The impulses produced by sensory nerve endings

THIS paper describes an amplifier used in conjunction with the capillary electrometer and some preliminary observations with it on the action currents set up in sensory nerve fibres by appropriate stimulation of their end organs. Since the introduction of the triode valve a number of workers have used valve amplification in conjunction with the string galvanometer for recording electric responses of very small intensity. The results of Forbes(l) and his co-workers and of Gasser and Newcomer(2) have shown how valuable such a combination may be for studying reflex effects in nerve. There is one serious limitation, however, in the ordinary type of string galvanometer which no amount of amplification can overcome, and that is the limitation imposed by the inertia of the moving system. Owing to the mass of the string the record of its movement does not give a true picture of the changes of electromotive force applied to it and the distortion, though of little account in the record of a muscle action current, is quite enough to obscure the true form of the much briefer response of a nerve fibre. With sufficient amplification this defect might be overcome by the use of a recording system of very high natural period (as in the oscillograph), but it cannot be overcome in the usual type of string galvanometer without very extensive alterations'. The magnitude of the distortion and the possibility of correcting the records by mathematical analysis have been dealt with very clearly by E rl a ng e r and Gasser(3) and more recently by Williams(4). The ideal instrument for recording nerve action currents is undoubtedly the cathode ray oscillograph devised by Erlanger and Gas s er, for in this the moving system is a stream of cathode rays, the inertia of which is completely negligible. At present, however, the intensity of the illumination from the ray is far too small to allow photographs to be made from a single excursion, and similar excursions must be repeated many times over before the plate or the eye is affected. As a result the cathode ray oscillo1 These objections would not, of course, apply to the beautiful instrument recently constructed by Prof. Einthoven, where the string moves in a vacuum and has a very high natural period.