Neural Pathway Between Heart and Brain Induces Syncope, Causes Healthy People To Faint With No Medical Reason

Most people faint at the sight of blood when they are having blood extraction or when they are exposed to a very intense emotional stimulus. New research offers a possible explanation for fainting that is not caused by an underlying medical condition.

Vasovagal Syncope

About 40% of people faint at some point in their lives. Most of the time, the reason why they pass out has no medical reason. Medical experts call this fainting vasovagal syncope because of the sudden drop in a person's heart rate and blood pressure. It reduces blood flow to the brain, shutting down the circuits that keep us conscious.

Studies during the 19th century connected this type of fainting to the vagus nerve, a tract that links the brain to internal organs such as the lung, heart, and gut. However, it was unclear which part of the vagus nerve is responsible for this type of swooning. As the longest and most complex of the cranial nerves, the vagus nerve is a major highway between the body and the brain.

It was previously thought that the vagus nerve was simply a way for the brain to control internal organs. Further studies on the nervous system showed that the vagus nerve is a two-way street. For instance, it was found that the gut can also affect the brain.

Syncope-Inducing Neural Pathway

Scientists tried to determine if the two-way connection is also true for the heart. They investigate the possibility that the heart also sends signals back to the brain, influencing its function and behavior.

Led by neurobiologist Vineet Augustine from the University of California San Diego, researchers used genetic tools and advanced imaging techniques to explore a cluster of sensory neurons in the vagus nerve of mice. By identifying the genes switched on in each neuron, the team identified a distinct population of cells that had not yet been studied.

The HYBRiD technology used to make certain tissues transparent was also utilized by Augustine and his team. It helped them show that cells form a fiber communication pathway that leads from the heart's ventricles to the area postrema, a part of the brainstem that induces vomiting.

Laser light was used to stimulate the pathway in mice, causing them to faint and demonstrate other signs observed in people who lose consciousness. These symptoms include dilation of pupils, rolling back of eyes, slowing down breathing and heart rate, and decreased blood pressure.

The study "Vagal sensory neurons mediate the Bezold-Jarisch reflex and induce syncope" also provides a better insight into how the brain and body typically work together to keep a person from passing out. As described by neurologist Robert Wilson from the Cleveland Clinic, this whole orchestra responds to the blood flow mechanism, telling the heart how to accelerate and how much to pump.

Understanding that orchestra has become significant since the onset of the COVID-19 pandemic. SARS-CoV-2 often affects a person's autonomic system, which regulates bodily functions, including breathing, blood pressure, and heart rate. Autonomic disorders did not get much attention in the past. After the pandemic occurred, a lot of patients suffering from long COVID experienced fainting, dizziness, and autonomic dysfunction, and this has become a significant health concern.