Scientists have discovered for the first time a reflex pathway between the heart and the brain associated with fainting.
Researchers from the University of California in San Diego, together with colleagues from the Scripps Research Institute, published their findings in the journal Nature, writes MedicalXpress.
As the basis of their research, they took the Betzold-Yarish cardiac reflex (BJR), first described in 1867.
The cardiac reflex is that it lowers the heart rate and causes a decrease in blood pressure and breathing, so according to scientists, it may be associated with fainting. But the neural pathways involved in the reflex have not been sufficiently studied.
The researchers focused on the genetics of a sensory cluster known as the nodal ganglia. This area is part of the vagus nerves, which transmit signals between the brain and visceral organs, including the heart.
Vagus sensory neurons (VSNs) project to the brainstem and are likely involved in BJR and syncope. The scientists discovered that these sensory neurons also activate another type of neuron called NPY2R.
Scientists stimulated this neural pathway in mice using optogenetics. The mice, which were moving freely, immediately fainted.
During these episodes, they recorded data from thousands of neurons in the mice’s brains, as well as heart activity and changes in facial features, including pupil diameter and displacement.
The researchers found that when NPY2R neurons were activated, the mice experienced rapid pupil dilation and the classic “eye rolling” that occurs in humans during unconsciousness.
They also recorded a slowing of the pulse, a decrease in blood pressure and breathing rate, as well as a reduced blood flow to the brain.
“We were amazed to see their eyes roll back around the same time that brain activity plummeted. Then, after a few seconds, brain activity and movement resumed“, the researchers said in conclusion.
Further testing showed that when NPY2R VSNs were removed from the mice, the Betzold-Jarish cardiac reflex and unconsciousness conditions disappeared.
Previous studies have shown that fainting is caused by reduced blood flow to the brain, but new evidence suggests that brain activity itself may play an important role.
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