The Golgi–Cajal dispute
In 1873 the Italian physician Camillo Golgi developed the "black reaction", a silver-nitrate stain that turns a few nerve cells dark. Golgi believed the nervous system was one continuous net, but the Spanish histologist Santiago Ramón y Cajal used the same stain to draw thousands of figures arguing that it was made of separate, individual cells. In 1891 Wilhelm Waldeyer named these cells "neurons" and summarised the neuron doctrine, and in 1906 Golgi and Cajal shared the Nobel Prize while still disagreeing. In 1897 Charles Sherrington named the junction between neurons the "synapse", and in the 1950s the electron microscope revealed the roughly 20-nanometre gap, confirming Cajal.
Structure and signalling
A neuron has dendrites that receive signals, a cell body containing the nucleus, and an axon that carries signals away. When incoming input crosses a threshold, sodium ions rush in and a millisecond electrical pulse — the action potential — travels down the axon. In 1952 Alan Hodgkin and Andrew Huxley described this mathematically using the squid giant axon, earning the 1963 Nobel Prize. Along axons wrapped in insulating myelin, the signal jumps between gaps and can exceed 100 m/s. At the axon terminal, neurotransmitters such as acetylcholine and dopamine are released into the synapse to pass the signal on. A single neuron may have thousands of synapses, and connections that are used often grow stronger — the basis of neuroplasticity, learning and memory.
Numbers, types and artificial neurons
In 2009 Suzana Herculano-Houzel's team dissolved brains and counted nuclei to estimate about 86 billion neurons in the human brain, showing that the familiar "100 billion neurons, ten times as many glia" figures had little basis. In 1962 David Hubel and Torsten Wiesel found neurons in the cat visual cortex that fire only for lines at particular angles, and some neurons are strikingly specialised, like the mirror neurons found in monkeys. The belief that adult brains make no new neurons was challenged in 1998 when Peter Eriksson's team reported neurogenesis in the human hippocampus, though its extent is still debated. Warren McCulloch and Walter Pitts's 1943 model of the neuron as a simple logic unit became the starting point of artificial neural networks and deep learning. When many neurons fire abnormally together, the result is an epileptic seizure.
FAQ
- Can neurons regrow?
- Most neurons are not replaced during life, but research suggests some new neurons form in adults in areas such as the hippocampus. How many in humans remains debated.
- Do we only use 10% of our brains?
- No. Brain imaging shows virtually every region is active over a day, and even small injuries cause deficits. The "10% myth" has no scientific basis.
Sources reviewed: 2026-10-26
References
- Golgi, C. (1873). Sulla struttura della sostanza grigia del cervello. Gazzetta Medica Italiana – Lombardia, 33, 244–246.
- Ramón y Cajal, S. (1899–1904). Textura del sistema nervioso del hombre y de los vertebrados. Madrid: Nicolás Moya.
- Sherrington, C. S. (1906). The Integrative Action of the Nervous System. New Haven: Yale University Press.
- McCulloch, W. S., & Pitts, W. (1943). A logical calculus of the ideas immanent in nervous activity. Bulletin of Mathematical Biophysics, 5(4), 115–133.
- Hodgkin, A. L., & Huxley, A. F. (1952). A quantitative description of membrane current and its application to conduction and excitation in nerve. Journal of Physiology, 117(4), 500–544.
- Hubel, D. H., & Wiesel, T. N. (1962). Receptive fields, binocular interaction and functional architecture in the cat's visual cortex. Journal of Physiology, 160(1), 106–154.
- Eriksson, P. S., et al. (1998). Neurogenesis in the adult human hippocampus. Nature Medicine, 4(11), 1313–1317.
- Azevedo, F. A. C., et al. (2009). Equal numbers of neuronal and nonneuronal cells make the human brain an isometrically scaled-up primate brain. Journal of Comparative Neurology, 513(5), 532–541.
- Kandel, E. R., Koester, J. D., Mack, S. H., & Siegelbaum, S. A. (Eds.). (2021). Principles of Neural Science (6th ed.). New York: McGraw-Hill.