THE SIGNIFICANCE OF FREQUENCY, NUMBER OF IMPULSES AND FIBER SIZE IN VASOMOTOR RESPONSES TO VAGUS AND DEPRESSOR NERVE STIMULATION IN THE RABBIT

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Abstract
In responses to stimulations of the depressor nerve of the rabbit in preparations uncomplicated by the presence of pressor effects, the curves of action potential area against stimulus strength and of % depression of blood pressure against stimulus strength, recorded coincidentally, have a similar form. The correction to be added to convert the curve of % lowering of blood pressure to a measure of afferent effect on the vasomotor centers is such as to suggest a direct proportionality between action current area and central effect. This would mean that the effect per impulse varied as the action potential area per stimulus for fibers of different size. If the area of potential varies as cross-sectional area of fibers, then the central effect per fiber varies as fiber mass. The effect of stimulation of vagus and depressor vasomotor fibers together is equal to the sum of their separate responses, both as to amplitude and time course. The vasomotor afferent fibers of lowest threshold in the vagus are purely depressor. Of the vagus afferent vasomotors in the range of threshold of the depressor nerve fibers, most are pressor. When both are stimulated at slow frequency, the depression of blood pressure is less than when only depressors are stimulated at lower strength, but a horizontal curve of blood pressure may be maintained in both cases; if the frequency is increased, a fall is followed by a rise with the stronger stimulation, while a pure depression is obtained with the weaker. The effect of frequency on stimulation of the vagus (or presumably of other mixed nerves) in giving a fall of pressure on slow stimulation, a rise on fast of the same strength, may be assigned to the stimulation of both pressor and depressor afferents in both cases, with different orders of temporal summation at the centers, such that the effect of increasing the frequency of pressor stimulation is greater than the effect on depressor stimulation. There is some indication that respiratory afferents, acceleratory and inhibitory, resemble the vasomotors in their summation and antagonisms. These statements apply to general systemic blood pressure effects, and the probability is recognized that this total may be made up of a combination of local reflexes whose analysis awaits further experiment.

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