Wave front analyses of impulses in tunicate heart.

KRIEBEL, MAHLoN E. Wave front analyses of impulses in tunicate hart. Am. J. Physiol. 218(4) : 1194-l 200. 1970.-The U-shaped, tubular hearts of Ciona are composed of a single layer of cells joined by apical tight junctions. Pacemakers at each end alternate in periods of activity, reversing the direction of blood flow. Hearts are attached by a raphe along one edge to the pericardium. Impulses do not pass the raphe and impulse spread was not altered after hearts were isolated and opened by cutting along the raphe. Conduction velocities were determined by placing the opened myocardium over a Plexiglas plate containing suction electrode openings spaced 1 mm apart. Conduction velocity in the heart axis increased (or decreased depending on the direction of conduction) linearly from 1 mm/set at the ends to 7 mm/set in the middle half of the heart (10 C). Wave fronts propagating along the heart axis were found to be parallel to the cell axis (except at the ends and bend) in both directions of conduction. Conduction velocity was found to be greatest parallel to the cell axis (77 mm/set at 65” to the heart axis) and least perpendicular to the cell axis (8.7 mm/set). These results demonstrate that cell shape influences impulse spread in an otherwise uniform sheet of excitable tissue.