Intrinsic cardiac nervous system

The Intrinsic cardiac nervous system (ICNS), also known as the heart's "little brain," is a complex network of neurons and ganglia embedded within the heart tissue that regulates cardiac function independently of the central nervous system. It modulates heart rate, conduction, and cardiac contractility in response to local and external stimuli.

Source: Wikipedia — Intrinsic cardiac nervous system (CC BY-SA 4.0)