IMPULSE PROPAGATION AND CONTRACTION IN THE TUNIC OF A COMPOUND ASCIDIAN

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Abstract
Diplosoma listerianum and D. macdonaldi (Fam. Didemnidae) have a network of cells ("monocytes") in the tunic which contain high concentrations of microfilaments and react positively with NBD-phallacidin, indicating the presence of F-actin. The tunic is contractile, especially in the areas around the cloacal apertures, which can be closed completely. Myocytes are concentrated in sphincter-like bundles around these openings, but also are found throughout the tunic. Electrophysiological recordings reveal a diffuse conduction system in the tunic propagating all-or-none impulses ("tunic potentials," TPs) through all parts with a conduction velocity of < 1.5 cm .times. s-1, and a refractory period of 1.6 s. TPs correlate one-for-one with contractions. The system is excitable to the touch, but is also spontaneously active, showing steady patterns of potentials as well as regular, ''parabolic'' bursts. The evidence suggests that the myocyte net itself conducts the impulses triggering the contractions. In the absence of conventional nerves and muscles, the system provides the colony with a way of regulating the effluent water current and hence the volume of a common cloacal space. The TP system is not ''wired in'' to the ascidiozooids either as a sensory or as a motor pathway. The tunic acts as an independent behavioral entity.