A part of the brain's ability to shield itself from the destructive damage caused by a stroke has been explained by researchers.It has been known for more than 85 years that some brain cells could withstand being starved of oxygen.Scientists, writing in the journal Nature Medicine, have shown how these cells switch into survival mode.They hope to one-day find a drug which uses the same trick to protect the whole brain.Treating a stroke is a race against time. Clots that block the blood supply prevent the flow of oxygen and sugar to brain cells, which then rapidly die.But in 1926, it was noticed that some cells in the hippocampus, the part of the brain involved in memory, did not follow this rule."They're staying alive when the prediction would say that they should die," said Prof Alastair Buchan from Oxford University who has investigated how they survive. I'm a survivorExperiments on rats showed that these surviving-cells started producing a protein called hamartin - which forces cells to conserve energy. They stop producing new proteins and break down existing ones to access the raw materials.When the researchers prevented the cells from producing hamartin, they died just like other cells.Prof Buchan said: "We have shown for the first time that the brain has mechanisms that it can use to protect itself and keep brain cells alive."Their aim is to develop a drug that can produce the same effect, which could be given when an ambulance arrived. This would buy the brain time until clot-busting drugs could be given in hospital.The researchers do not know why these cells have this defence, but other nearby cells in the hippocampus do not. There are differences in function. The cells that die are known as CA1 cells which are very plastic and are involved in laying down memories whereas the surviving, or CA3, cells are less adaptable.Speaking to BBC News online, Prof Buchan said the focus of this research was on "ways to keep brain cells alive" which could have impacts beyond stroke - such as in Alzheimer's disease and spinal cord injuries.Commenting on the study, Dr Clare Walton from the Stroke Association said: "Previous research has shown that some brain cells are naturally more resilient than others, and this study has identified a particular protein in the cells that is responsible."In the future, researchers could try to turn on this protein in other, less resilient brain cells to reduce the brain damage caused by stroke."The findings of this research are exciting, but we are still a long way off from developing a new stroke treatment."