The “Iterative” Nature of the Vagus Nerve Fibers to the Heart.

Abstract

If the right vagus nerve of a turtle be stimulated electrically, the degree of inhibition produced in the heart depends upon a number of factors. If we consider only those introduced by changing the nature of the stimulation used, the inhibitory effect will depend upon (1) the shape of the time-voltage curve of the shock used, (2) the strength of the individual shocks used, (3) the number of stimuli applied to the nerve, and(4) the frequency of stimulation. Since such variables must act through changes in the nerve fiber responses and since such responses follow the all-or-none law of fiber response, the following argument must hold. The strength of stimulus applied must, of course, be great enough to reach the threshold of one or more vagus inhibitory fibers before any cardiac inhibitory effects will appear. As the shock strength is increased above this threshold strength, there will be a resultant increase in the number of nerve fibers stimulated. There will also be an increased inhibitory effect upon the heart. The inhibition may attain a certain maximum which is probably achieved in consequence of the stimulation of all of the cardiac inhibitory fibers in the nerve. The effect of an increasing number of stimuli in producing an increased inhibitory effect can be thought of only as an increase in peripheral effect. The effect of changing the stimulus frequency is such that increasing the frequency of stimulation up to a certain amount (between 10 and 40 stimuli per second in most cases) causes an increased inhibitory effect upon the heart. With increasing rates of stimulation the inhibitory effects become no greater but rather become less. The decrease in effect with an increase in rate is probably related to a Wedensky type of inhibition which develops in the nerve (or the end mechanism) with too rapid stimulation.