Whether you’re hot, hungry or perhaps a member of the Household Cavalry standing guard for hours, 40% of people will faint at least once in their lifetime.
Known in the medical world as syncope, fainting – a brief loss of consciousness – has baffled doctors for hundreds of years.
No one knows exactly how it happens.
Until now.
As a rule, it is generally accepted that the brain rules the body, sending out missives to the limbs and organs to do its bidding, the supreme ruler.
And while it has long been understood that the basic mechanism behind fainting is a lack of blood to the brain, no one has been able to determine what was causing it.
Now however, researchers have discovered a new nerve pathway that works in the opposite direction, allowing the heart to rule the head – and in this case, restricting blood flow.
By activating this previously undiscovered group of sensory neurons in mice, scientists found they immediately fainted.
‘What we are finding is that the heart also sends signals back to the brain, which can change brain function,’ said senior author Assistant Professor Vineet Augustine, from the University of California San Diego.
‘Our study is the first comprehensive demonstration of a genetically defined cardiac reflex, which faithfully recapitulates characteristics of human syncope at physiological, behavioral and neural network levels.’
For decades scientists have suspected a particular mechanism known as the Bezold-Jarisch reflex (BJR) could be connected to fainting, given its involvement in heart rate, blood pressure and breathing. This has been difficult to prove however since the nerve pathways its uses are not well known.
But the latest study identified a specific cluster of neurons in the vagus nerve – the main nerve behind mood, immune response, digestion and heart rate – that are closely related to the BJR. It was these neurons that, when stimulated, led to the mice fainting.
As seen in humans, the mice quickly bounced back.
‘Neurons in the brain are very much like extremely spoiled children,’ said Dr Jan Gert van Dijk, clinical neurologist at Leiden University Medical Centre in the Netherlands who was not involved in the study.
‘They need oxygen and they need sugar, and they need them now. They stop working very quickly if you deprive them of oxygen or glucose.’
Fainting typically only lasts 20 to 40 seconds – nerve cells begin to die after two to five minutes if starved of oxygen.
Speaking to Nature, said Dr van Dijk, added: ‘If you add oxygen again, they’ll simply resume their work and do so just as quickly.’
Now that scientists believe they have discovered how people faint, the next mystery to solve is why, and what triggers the newly-discovered neurons.
‘It’s been one of the biggest riddles of my entire career,’ said Dr van Dijk.
The study is published in the journal Nature.
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