The discharge of impulses in motor nerve fibres

IT is generally agreed that the motor nerve fibres perform their function in the body by conducting impulses of the normal type, but there is little agreement as to the way in which these impulses are discharged. According to one line of argument a powerful contraction is brought about by more or less synchronous volleys of impulses occurring at the rate of 50-80 a sec. in each motor neurone; according to another the rate of discharge is very much higher and the apparent synchronisation revealed by the electromyogram is caused by the properties of the recording instrument or of the muscle. What happens with a relatively weak contraction is too uncertain even to have become a subject for controversy. In the present investigation we have obtained direct records of the discharge in individual motor nerve fibres innervating the diaphragm. We cannot assume that our results can be applied to every kind of reflex contraction, but for this particular case they seem so definite that it is unnecessary to set out the whole of the evidence which has been brought forward by indirect methods in the past'. The phrenic is an ideal nerve for investigations of the activity of the motor centres, for the respiratory discharge needs no external stimulus to maintain it, its intensity can be varied by controlling the air supply to the lungs and it occurs at definite intervals separated by periods of inactivity. The electric responses in the whole nerve trunk have been recorded by Dittler and Garten (3) with the string galvanometer, and by Ga s s e r and N ew c om e r (4) with the same instrument used in conjunction with a valve amplifier. Their records (made from the dog) show that in each period of inspiration the electric response takes the form of a series of oscillations at the rate of about 70 a sec. Each oscillation resembles