Human Sperm (IMAGE) University of Gothenburg Caption The tail is a highly complex machine that consists of around a thousand different types of building blocks. The most important of these are called tubulins, which form long tubes (microtubules). The tubes are found inside the sperm tail. Thousands of motorproteins - molecules that can move - are affixed to these tubes. By being fixed to one microtubule and "walking on" the adjacent microtubule, the motorproteins in the sperm tail pull and the tail bends, enabling the sperm to swim. Credit University of Gothenburg 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.