Search

The elastic theory of flexural waves in thin rods accurately predicts the velocity of flagellar bending waves over a wide range of viscosities. This shows that flagella behave as a purely mechanical system for the transmission of these waves. An evaluation of the total bending moment reveals that this moment occurs in phase over the entire length of a flagellum. From this it is concluded that each contractile fiber in the flagella is activated simultaneously over its whole length. The magnitude of the bending moment decreases linearly along the flagellum. This is most easily explained by a sliding filament hypothesis in flagella with the elementary 9 + 2 fibers. The expression found for the bending moment explains logically that the wave velocity in flagella is determined by their mechanical properties and the outside viscosity only.

The tubular heart of tunicates is composed of a single layer of myoendothelial cells. The direction of contraction reverses every few minutes. The conduction times in both directions are the same. Conduction velocity was greatest in the middle of the arms of the V-shaped heart and slowest in the apex. The greater the heart length, the greater was the conduction velocity. The Q(10) of conduction velocity was 2-2.3. Removal of the raphe which attaches the heart to the pericardium and removal of a line of undifferentiated cells opposite the raphe did not change the conduction velocity or prevent the heart from reversing the direction of conduction. The median resting potential of 42 cells was -71 mv and the median action potential was 75 mv. At 20 degrees C the duration of the action potential was 1.2 sec and the maximal rate of depolarization was 3-10 v/sec. An increase in the beat frequency produced by electrically stimulating the heart decreased the resting potential, rate of rise, the duration, and the overshoot of the action potential. The shape of the action potential was sometimes different in the two directions of conduction. The electrophysiological evidence indicates only one cell type and suggests that the mode of the spread of excitation is by local current flow from cell to cell.

The tubular ascidian heart is composed of a single layer of cells joined together by apical (zonulae occludentes) and spot (maculae occludentes) nexuses. Intercalated discs or desmosomes were not observed in this tissue. Rectangular pulses of current were applied across the opened and flattened myocardium. Assuming that all the transepithelial current flowed through a uniform gap between cells, the resistivity in the gap must be very high compared to that in bulk solution. It is likely, therefore, that the gap width is of the order of an ionic radius or smaller. Assuming that all the transepithelial current flowed through the cells and that the inner and outer membranes had the same resistivity, the membrane resistivity was about 210 ohms cm(2) and the membrane capacitance was about 1.6 microF per cm(2). The myocardial cells were found to be in electrical continuity with each other through the nexuses since current could be passed through a strip of myocardium in a sucrose gap. Assuming that the longitudinal resistance of the cytoplasm was negligible, the cell-to-cell resistivity of the nexuses was 0.2 ohm cm(2). It is concluded that the nexuses provide a low resistance pathway between cells and a transepithelial barrier.