The Heart’s ‘Small Brain’: How It Shields Us from Stress

Every heartbeat is synchronized not only by the brain but also by a unique nervous system located within the heart itself. Recent studies on mice are unraveling how this intricate system maintains a steady heartbeat, even under extreme stress. This research challenges prior assumptions that all cardiac neurons serve the same function.

“The key is to maintain heart function regardless of external conditions because a malfunctioning heart can be fatal,” states Louis Chan, a neuroscientist at Yale School of Medicine and co-author of this groundbreaking study.

The results revealed today are cell¹ and could pave the way for improved treatments in heart disease management.

Understanding the Heart’s Core Functions

Often referred to as the “second brain” of the body, the heart contains its own mini-brain known as the intrinsic cardiac nervous system. This complex network of neurons resides in the fat pad surrounding the heart and communicates with the brain to regulate heart function. However, these cardiac neurons are extremely rare, comprising only about 0.01% of the cells in certain heart tissues, which complicates efforts to pinpoint their specific roles, according to Zhang.

Mouse study reveals how brain signals contribute to heart attack damage

To close this research gap, the team genetically modified mice to label all cardiac neurons. Through gene sequencing and high-resolution imaging techniques, they identified markers for two distinct subtypes of neurons.

When stimulated, one group known as MP⁺ neurons was found to slow the heart rate. Removing these neurons led to heart failure, indicating their critical role in regulating heartbeat. While they can slow down your heart, they are also essential for maintaining a stable rhythm.

Understanding Stress Response in the Heart

The second subtype, Ddah1⁺ neurons, revealed even more intriguing properties. Stimulating or disabling these neurons appeared not to affect the mice significantly. “They seemed indifferent,” Chan recounts, “and survived long periods without issue—very puzzling.”

However, during a blood pressure measurement, Qian Xu, a graduate student and co-author of the study, observed a sudden failure in the Ddah1⁺ neurons. The research revealed that two-thirds of the mice lost Ddah1⁺ neuronal function during this process while control mice remained unaffected. “Just before an animal succumbed, its heart rate dramatically dropped and didn’t recover,” Chan explains.

Further findings suggested that stress might trigger the death of these neurons. In experiments stressing the mice—by placing them in a narrow tube with a tail cuff—most again succumbed. Deleting Ddah1⁺ neurons appeared to increase heart vulnerability to stress, while stimulating them enhanced survival among stressed mice.