equally adamant that injury potentials were a minor matter and that they would add only a small part to
the resting potentials. As usual both parties were partially right and partially wrong. In fact Du Bois-
Reymond was not even fully correct in his interpretation of his primary observation of the nerve
impulse. He visualized it as being due to localized masses of "electromotive particles" on the surface of
the nerve, a concept seemingly related to the then known mechanism of metallic conduction along a
wire. The old objections still applied--the resistance of the nerve was too high and it lacked appropriate
insulation. Nonetheless, the impulse was there, a fact easily verified with the equipment then available.
In a technical triumph for that time, von Helmholtz, a colleague of Du Bois-Reymond in Berlin,
succeeded in measuring the velocity of the nerve impulse, obtaining a value of 30 meters per second, in
full agreement with "instantaneous" measurements on currents in a wire--this was a different
phenomenon entirely. The problem was given to another of Du Bois-Reymond's students, Julius
Bernstein. He repeated and confirmed von Helmoltz's velocity measurement, at the same time making
precision measurements on the nerve impulse itself. His studied led to the proposal in 1868 of his
theory of nerve action and bioelectricity in general, which has come to be the cornerstone of all modern
concepts. The "Bernstein hypothesis" postulated that the membrane of the nerve cell was able to
selectively pass certain kinds of ions (atoms with electric charges resulting from dissolving salts in
water). Situated within the membrane was a mechanism that separated negative from positive ions,
permitting the positive ones to enter the cell and leaving the negative ions in the fluid outside the cell.
Obviously when equilibrium had been reached an electrical potential would then exist across the
membrane-the "transmembrane potential." The nerve impulse was simply a localized region of
"depolarization," or loss of this transmembrane potential, that traveled down the nerve fiber with the
membrane potential being immediately restored behind it. This was a most powerful concept, in that it
not only avoided the problems associated with electrical currents per se, but it relied upon established
concepts of chemistry and satisfactorily explained how the impulse could be observed electrically and
yet not be electrical in nature. Bernstein, realizing the power of the concept, postulated that all cells
possessed such a transmembrane potential, similarly derived from separation of ions, and he explained
Matteucci's current of injury as being due to damaged cell membranes "leaking" their transmembrane
potentialsintellectually, a most satisfying explanation at that time.
Fig. 1.6. Electricity and nerves. Top: the sample anatomy of a single nerve cell or neurone. The cell
body is either in the brain or the spinal cord, and the nerve fiber is in the peripheral nerves such as in
the arms and legs. Middle: the concept of Du Bois-Reymond-electrical "particles" moving along the
ELECTROMAGNETISM & LIFE - 16