THE MEMBRANE THEORY
Now let us give the biological battery Galvani discovered a somewhat closer look. Presumably you can imagine the 70 mv potential between the cut and normal surfaces of the nerve-muscle preparation (Fig. 4) and the busy traffic of negative and positive ions moving in opposite directions under its influence. What maintains this potential in the biological battery? Furthermore, why does a flow of ions crossing that nerve resting on muscle (A) rouse it into action? The key to the answer for both questions is a single explanatory concept--the so-called membrane theory. This theory is in its way comparable to the electron theory in physics; the electron theory "explains" much of what we know about the physical universe while the membrane theory similarly "explains" several fundamental properties of nerves and muscles.
Galvani did not know that his frog muscle actually was made up of thousands of long cylindrical cells called muscle fibers, bound together in a bundle. The true situation was not discovered until many years after his death. He did not realize, therefore, that the single structure he called "muscle" was in fact a collection of