Which nerves did Ivan Pavlov study to understand the control of the heartbeat?

To understand the control of the heartbeat, Ivan Pavlov conducted research on the cardiac nerves and the vagus nerves.

Through the careful dissection of fine cardiac nerves, Pavlov demonstrated that the strength of the heartbeat is controlled by nerves originating from the cardiac plexus. Additionally, by stimulating the severed ends of the cervical nerves, he was able to observe and document the specific effects that the right and left vagal nerves have on the heart.1