PACEMAKER INTERACTION INHYDRA

Abstract

Previous reports have described in detail the characteristics of two pacemaker systems in hydras. Each of these, the rhythmic potential (RP) system located near the base of the polyp, and the contraction burst (CB) system at the sub-hypostome, has its own conducting system running the length of the animal, so that the impulses originated at either end spread throughout the organism. It has been postulated that these conducted impulses of each system can reach and influence both the pacemakers of its own, and those of the other, system. The over-all behavior of the hydra would thus be the integrated responses of the effectors responding to these interdependent coordinating systems. Certain evidence favoring this hypothesis is presented. It is evident that there are interactions between the two systems, especially in the control of contraction burst initiation. During periods in which RP impulses are appearing at short, regular intervals, contraction bursts are characteristically absent. Paradoxically when RP frequency is low, CB firing is also minimal. Normal CB activity levels are associated with intermediate RP frequencies. There is also a frequency correlation between RP rate and the onset of individual contraction bursts. The interval between RP impulses is longer just before the first CB pulse than it is midway between bursts. It is shortest just following the burst. The longest RP intervals occur during contraction bursts. It appears that the initiation of endogenous CB activity is influenced by RP frequency in a manner comparable to the triggering of the distinctive locomotor CB after RP pacemaker stimulation. Comparable work with other coelenterates has stressed the importance both of endogenous rhythmicity and ordered interaction between separate pacemaker-driven coordinating systems. Hydras are not exceptional to this pattern, but they do show distinctive features compared to strictly sessile polyps. Further investigation may disclose how the so-far unique RP pattern of repeated individual pulses and the absence of pulse bursts or trains is responsible for hydras' distinctive behavior.