Nerve Cells and Muscle Fibers in Hydra (IMAGE) Kiel University Caption Hydra's nerve cells (in green) generate electrical impulses that cause contractions of muscle fibers (shown in red) in the gastric cavity wall. Credit Image: Christoph Giez, Dr. Alexander Klimovich Usage Restrictions None License Licensed content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.