Microtubules are protein structures found within cells. They are generally long and form a structural network within the cell's cytoplasm, but in addition to structural support microtubules are used in many other processes as well. They form a substrate on which other cellular chemicals can interact, they are used in intracellular transport, and are involved in cell motility. The assembly and disassembly of microtubules into their subcomponent tubulin is one way in which cells can change their shape. A noteable structure involving microtubules is the spindle used by eukaryotic cells to segregate their chromosomes correctly during cell division.
Roger Penrose has proposed that the hollow cores of microtubules inside neurons form an environment capable of supporting quantum-scale information processing and in conscious awareness. There is no evidence to support this proposition at this time, and it is not widely accepted.