Otto Loewi, an Austrian pharmacologist, proved in 1921 that nerve cells communicate using chemicals rather than purely electrical signals. He shared the 1936 Nobel Prize in Physiology or Medicine with Henry Dale, a British physiologist who had independently identified the specific chemical involved. Together, their work established that the nervous system runs on chemistry, not just electricity.
The Idea Before the Proof
Before Loewi’s breakthrough, scientists already suspected that chemicals might play a role in nerve signaling. In 1904, a Cambridge student named Thomas Renton Elliott observed that the effects of stimulating certain nerves looked remarkably similar to the effects of injecting adrenaline. He suggested that nerves might work by releasing adrenaline-like substances at their endings. It was a bold idea, but Elliott lacked the experimental tools to prove it, and many prominent scientists remained skeptical. The dominant view held that nerve signals were purely electrical, passing directly from one cell to the next like current through a wire.
This disagreement would eventually split neuroscience into two camps, colorfully known as the “soups” (those who believed in chemical messengers) and the “sparks” (those who insisted on electrical transmission). The debate persisted for decades.
Loewi’s Famous Frog Heart Experiment
The experiment that settled the question came to Loewi, by his own account, in a dream. He set up two frog hearts in separate fluid-filled chambers. One heart still had its vagus nerve attached (a major nerve that controls heart rate). The other heart had been disconnected from all its nerves.
Loewi stimulated the vagus nerve on the first heart, which slowed its beating. That part was already well understood. The clever step came next: he took the fluid surrounding that first heart and applied it to the second, denervated heart. The second heart slowed down too, even though no nerve had been stimulated. Something in the fluid, released by the nerve, was carrying the signal.
In a follow-up experiment, Loewi stimulated a different nerve (the accelerator nerve) on the first heart, making it beat faster. When he transferred that fluid to the second heart, it also sped up. The chemical message worked in both directions: slowing and accelerating.
Loewi called the inhibitory substance “Vagusstoff,” meaning “vagus stuff.” It was later identified as acetylcholine, a molecule that remains one of the most important chemical messengers in the human body.
Henry Dale’s Contribution
While Loewi demonstrated that a chemical could carry a nerve signal, Henry Dale spent years pinning down exactly what that chemical was and where it acted. Dale had actually discovered naturally occurring acetylcholine in animal tissue back in 1913, years before Loewi’s experiment, but at the time nobody knew what it did in the body.
After Loewi’s 1921 experiment pointed to a chemical messenger, Dale and his colleagues methodically showed that acetylcholine functioned as a neurotransmitter at multiple points in the nervous system: at the junctions where nerves meet muscles, at the relay stations within the autonomic nervous system (which controls involuntary functions like digestion and heart rate), and at the nerve endings that regulate organs like the heart and lungs. This work transformed acetylcholine from a laboratory curiosity into a cornerstone of neuroscience.
The Nobel committee recognized both men jointly in 1936 “for their discoveries relating to chemical transmission of nerve impulses.”
How Chemical Signaling Actually Works
The process Loewi and Dale uncovered is now understood in fine detail. When an electrical impulse travels down a nerve cell and reaches its tip, it triggers the opening of tiny channels that let calcium ions flood into the cell. Calcium concentration inside a resting nerve ending is extremely low (roughly 100 nanomoles per liter), so when these channels open, calcium rushes in rapidly along a steep gradient. That surge of calcium is the “go ahead” signal: it causes small packets of chemical messengers to fuse with the cell membrane and spill their contents into the narrow gap between nerve cells, called the synapse.
Those chemical messengers (neurotransmitters) drift across the gap and lock onto receptors on the next cell, triggering a new electrical or chemical response. The whole process takes a fraction of a millisecond. After the signal is delivered, the neurotransmitter is either broken down by enzymes or recycled back into the sending cell, resetting the system for the next signal.
Chemical and Electrical Synapses Coexist
Loewi’s discovery didn’t mean the “sparks” camp was entirely wrong. The brain does use electrical synapses, called gap junctions, where cells connect directly and pass signals almost instantaneously with no chemical intermediary. These electrical synapses are especially common in circuits that need extreme speed, like escape reflexes in fish. For years, scientists believed electrical synapses were mostly limited to invertebrates and cold-blooded animals, but research has since revealed that they’re more widespread in the mammalian brain than anyone originally expected.
Still, chemical synapses dominate in the human nervous system. They’re slower than electrical ones by a tiny fraction, but they offer something electrical synapses cannot: flexibility. Because chemical signals pass through receptors that can be tuned, amplified, or dampened, they allow the brain to learn, adapt, and fine-tune its responses. The chemical system Loewi first glimpsed in a frog heart turned out to be the primary language of the entire nervous system.