The relation of structure to the spread of excitation in the frog's sciatic trunk

In the past a great many attempts have been made to explain quantitatively the spread of current and of excitability about electrodes on a frog's nerve trunk in terms of the electrical properties of a simple cable consisting of a resistant (and capacitative) cylindrical membrane separating a conducting core from a conducting outside medium. We have shown (Rashbass & Rushton, 1949b) that in at least three important respects the excitability of the sciatic trunk does not behave as it would if this were so. For the excitation does not always arise at the cathode, the excitability does not fall away exponentially on either side of a single electrode, nor are the results the same when the nerve is stimulated by bipolar electrodes as by the symmetrical tripolar arrangement. In the present paper we attempt to show how these results can be explained in terms of the electrical properties of the structures of the nerve trunk. In 1926 Bishop, Erlanger & Gasser drew attention to the appreciable electrical resistance (and capacity) of the nerve's connective tissue sheath, and suggested that 'many of the properties of the nerve as usually measured may be in fact the property of this non-nervous structure'. Although since then various workers (see Discussion) have given differing significance to the part played by the epineurium, its contribution to the spatial characteristics of nerve has not been extensively investigated. In the first part of this paper we show that when the epineurial connective sheath has been removed, the nerve follows closely the expectations of the simple cable theory, and in the second part we show by what property the connective tissue brings about those deviations from the simple theory which are exhibited by the unstripped nerve.