The authors have agreed that this Chapter should not be an exhaustive compilation of all papers. Instead they intend to submit the essential modern knowledge necessary to neurobiologists working with peripheral nerve of frogs which, in their opinion, is an essential test object to develop the theoretical and experimental basis of the function of excitable membranes of vertebrates. They want, in particular, to present a chapter which in the future can be used to familiarize newcomers with the methods and theories used in this field. Since there is almost no work on unmyelinated amphibian fibers, the discussion is almost exclusively on myelinated nerve.

Electrophysiology of the Peripheral Myelinated Nerve

A method similar to the sucrose-gap technique introduced be Stapfli is described for measuring membrane potential and current in singly lobster giant axons (diameter about 100 micra). The isotonic sucrose solution used to perfuse the gaps raises the external leakage resistance so that the recorded potential is only about 5 per cent less than the actual membrane potential. However, the resting potential of an axon in the sucrose-gap arrangement is increased 20 to 60 mv over that recorded by a conventional micropipette electrode when the entire axon is bathed in sea water. A complete explanation for this effect has not been discovered. The relation between resting potential and external potassium and sodium ion concentrations shows that potassium carries most of the current in a depolarized axon in the sucrose-gap arrangement, but that near the resting potential other ions make significant contributions. Lowering the external chloride concentration decreases the resting potential. Varying the concentration of the sucrose solution has little effect. A study of the impedance changes associated with the action potential shows that the membrane resistance decreases to a minimum at the peak of the spike and returns to near its initial value before repolarization is complete (a normal lobster giant axon action potential does not have an undershoot). Action potentials recorded simultaneously by the sucrose-gap technique and by micropipette electrodes are practically superposable.

https://rupress.org/jgp/article-pdf/45/6/1195/1242867/1195.pdf

Membrane potentials of the lobster giant axon obtained by use of the sucrose-gap technique.

Speculating in his Lecons sur l’Histologie du Systeme Nerveux on the physiological function of myelin and nodes in conduction in myelinated nerve, Ranvier (1878) concluded that the myelin sheath, because of its lipid makeup, would protect the axon cylinder against compression. While this conclusion does not play a prominent part in current thinking, his second conclusion does—namely, that the myelin might serve as a nonconducting envelope to surround the conducting axon and insulate it electrically from the external conducting medium. Referring to the analogy between conduction in a myelinated nerve and conduction in a marine telegraph cable, Ranvier wrote: 
But what is the role of the myelin sheath itself? It clearly has a protective role; it preserves the axis-cylinder from compression. As it is liquid, or almost liquid, pressures exerted on it are transmitted in every direction and are thus distributed over many points, so that their constrictive action on the axis-cylinder is much reduced. The myelin has also perhaps another role; it is probably an insulating sheath. One knows that electric wires that are immersed in a conducting medium must be insulated from this medium by a nonconducting sheath; it is on this principle that the construction of submarine cables rests. It would be possible—certain facts lead one to believe—that the transmission of sensory or motor impulses has some analogy with the transmission of electricity, and maybe it is convenient that each nerve tube is insulated so that this transmission is more effective. I do not say, I wish you to note, that this insulating myelin sheath is necessary for the transmission of impulses, since we will see to the contrary, in the next lesson, that this transmission is achieved equally well by nerve fibers that lack myelin; nevertheless, I think that this insulation serves to make it more perfect.

Physiological Basis of Conduction in Myelinated Nerve Fibers

Chapter 5 – voltage clamp techniques*

A unity gain cathode follower

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.

Measurement of the Action Potential of Myelinated Nerve Fiber

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Measurement of the Action Potential of Myelinated Nerve Fiber