Measurement of the action potential of myelinated nerve fiber.

THE PEAK VALUE of the action potential of the vertebrate myelinated fiber has been previously recorded by a number of investigators. (1-5 and others). Values range from about 60-155 mv depending on the method employed. These methods may be divided into three general categories. The action potential at the node of a single fiber may be estimated from a knowledge of the action current in the axis-cylinder and the resistances in the various parts of the nerve fiber. Action currents have been determined by measuring the potential drop across a low resistance shunting the isolated section of a nerve fiber (2) and the resistances involved have been measured to a first approximation (cf. also 6, p. 32), but the uncertainty in the latter measurements lends considerablelimitation to the accuracy of the action potential estimated by this method. A second method adopted by Huxley and Stgmpfli measures the peak value of the action potential by determining the amplitude of a rectangular voltage pulse just necessary to buck out the action potential of the fiber. The method suffers from errors introduced by the capacitative flow of current across the myelin sheath (see e.g. 6, fig. 33). The third method measures the action potential of a single fiber by introducing a hyperfine glass pipette electrode into the fiber (4, 5). Errors due to capacitative losses between the fluid inside the microelectrode and the external solution are probably adequately compensated for by the use of capacitative positive feedback in the amplifier (7-9). However, after the microelectrode is introduced into the fiber there occurs an apparent prolongation of the time required for the impulse to jump across the internode impaled and a rapid deterioration of action potential suggesting damage to the fiber. Thus, the action potential recorded by this method is also subject to some doubt. Most of these difficulties are avoided by connecting the isolated single fiber directly to the grid of a high impedance amplifier developed for use with microelectrodes.Capacitative effects can be minimized by exposing the isolated internode to dry air and by utilizing a positive feed-back in the amplifier. Injury effects are avoided by using the fiber internode itself as a micro-electrode extension connecting the amplifier grid to the inside of the active node.