Answer: Otto Loewi conducted experiments on frog hearts that demonstrated that a chemical was released by nerves that can influence heart rate.
Otto Loewi (1873-1961) was a German pharmacologist. He is best known for his characterization of acetylcholine as the chemical substance that influences the rate of a heart beating in frogs, an experiment that was first published in 1921.
Prior to Loewi’s experiment, it was unclear as to whether chemical or electrical stimulation was necessary for sending signals between neurons. On one hand, it was known that electrically stimulating the nerve attached to a muscle can cause a contraction. At the same time, it was known that exposing the muscle to certain chemical substances can also cause a contraction of the muscle. However, it wasn't clear if there was a connection between these two observations. Loewi demonstrated that there was a correlation.
According to urban legend, Loewi was inspired to conduct his experiment in the middle of the night, as if the design had come to him in a dream. He leapt out of bed, and went to his laboratory, where he dissected out the hearts of two frogs. One of them was still attached to the vagus nerve, or cranial nerve IX - the cranial nerve that is responsible for parasympathetic nervous system activity. The hearts were bathed in a physiological saline solution. By stimulating the vagus nerve, Loewi observed that the heart rate slowed down, as expected.
Loewi then took some of that saline solution and bathed the second heart with that same solution. Loewi reasoned that if a chemical was released by the vagus nerve stimulation, then the second heart would respond as if it's vagus nerve was activated. The heart rate of the second heart slowed. In this experiment, he demonstrated that a chemical substance was important for the signal that is passed between the nerve and the heart muscle.
He named the chemical ”Vagusstoff”. This substance was later found to be acetylcholine. Because of his work characterizing the neurotransmitter and describing the physiological changes associated with the chemical nature of nerve signaling, he was awarded the Nobel Prize in Physiology or Medicine in 1936. This prize was shared with his friend, Sir Henry Dale.