Abstract
The larva of ascidians represents a minimal form of chordates. Its tail has bilateral muscle bands, each composed of about 20 cells, and whose activities are controlled by 6-10 motor neurons. This simplicity as well as the recent extensive accumulation of genomic data provides an opportunity to study a detailed mechanism for regulating the complex locomotion patterns in chordates. In order for investigating the relationship between locomotion patterns and muscle activities, we tried a combinatorial approach to tail-beating patterns of Ciona intestinalis larva, using a high-speed video camera (HSVC), extracellular suction electrode for field potential recording from muscle bands, and intracellular microelectrode for recording membrane potential from the muscle cell. This study revealed that (1) the activities of muscle bands are considerably varied with respect to the rhythm, pattern, and velocity of tail bending. (2) As larva matured, the propulsive movement became efficient, and the behavioral modes could be complicated. (3) The changes in muscle field potential during a swimming train corresponded well with the beating patterns simultaneously recorded under HSVC; which promised a correlation between the observable locomotion patterns and intrinsic physiological properties of the tail. (4) Stepwise current injections into a muscle cell did not evoke an 'all-or-none' response, but a 'graded' one. These illuminate an intrinsic mechanism of the larva to show variable locomotion patterns using a pair of single layers of apparently-uniform muscle cells. [Jpn J Physiol 55 Suppl:S118 (2005)]
