Co-Ordinating Systems and Behaviour In Hydra: I. Pacemaker System of the Periodic Contractions

Abstract

1. A technique of recording electrical activity from an intact, essentially normal specimen of Hydra is described. 2. The existence of regularly recurring co-ordinated longitudinal contractions of the ectodermal muscles is confirmed and re-emphasized. 3. Such contractions are found to consist of a patterned series of individual coordinated contractions, each preceded by a single large, compound potential. The overall contraction, consisting of a variable number between 5 and 12 or more contractions depending on species, is thus called a contraction burst. 4. These contraction-burst potentials originate endogenously; they are considered to be the most important effector activity in Hydra. 5. Contraction-burst potentials originate in the hypostome and are conducted throughout the column at approximately 15 cm./sec. 6. Contraction-burst patterns have been studied quantitatively in two species, showing interspecific differences between both regular contraction bursts and those associated with locomotion. 7. Certain extrinsic and intrinsic variables affect contraction-burst frequency. Daylight, and nutritional state, both modify this rate, with the former giving rise to a circadian activity cycle under natural conditions. 8. Single electric shocks usually cause a single co-ordinated muscle contraction. Such stimuli can markedly reduce endogenous contraction-burst activity. 9. Sudden illumination interrupts contraction bursts temporarily, even halting those in progress. Blue light is most effective. This stimulus has been used as a tool to investigate the properties of the pacemaker systems concerned with contraction bursts. 10. The nature and properties of these pacemakers is discussed.