Actually it’s not muscles. It’s a network of ganglia, neurotransmitters, protein and support cells, just like in our brain - aka the heart brain / intrinsic cardiac nervous system.

For example: we realize we are in danger. This is intuitive and sensed with the heart. As a reaction, the heart sends this to your brain, who can send signals to the body to make it ready for action / be alert.

The heart is a big part of your intuition, based on experience, while your brain is the more rational one.

If you’re ever interested in what they’ve researched so far, you should read this.