Whether as a result of heat, hunger, standing for too long, or merely at the sight of blood or needles, 40% of people faint at least once in their lifetime.

But exactly what causes these brief losses of consciousness — which researchers call ‘syncope’ — has remained a mystery for cardiologists and neuroscientists for a long time.

Now, researchers 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 November1, 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.