A silver stain that blackened one cell in a hundred settled the most important question in neuroscience: is the nervous system one continuous web, or made of separate cells?
History of Neuroscience
Intermediate
Golgi, Cajal and the Neuron Doctrine
The Editors
January 17, 2026
3 min read
In plain English
In the late 1800s, scientists couldn't tell whether nerve tissue was a continuous mesh or made of separate cells. Camillo Golgi invented a stain that showed single nerve cells in full detail, and Santiago Ramón y Cajal used it to prove that neurons are separate units that touch but don't fuse. They shared a Nobel Prize — and disagreed to the end.
Intermediate level: Builds on the basics. Introduces the real vocabulary and the mechanisms behind it.
Concepts to know first
By the mid-19th century, cell theory held that living things are made of cells. But nervous tissue was a puzzle: under the microscope it looked like a dense tangle, and many scientists believed nerve fibres fused into a continuous network, the reticulum, through which signals flowed freely.
The black reaction
In 1873, working in a makeshift laboratory in a hospital kitchen, the Italian physician Camillo Golgi discovered that hardening brain tissue in potassium dichromate and then soaking it in silver nitrate stained a small fraction of nerve cells — perhaps one in a hundred — completely black, from cell body to the finest branch. He called it la reazione nera, the black reaction. For the first time, individual neurons could be seen whole against a clear background.
Golgi himself, however, interpreted what he saw as support for the reticular theory.
Cajal's revolution
From 1887 the Spanish histologist Santiago Ramón y Cajal improved Golgi's method — notably by staining young tissue, where fewer insulating sheaths obscured the fibres — and studied almost every part of the nervous system. He concluded that neurons are separate, independent cells whose branches come very close to one another but do not fuse. He also proposed the law of dynamic polarisation: signals flow in one direction, from dendrites to cell body to axon. In 1891 Wilhelm Waldeyer gathered these ideas under the name neuron doctrine and coined the word "neuron".
A Nobel Prize for two rivals
Golgi and Cajal shared the 1906 Nobel Prize in Physiology or Medicine. In his Nobel lecture Golgi defended the reticular theory; Cajal used his to refute it. Final proof came only in the 1950s, when the electron microscope showed a gap of about 20 nanometres — the synaptic cleft — between neurons. Cajal had been right. (Rare electrical synapses do link some neurons directly, giving Golgi a small consolation.)
Everyday examples & things to try
Analogy
Why staining only a few cells helped
Imagine trying to see a single tree in a dense forest from above — you can't. Now imagine a dye that paints just one tree in a hundred bright black, trunk to twigs. Suddenly you can see what a tree looks like. That's what Golgi's stain did for neurons, and its randomness was its strength.
Key takeaways
- The reticular theory held that nerve tissue is one continuous web.
- Golgi's 1873 silver stain revealed single neurons; Cajal used it to show they are separate cells.
- The neuron doctrine was confirmed in the 1950s by electron microscopy of the synaptic cleft.
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