath and the surface membrane at the node of Ranvier of the myelinated nerve fiber behave, to a weak penetrating current, like a condenser with an ohmic resistance connected in parallel (1-5). The absolute values of these capacities and resistances have been estimated already with some accuracy. In order to analyze the process of action potential production at the nodal membrane, it is desirable to obtain more accurate measurements on the physical constants of the membranes at rest. In the present investigation, the physical constants of the myelin sheath were measured by recording the current that passes through this membrane when two nerve impulses, each coming from an end of a single fiber preparation, collide at the internode under investigation. This collision of impulses (6) makes the spatial distribution of the potential inside the myelin sheath uniform and consequently increases the accuracy of the measurement. Measurements of the capacity and the resistance of the resting nodal membrane were made, using the collision technique, on a node treated with a sodium-free Ringer or with a diluted cocaine-Ringer solution. The effects of several chemicals and of temperature changes upon the physical properties of the nerve membrane were investigated by the same technique.