Human brains have unsuspected oxygen reserves, say US researchers. These enable our brains to cope with arduous tasks without having to rely on boosting the blood supply to the brain.
Until now it was thought that no oxygen is stored in brain cells, say the researchers. Whenever part of the brain is active, blood flow to that region increases, and oxygen use was assumed to be the trigger.
鈥淚t is just accepted,鈥� says Mark Mintun of the Washington University School of Medicine.
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However, Rob Turner, at the Functional Imaging Laboratory at University College London, says other researchers have already realised that levels of oxygen in brain tissue are not zero. 鈥淭his is no more than another experimental repetition of something that has been current for around two years,鈥� he says.
Mintun insists the work is new. 鈥淧eople were uncomfortable with the model, but we鈥檙e the first to take it on directly and say 鈥榟ere鈥檚 human evidence that the model is not exactly right,鈥� he says.
Imaging which parts of the brain are active by measuring blood flow is the basis of a technique called positron emission tomography (PET).
Mintun and his colleagues used PET to image the brains of volunteers carrying out a simple task, both in normal conditions, and with slightly reduced oxygen. 鈥淚t was like we instantly transported them to 14,000 feet,鈥� says Mintun. They expected to see a much larger increase in blood flow when oxygen was scarce, but there was no change.
The only way to explain the results was if there was a reserve of oxygen in the brain that cells could use straight away, without having to rely on blood to bring it. 鈥淭he brain does have increased blood flow, but it must not be for delivering oxygen,鈥� says Mintun. 鈥淲e need to look elsewhere for the trigger.鈥�
But he says that PET imaging is still a valid technique, because even if oxygen use is not causing the surge of extra blood, the trigger still seems to be linked to brain activity. 鈥淏lood flow is telling us something about brain activation maybe even more directly than we thought,鈥� he says.
In other work, so far unpublished, Mintun and his colleagues ran mathematical models using data from rats鈥� brains. They found that rats do not have the same oxygen stores that people do. 鈥淭hey really exhaust their oxygen reserves rapidly,鈥� says Mintun.
He speculates that while human brains are more immediately flexible to different levels of activity, rats brains have perhaps evolved not to waste resources. 鈥淭hey are living on the edge a little more,鈥� he says.
But Turner is sceptical. 鈥淭here鈥檚 no reason to believe that the delivery of oxygen to the brain is any different for humans than for other animals.鈥�
More at: Proceedings of the National Academy of Sciences (vol 98, p 6859)