Abstract
Certain phenomena observed in the electrocardiogram have forced the question: May reflex augmentation of inhibitory tone during the systolic rise in arterial pressure produce effects which wax and wane within the time of a single cycle, or are the reflex effects smoothed out so that little change in the degree of inhibition occurs within a single cycle?
The experimental results of Brown and Eccles 1 suggest that the first alternative may be the true one. In their experiments, however, one (or 2) stimuli were applied to the vagus nerve and time was then allowed for full recovery from inhibition before another stimulus was given. If a stimulus be applied every 2 cycles, or less, conditions as they sometimes obtain in the human heart are more closely approximated. A curve illustrating the time course of vagus slowing is shown in Fig. 1. Make-break shocks were applied about every 650σ to the peripheral end of the cut right vagus of a dog under urethane anesthesia. The other vagus was also cut. Electrocardiograms were taken. The uninhibited cycle was 350σ. Several experiments were carried out with similar results.
The curve shown rests upon a background of inhibition resulting from the frequency of nerve stimulation. The zero point on the ordinates is an arbitrary point of reference. The latency of inhibitory effect is 170σ the ascending limb of the curve is about 300σ.
In several experiments the rate of vagus stimulation was only slightly slower than the inhibited sinus rate. Under these conditions the heart slowed or accelerated, depending upon the time in the cycle at which the stimuli fell.
Only when the rhythm of the physiological inhibitory reflex pulses differs from the rhythm of the pacemaking structure can a waxing and waning of inhibitory effects be demonstrated in the human heart.
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