Coordination of the heart and gill rhythms inLimulus

Abstract

1. WhenLimulus is exposed to hypoxia both heart rate and ventilation rate decrease together (Fig. 1, Fig. 2A). Hypoxia ultimately leads to cessation of ventilation and concomitant bradycardia. 2. When oxygen is reintroduced into an oxygen free aquarium ventilation resumes rapidly, with a parallel increase in heart rate (Fig. 1, Fig. 2B). 3. Covariation of heart and gill activity similar to that in hypoxia experiments also occurs during the normal respiratory behavior patterns ofLimulus, such as intermittent ventilation, swimming, hyperventilation and gill cleaning. 4. The covariation of heart and ventilation rates is especially evident during transitions of intermittent ventilation (alternating periods of apnea and ventilation, Fig. 3). 5. Covariation is also evident during the large increases in ventilation frequency which occur during hyperventilation and swimming (Fig. 4). 6. Gill cleaning is a centrally determined motor sequence which consists of rhythmic flicking of the inner lobes of a gill plate between the book gill lamellae of the plate on the opposite side. During this behavior there is a marked slowing of the heart rate which is at least as great as the decrease in rate seen during periods of apnea (Figs. 5 and 6). 7. Changes in heart rate associated with ventilatory activity do not appear to be caused by the metabolic demand resulting from such activity (Fig. 7). 8. In addition to frequency covariation of the heart and ventilation rates, there can also be phase coordination of the two rhythms. When the two are close to the same frequency or to harmonic frequencies, the heart often maintains a phase preference with respect to the concurrent gill interval over a considerable period of time (Fig. 8). 9. These results suggest that there are common tonic inputs to both the cardiac ganglion and the central pattern generators for the various ventilatory behaviors, which modulate the frequencies of both simultaneously. Both the frequency covariation and phase communication between the two systems may serve to increase the efficiency of the respiratory-circulatory interactions.