Soups and Sparks: The Discovery of Chemical Transmission

Otto Loewi's dream experiment with two frog hearts proved that nerves talk with chemicals — and set off a decades-long 'war of the soups and the sparks'.

Once it was clear that neurons are separate cells, a new question arose: how does a signal cross the gap? Physiologists split into two camps. The sparks argued the electrical impulse jumped directly across. The soups argued the nerve released a chemical messenger.

Loewi's dream

The German pharmacologist Otto Loewi said the decisive experiment came to him in a dream on the night before Easter Sunday 1921. He woke, scribbled notes he couldn't read in the morning, and was lucky enough to dream it again.

He took two frog hearts, kept beating in salt solution. Stimulating the vagus nerve of the first heart slowed it. He then transferred fluid from the first heart to the second — which also slowed, even though its own nerve hadn't been touched. Something released by the nerve into the fluid carried the message. He called it Vagusstoff ("vagus substance").

Acetylcholine

The British physiologist Henry Dale had already studied a chemical called acetylcholine and its effects; work in the following years identified Vagusstoff as acetylcholine, which Dale and colleagues also showed transmits signals from nerves to skeletal muscles. Loewi and Dale shared the 1936 Nobel Prize. Acetylcholine was the first neurotransmitter identified.

The war ends

The sparks held out longest for synapses in the brain and spinal cord, where transmission is very fast. The Australian physiologist John Eccles was a leading spark champion — until his own intracellular recordings in the early 1950s convinced him that central synapses are chemical too. Both sides were partly right: most synapses are chemical, but electrical synapses (gap junctions) do exist.