. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. CARDIAC NERVES OF PANULIRUS ARGUS 165 2. Cardio-accelerators The investigation of the cardio-accelerators was not as extensive as that of the cardio-inhibitor. The pseudochronaxies of both cardio-accelerator nerves (Fig. 2) were not significantly different from those obtained for the cardio-inhibitor. A sec- ond threshold was found, though not as commonly as with the inhibitor. Compari- sons of the thresholds at different frequencies were not made. From the responses in Figure 6 and the similarity of the shape of the frequen
. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. CARDIAC NERVES OF PANULIRUS ARGUS 165 2. Cardio-accelerators The investigation of the cardio-accelerators was not as extensive as that of the cardio-inhibitor. The pseudochronaxies of both cardio-accelerator nerves (Fig. 2) were not significantly different from those obtained for the cardio-inhibitor. A sec- ond threshold was found, though not as commonly as with the inhibitor. Compari- sons of the thresholds at different frequencies were not made. From the responses in Figure 6 and the similarity of the shape of the frequency- acceleration curves, it seems fairly clear that the two cardio-accelerators act in the same manner on the heart, though it is not possible to say that they have the same quantitative effect. In Figure 9A twice as many fibers were probably stimulated in the first cardio-accelerator as in the second. A later series of stimuli applied to. i MINUTE 3V IMS 16 2 2V IMS 38 3 FIGURE 8. Cardio-accelerator; frequency varied. A. Cardio-accelerator 1 ; stimulation at 3 volts, 1 millisecond. B. Cardio-accelerator 2; stimulation at volts, 1 millisecond. the first cardio-accelerator produced a curve of acceleration with a maximum re- duced to approximately that of the second accelerator. In several instances stimulation of the cardio-accelerators restored a regular beat to an irregular or inactive heart. In such cases, the maximum rate obtained (95-125 per minute) was comparable to the maximum rate obtained from a normally beating heart upon stimulation (74-140 per minute). As Wiersma and Novitski (1942) found, however, the maximum was in many cases correlated with the normal heart rate. The cardio-accelerators usually destroyed an alternating rhythm by equalizing succeeding beats, redistributing a given number of impulses into equal bursts rather than by only increasing the number of impulses in the weaker bursts (Fig. 6). The effects of varying frequencies of stimulation are shown i
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