Electrophysiology of the swimming system of hydromedusae and the effects of atropine-induced crumpling.

1. In order to evaluate how crumpling behavior influences the swimming system in Liriope tetraphylla, extracellular recordings of muscle action potentials related to both behaviors were performed. 2. The swimming pattern of Liriope consists of irregular bursts of muscle potentials. A correlation was obtained between number of muscle potentials and burst length and between burst and interburst periods. The swimming pattern could be predicted from the burst length since a long burst was followed by a short period of quiescence and by a burst as long as the preceding one. 3. The addition of atropine to induce radial muscle contraction first caused an increase in swimming activity and then an irreversible reduction. The enhancement of swimming activity was due to the increase of burst length and of the frequency of swimming bursts (due to the decrease of the interburst period), and the reduction of swimming activity, to the enhancement of the interburst period and to the reduction of the burst length. 4. It is proposed that, when facing a noxious stimulus, Liriope will first "escape" from it by enhancing its swimming activity and, if the stimulus persists, blockade of the pacemaker system becomes accentuated as the radial muscle contractions become more sustained.