December 13, 2021
Keio University
Konan University
University of Tsukuba
In a joint research project, Taichi Akahoshi (third-year doctoral student) of the Graduate School of Science and Technology, Keio University, Madoka Utsumi (fourth-year undergraduate student) of the Faculty of Science and Technology at the same university, Professor Kotaro Oka, Associate Professor Koji Hotta, and their colleagues, along with Professor Takehiro Kusakabe of the Faculty of Science and Technology and the Konan Institute for Integrative Neurobiology at Konan University, Kohei Onuma (postdoctoral researcher), Makoto Murakami (undergraduate alumnus), and Assistant Professor Takeo Horie of the Shimoda Marine Research Center, Faculty of Life and Environmental Sciences, University of Tsukuba, have revealed that the rhythmic spontaneous movements in the early development of ascidians are controlled by just a single pair of motor neurons. The embryos of vertebrates such as fish and amphibians exhibit spontaneous movements at an early stage of development before they begin to swim. While it has been suggested that these spontaneous movements are generated by the rhythmic neural activity of a group of neurons in the spinal cord, when and how they are acquired has remained a mystery. In this study, we tackled this mystery using the ascidianCiona intestinalisType A, which is the closest relative to vertebrates and for which cell lineage and neural connectivity information in the larval stage are known. As a result, we discovered for the first time in the world that a single pair of motor neurons, MN2, in the region corresponding to the spinal cord ofCiona intestinalisType A larvae, is both necessary and sufficient to generate the initial motor rhythm with a period of tens of seconds, and that changes in the membrane potential of MN2 correspond to the muscle contractions of the tail. The motor neuron MN2 is considered to be a key component of the neural circuit (central pattern generator, CPG) that produces the alternating left-right tail-beating movements during the swimming stage. This research finding is a significant discovery that contributes to elucidating the development of neural circuits that generate autonomous and rhythmic movements common to animals, such as swimming and walking. The research results were published online inScience Advanceson December 10, 2021 (US Eastern Time).
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