• By the turn of the 20th century, physiologists were fairly certain that neurons communicated with each other and with target organs by means of a chemical substance. But the question was, how to prove it.
• Acetylcholine and norepinephrine had already been identified as putative (suspected) neurotransmitters.
• In 1921, Otto Loewi came up with the answer, as he told it, in his sleep.
• Loewi's experiment
  • Loewi arranged two frog's hearts so that the baths they were in could be circulated through both preparations by way of a pump that could be turned on and off.
  • One heart still had the vagus nerve attached. Stimulation of the vagus nerve slows the heart rate.
  • If the pump was turned off, and one heart was stimulated, there was no effect on the second heart.
  • If the pump was turned on, and one heart was stimulated, after a delay the second heart was also affected.
  • This showed that something released by the nerve and that could circulate in the bath must be influencing heart rate. This had to be a chemical substance.
• Loewi was convinced the chemical substance was acetylcholine, since direct application of acetylcholine to the heart muscle also caused it to slow.
• But he couldn't prove it, so he called the chemical substance vagusstoff.
• Later it was demonstrated that vagusstoff was, in fact, acetylcholine.

Baseline Condition		Control Condition		Treatment Condition
NO PUMP NO STIMULATION OF VAGUS
heart A	heart B	heart A	heart B	heart A	heart B
There is no pumping of fluid between the two chambers and no stimulation of the vagus nerve. The hearts will continue beating for several hours under these conditions.		Vagus nerve to heart A is stimulated but the pump is not on, so no fluid is circulated to heart B. Result: Vagus stimulation slows heart A, but heart B remains unchanged.		Vagus nerve to heart A is stimulated and the pump is circulating fluid from bath A to bath B. Result: Vagus stimulation slows heart A, and heart B slows down as well.

Note: Yes these are human hearts. Try finding an animated gif of a beating frog's heart on the Internet! I drew the pump myself. Consider it in the public domain.

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