Unlocking the Mysteries of the ‘Mini Brain’ Inside Your Heart

One of the longstanding mysteries of the human heart has to do with its complex nervous system—or the heart’s “mini brain,” as it is often described. Scientists have known for years that this system plays a role in heart function, notably to help manage the heart rate, but had little insight into exactly how it works and how important it is, in numerous ways.

A recent study involving mice, published in the journal Cell, sheds compelling new light on the intrinsic cardiac nervous system (ICNS), the complex network of approximately 40,000 neurons that are embedded in the fat pads surrounding the heart. The study suggests that this “mini brain” is in fact essential for survival—and the findings may inspire new therapies to correct arrhythmias and other heart abnormalities.

The ICNS was previously thought to function mainly as a relay station for the autonomic nervous system, a whole-body system that handles subconscious tasks like heartbeat and breathing and is controlled primarily by the central nervous system, which is located in the brain and spinal cord. Yet the new study found that it does much more than just pass signals from the brain to the heart: The ICNS actively regulates cardiac function, with two different populations of neurons that perform distinct jobs. Npy+ neurons maintain normal cardiac performance in everyday conditions—controlling heart rate, baseline cardiac function, and coronary blood flow, for instance—while the other group, Ddah1+ neurons, are essential for cardiac stability and protect the heart from arrhythmias during intense physical or emotional stress. 

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“Without these local neural circuits, the heart cannot properly adapt to changing physiological demands,” explains study coauthor Rui Chang, an associate professor in the departments of neuroscience and cellular and molecular physiology at the Yale University School of Medicine.

The neurons within the ICNS can communicate with each other and with other neurons controlled by the central nervous system. As a result the ICNS is able to make continuous adjustments to the heart’s mechanical and electrical activity in response to shifting conditions—for example, increasing your heart rate when your brain detects that you need to be on high alert “like when you’re being chased by a tiger,” says Deepak L. Bhatt, a professor of cardiovascular medicine at the Icahn School of Medicine at Mount Sinai in New York, or slowing it down when you’re no longer in danger.

But the ICNS isn’t simply taking direction from other systems. “These signals are bidirectional—the brain and the heart talk to each other,” says Majid Basit, a cardiologist with Memorial Hermann Health System in Houston. It takes in information from the brain and body and then makes its own adjustments independent of the central nervous system. This allows the heart to adjust more quickly than it would through the brain-to-heart communication pathway mediated by the autonomic nervous system.

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For example, Basit says, the ICNS helps to maintain the right amount of blood flow from the heart in response to factors like exercise, emotion, or even changes in posture. It has further been shown to adjust the heart’s ability to generate force as it contracts, ensuring it can adequately pump blood throughout your body. And the ICNS helps the heart relax after squeezing and coordinates the beating rate with electrical impulses.

Understanding these complexities could inspire promising therapeutic interventions for when the heart isn’t operating properly. The goal is to develop future therapies that could selectively modulate specific neural circuits rather than broadly stimulating or suppressing the autonomic nervous system, Chang says. This would help doctors address a cardiac problem—such as heart arrhythmias from atrial fibrillation, heart failure, heart attack, or other disorders—directly and precisely without creating collateral damage. 

Current treatments for heart disease tend to target the heart muscle, blood vessels, or the electrical conduction system. With use of certain medications, for example, these approaches can bring about unwanted side effects such as kidney impairment, bleeding risks, dizziness, fatigue, and other problematic symptoms.

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“Our work points toward the possibility of selectively modulating the neurons involved in a particular cardiac function while preserving others,” says Chang. This might involve electrical stimulation of specific nerve branches with better tuned stimulation patterns, or drugs that act on molecular targets that are rich with particular neurons.

Another possibility that’s currently being researched is a low-intensity, focused ultrasound, which Chang notes could be combined with specific drugs to modulate the action of certain neurons. In addition, stimulation of the vagal nerves—which connect the brain with various internal organs, including the heart—may be helpful in some instances as it “stimulates the heart using gentle electrical currents,” says Basit. Down the road, gene-based interventions could be developed to selectively target specific types of nerve cells within the ICNS.

Ultimately, the new research raises the prospect of a future in which scientists can harness the power of the heart’s “mini brain” to keep the heart functioning properly—and fine-tune what isn’t working optimally.