What causes fainting? Scientists finally have an answer.

From the article:

[R]esearchers have discovered a neural pathway, which involves a previously undiscovered group of sensory neurons that connect the heart to the brainstem. The study, published in Nature on 1 November, shows that activating these neurons made mice became immobile almost immediately while displaying symptoms such as rapid pupil dilation and the classic eye-roll observed during human syncope.

The authors suggest that this neural pathway holds the key to understanding fainting, beyond the long-standing observation that it results from reduced blood flow in the brain. “There is blood flow reduction, but at the same time there are dedicated circuits in the brain which manipulate this,” says study co-author Vineet Augustine, a neuroscientist at the University of California, San Diego.

…

Using a new technique that combines high-resolution ultrasound imaging with optogenetics — a way of controlling neuron activity using light — the researchers stimulated the NPY2R VSNs in mice while monitoring their heart rate, blood pressure, respiration and eye movements. This approach allowed the team to manipulate specific neurons and visualise the heart in real time. “This was not possible before, because you needed to figure out the identity of these neurons,” says Augustine.

When the NPY2R VSNs were activated, mice that had been freely moving around fainted with a few seconds. While passed out, the mice displayed similar symptoms to humans during syncope, including rapid pupil dilation and eyes rolling back in their sockets, as well as reduced heart rate, blood pressure, breathing rate and blood flow to the brain.

“We now know that there are receptors in the heart that when made to fire, will shut down the heart,” says Jan Gert van Dijk, clinical neurologist at Leiden University Medical Centre in the Netherlands.

In humans, syncope is usually followed by a rapid recovery.