Electrophysiological Studies on Initiation and Reversal of the Heart Beat in Ciona Intestinalis

Abstract

1. Electrophysiological techniques have been used to define pacemaker characteristics and organization in the heart of the tunicate Ciona intestinalis. The frequency of spontaneous beat initiation was regular or irregular for a given period of time; in any single heart, these frequency modes intermittently changed from one to the other in no regular order. Reversals could take place immediately, after a pause, or after collisions. 2. Trains of electrical stimuli applied to the ends of the heart could drive propagated contractions at a frequency of up to 2.6/sec, and dominance could be controlled by altering the frequency and/or intensity of the stimuli. Each end gave a characteristic response to increasing frequency of driving, and the threshold for one-to-one driving was frequency-dependent. 3. When the middle of a heart was ligatured, the two ends were independently active. Visceral ends exhibited regularly varying levels of high and low frequency with a period of several minutes, whereas the frequency levels of the hypobranchial ends varied irregularly. 4. Intracellular resting potentials were approximately -50 mV., and propagated action potentials associated with contractile events did not show ‘overshoot.’ Partial electrical responses were affiliated with weak or failing mechanical events, and complex, double-peaked intracellular potentials were often associated with collisions. In some cases a cell could show electrical activity that was completely dissociated from the mechanical response of the whole heart. Excitation thus probably spreads passively between cells, and multiple foci of activity may co-exist at any given time. 5. From electrophysiological and anatomical data, an hypothesis is proposed to explain the observed mechanical activity on the basis of the interactions of individual cells and higher-order interactions among groups of cells.