2023-11-04 17:10:00
Emotion, prolonged upright position, getting up suddenly, taking certain medications… There are different causes of fainting depending on the MSD Manual. But what actually happens in the body when a person loses consciousness? Researchers from l’University of California San Diegoof Scripps Research Institute and other institutions have answered this question. Their work was published in the journal NatureNovember 1, 2023.
Neurons linked to fainting brought to light
During their experiments, the scientists studied the neuronal mechanisms linked to the Bezold-Jarisch reflex (BJR). It is a cardiac reflex that manifests itself as a reduction in heart rate, blood pressure and respiration, most often in cases of dehydration or prolonged standing and stillness. Until now, researchers thought it might be associated with fainting, but without knowing the neural pathways involved.
To better understand them, scientists have worked on how vagal sensory neurons (or VSNs, present in the head and neck and linked to the vagus nerve) are associated with fainting. Specifically, they discovered that VSNs expressing the neuropeptide Y receptor Y2 (NPY2R) are linked to the BJR.
So, when testing with mice, they observed that by activating VSNs expressing NPY2R, the mice immediately fainted. Additionally, at the same time, the rodents showed dilated pupils, eye rolling – similar to that of humans when they faint -, a sudden stop in heart rate, blood pressure and breathing rate. Finally, the team also noticed a reduction in blood flow to the brain.
Fainting: the body also sends signals to the brain
“We were amazed to see eyes rolling back just as brain activity was decreasing, explain the researchers. Then, following a few seconds [d’évanouissement]brain activity and movement returned.”
By studying the brain and the signals from NPY2R-expressing VSNs, scientists observed connections between the body and the brain when a person faints. “Traditionally, neuroscientists have thought that the body follows the brain, but it is now clear that the body sends signals to the brain,” they write. The heart also sends signals back to the brain, which can change brain function.”
In other words, the communication between the heart – therefore the body – and the brain goes in both directions and is visible by the fact that the signs (eye rolling, blood pressure, etc.) occur at the same times.
In another part of the trial, scientists removed VSNs expressing NPY2R from mice. Results: they no longer lost consciousness. These VSNs therefore play a very important role in fainting.
In the future, the team plans to continue research into other health issues, including syncopewhich is a sudden and brief loss of consciousness, of a few seconds to less than 3 minuteswith total loss of tone.
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