The neuron doctrine, championed by Spanish histologist Santiago Ramón Cajal, established that neurons are the fundamental units of the nervous system.
The nervous system consists of –
- The central nervous system (brain + spinal cord)
- Peripheral nervous system (the rest of the system)
There are two types of cells in the nervous system –
- Neurons
- Glia
Neurons receive, process, and transmit electrochemical signals within the body. Some neurons, called sensory neurons, also receive information from the environment. They’re the fundamental units of the nervous system. Glia supports the neurons by insulating and nourishing them, and they outnumber the neurons by 10:1.
Studying neurons was a challenge in the past because they are too small. After the microscope was invented, it was still very difficult to observe them because studying neurons in a microscope required very thin slices of brain tissue (thinner than the diameter of the cells). The brain’s consistency (similar to Jell-O) makes it extremely difficult to create such slices.
The study of cells under the microscope is called histology. Histologists eventually developed a way to “harden” the brain without disturbing its cell structures by using formaldehyde. Unfortunately, it was still not possible to see the individual cells in the brain slices because the brain has a very uniform cream-colored appearance even under the microscope.
The solution to this was the discovery of various stains, such as the Nissl stain and the Golgi stain. Stains are dyes that color sample slices in specific ways. For example, the Nissl stain colors the nucleus (central body) of both neurons and glia, but it also colors the area surrounding the nucleus of the neuron. Thus, it enabled distinguishing neurons and glia from each other.
Golgi stain, discovered by Italian histologist Camillo Golgi, stains a small percentage of neurons in a sample, but makes them even more clearly visible. It shows that the neurons have two parts –
- Cell body or soma
- Neurites
Neurites usually contain (a) one axon (the output channel) and (b) multiple dendrites (the input channels).
Golgi believed that the neurites of different neurons were fused together to form a continuous network, much like the circulatory system (i.e., blood vessels). This is called the Reticular theory.
In contrast, Spanish histologist Santiago Cajal argued that neurons are separate cells, the neurites of different neurons are not continuous, and they communicate by contact, not continuity. This is called the Neuron doctrine.
Both Golgi and Cajal shared the Nobel Prize in 1906, but remained rivals in their ideas. Decades later, in 1950, with the discovery of the electron microscope, it was finally confirmed that Cajal was correct.