For years, the reasons behind sudden fainting spells have eluded medical professionals. Often occurring when individuals are unwell, in overcrowded spaces, or in extreme heat, fainting, medically termed syncope, typically results from an abrupt drop in blood flow to the brain. However, a groundbreaking study by the University of California San Diego, has unveiled a set of sensory neurons that link the heart directly to the brain, suggesting that our hearts might have more influence over our consciousness than previously thought. The findings were published in Nature.
Assistant Professor Vineet Augustine and his team have identified a genetically defined cardiac reflex within mice that mirrors human syncope in terms of physiology, behaviour, and neural responses. The discovery contradicts the long-held belief that the body is merely a follower of the brain’s commands. Instead, it appears the heart communicates with the brain to potentially trigger fainting spells.
During the experiments, the researchers observed that when these neurons were activated, the mice exhibited rapid pupil dilation and eye rolls, symptoms commonly associated with human fainting. The study further indicates that, while reduced blood flow to the brain is a factor, the brain itself may play a significant role in the process of fainting.
The findings present a neural pathway involving sensory neurons that connect the heart to the brainstem. These neurons, called NPY2R VSNs, are part of the vagus nerve, which extends from the brain to various organs, including the heart. The researchers used advanced techniques such as single-cell RNA sequencing and optogenetics to stimulate these neurons in mice, causing them to exhibit syncope-like symptoms.
This study not only provides insights into the mechanisms behind fainting but also opens up new avenues for treating cardiac-related syncope. Cardiologist Kalyanam Shivkumar from the University of California, Los Angeles, suggests that understanding these pathways could inspire novel treatment approaches.
The researchers recorded the activity of thousands of neurons across different brain regions as the mice experienced induced fainting. They noticed a decrease in brain activity in all areas except the periventricular zone of the hypothalamus. Manipulating this specific brain region affected the duration of the fainting spells, implying that a coordinated neural network, including the NPY2R VSNs and the periventricular zone, regulates fainting and the subsequent recovery.
The revelation of the NPY2R VSNs marks a significant milestone in understanding the complex interplay between the heart and the brain. Clinical cardiologist Richard Sutton from Imperial College London views this as a thrilling advancement that, with further research, could potentially unravel long-standing cardiac enigmas.
The study opens up the question of what exactly causes these neurons to fire and induce fainting. This inquiry is crucial for Jan Gert van Dijk, a clinical neurologist at Leiden University Medical Centre, who has pondered this enigma throughout his career.