First coined in the 1960s, the term “neuroscience” refers to the scientific study of the nervous system, including our fascinating brain, from its most fundamental aspects, such as molecules and cells, to the integrative dimensions that underlie our cognitive and behavioural functions. Although much remains to be discovered, giant steps have been made in this field over the past few decades. To appreciate how far we have come, nothing is better than looking back at some of the milestones that have marked this history.
The colouring technique that changes everything. In the 1830s, German scientists Matthias Jakob Schleiden and Theodor Schwann laid the foundation for the theory that would become the basis of biology, the cell theory, which presents the cell as the structural and functional unit of all living organisms. However, the brain tissue could not be marked the same way as other tissues in the body, which prevented scientists from checking whether it too corresponded to the cell theory.
In 1873, the Italian physician Camillo Golgi discovered, by accident, how to colour neurons and their arboreal extensions, thus observing them under the microscope. The discovery in question, called “black reaction” by Golgi, is a chemical reaction that will eventually be called colouration, “Golgi,” or “silver nitrate” method. It is unknown why, but this chemical reaction colours only a tiny proportion of the neurons, which allows the contours of the reacting cells to be clearly seen, as they do not overlap. However, the Italian doctor’s technique was difficult to perform. The Spanish neuroanatomist and histologist Santiago Ramon y Cajal succeeded in standardizing it a few years later, enabling him to produce thousands of drawings from his observations that gave the first idea of how the brain works.
Cajal’s colossal contribution. After improving the black reaction procedure, the Spanish neuroanatomist and histologist confirmed that the ends of neurons are indeed free. This was the first step in what would become the neuron theory in which these cells represent the basic structural and functional unit of the nervous system. This basic theory was refined in the late 19th century by the German anatomist Heinrich Wilhelm Waldeyer, to whom we owe the term “neuron,” and it would eventually take over the competing theory of the time, the reticular theory, which presented the nervous system as a single continuous network.
In addition to accurately illustrating the three morphological parts of the neuron (the cell body, the dendrites and the axon), Cajal described the different stages of growth and laid out the basic principles of neuronal theory; in particular, he foresaw that the mode of communication between neurons was by contact involving a nerve signal. The English physiologist Charles Sherrington gave the connection between two neurons the name “synapse” in 1887. Still, its functioning was elucidated in the middle of the 20th century by the biophysicist Bernard Katz. Using the transmission electron microscope, which allows the observation of elements smaller than cells, Katz found that it is by means of a chemical message, i.e. by the release of neurotransmitters, that nerve impulses are transmitted from one neuron to another, in other words, that neurons communicate with each other.
1906: the Nobel of controversy. Before the neuron theory was recognized, the reticular theory of von Gerlach, laid out in 1871, dominated, arguing that the brain is made up of a single network of fibres and fused cells in which thought is born and circulates. In 1906, Golgi and Cajal both received the Nobel Prize in Physiology for their respective work on the nervous system. During the ceremony, Golgi attacked his colleague, criticizing him for defending the idea that the nervous system is made of distinct cells and not of a unique and continuous network.
This event will be the spark plug of a controversy that will last for several years between the supporters of the reticular theory and those of the neuronal theory. The “neuronists” finally won out in the 1920s and 1930s, when the work on the functions of neurons by the two English scientists Charles Sherrington and Edgar Adrian confirmed the idea that the neuron is not only the basic anatomical unit of the nervous system but also its central functional unit. Their work earned them the Nobel Prize in Medicine or Physiology in 1932.
Medical imaging, a new era in neuroscience. Our knowledge of the brain took a great leap forward in the 1990s with the advent of functional magnetic resonance imaging (fMRI), which allows us to visualize its structure and functioning live and without danger. This advance confirmed that the brain has an extraordinary capacity to evolve and adapt at any age, a characteristic called cerebral (or neuronal) plasticity or neuroplasticity. Driven by cognitive activity, brain plasticity means that neuronal connections are created or strengthened, and others are weakened or eliminated, thus changing the brain’s architecture and functioning.
This modern chapter in the history of neuroscience has also shed light on the central mechanisms and factors at work in learning (see Neuroscience: Learning in 4 Steps) as well as on the nature of certain neurological problems, including dyslexia, dyscalculia and dementia, commonly referred to as the “3 Ds”. Note that various sophisticated devices, such as eye-tracking systems or electroencephalographies, have been added to fMRI to finely decode our meninges. That’s not to mention that as of 2019, the world of neuroscience has a great addition: the world’s most powerful MRI machine that can produce images of our gray matter 100 times more accurately than a conventional fMRI. About a five-story building, the giant cylinder is located in France at the NeuroSpin center, headed by neuroscientist Stanislas Dehaene.
It is not only neurons… Thanks to technological progress in neuroscience, we know, for example, that there are more than 1000 different types of neurons. We also know that neurons are not the only brain cells that deserve our attention. Since the early 2000s, with the sophisticated microscopes developed for molecular biology, scientists have been able to see at work the glial cells (or glia) that surround neurons. For every 85 million neurons, our brain has 100 million glial cells, and researchers have found that they play a much more important role than previously thought. Divided into three groups, they can either make myelin, the cover that protects neurons, or defend the immune functions of the nervous system or work to support, nourish and protect neurons. This new field of exploration has prompted some neuroscientists to predict that a new understanding of the brain, less centred on neurons and making more room for their neighbouring cells, could soon be born.
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