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1 Department of Zoology, University of Washington, and the Marine Biological Laboratory, Woods Hole, Massachusetts 02543
1. The primary pacemakers in the tunicate heart are located near the cardiovascular junctions close to the raphe that connects the V-shaped heart to the pericardium. However, small isolated pieces of the ring of myocardial cells at the ends of the heart were found to have nearly equal pacemaker capabilities. Arrhythmia in one primary pacemaker region was found to result from activity of two or more centers.
2. Following isolation of a small piece of tissue in sea water, 20-50% of the cells were observed to pulsate and usually within a few minutes all cells contracted synchronously. Reversals in the direction of conduction in strips of myocardium without any primary pacemaker region were observed.
3. The C center located in the middle of the heart was found to be dormant during normal heart activity but in opened animals it was activated by increasing the blood pressure.
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