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Illustrations of two contrasting theories of the brain’s composition. These are the reticular theory, left, and the neuron doctrine that were proposed. (Click bottom source link for a better viewing of this photo to compare and contrast the theory's)

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1. Reticular Theory vs. Neuron Doctrine (Early 1800s–1891)

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1. Reticular Theory vs. Neuron Doctrine (Early 1800s–1891)

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In the late 19th century (1800s) the scientific community was divided over the structural organization of the nervous system. The main theory, the reticular theory by Camillo Golgi, stated that the brain was a continuous network of interconnected fibers, functioning as a unified whole (Golgi, 1873). Golgi's belief was based on his observations using a staining technique he developed, which appeared to show a seamless web of neural tissue. We will touch on this more in the next section.

Also, Santiago Ramon and Cajal introduced the Neuron Doctrine, asserting that the nervous system is composed of discrete, individual cells called neurons (Cajal, 1891). Through meticulous observations and detailed drawings, Cajal demonstrated that neurons are separate entities that communicate at specific junctions (Shepherd, 1991). This finding fundamentally changed our understanding of neural structure and established base knowledge for modern neuroscience.

Sources:

Cajal, S. R. (1891). Textura del sistema nervioso del hombre y de los vertebrados. Madrid, Spain.

Golgi, C. (1873). On the Structure of the Nervous System. Italian Academy of Sciences.

Shepherd, G. M. (1991). Foundations of the Neuron Doctrine. Oxford University Press.

Hunched Over a Microscope, He Sketched the Secrets of How the Brain Works - The New York Times

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Early Illustrations of the Nervous System by Camillo Golgi and Santiago Ramón y Cajal

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Beautiful diagrams exploring the nervous system from two pioneers in the field.

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Early Illustrations of the Nervous System by Camillo Golgi and Santiago Ramón y Cajal are still in some of today's neuroscience textbooks. This photo specifically is pyramidal cells stained with the Golgi method.

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2. Golgi’s Silver Staining Technique (1873)

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2. Golgi’s Silver Staining Technique (1873)

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In 1873, italian scientist Camillo Golgi developed the black reaction. The black reaction is a groundbreaking staining method that used silver nitrate to visualize nervous tissue under a microscope (Golgi, 1873). This technique selectively stained a limited number of neurons in their entirety, revealing their complex structures against a transparent background. This allowed scientists for the first time to observe the detailed morphology of individual neurons (Public Domain Review, 2017). They were able to view a lot including their cell bodies, dendrites, and axons.

Golgi's method was revolutionary, providing unprecedented insights into the architecture of the nervous system. However, his interpretation of the results led him to support the Reticular Theory, believing the stained structures formed a continuous network.

Sources:

Golgi, C. (1873). On the Structure of the Nervous System. Italian Academy of Sciences.

Public Domain Review. (2017). Illustrations of the Nervous System. Retrieved from https://publicdomainreview.org/collection/illustrations-of-the-nervous-system-golgi-and-cajal

What is the Golgi stain? — Brain Stuff

Hunched Over a Microscope, He Sketched the Secrets of How the Brain Works - The New York Times

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Early Illustrations of the Nervous System by Camillo Golgi and Santiago Ramón y Cajal

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Beautiful diagrams exploring the nervous system from two pioneers in the field.

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More early Illustrations of the Nervous System by Camillo Golgi and Santiago Ramón y Cajal. This photo specifically is a illustration of nerve cells in a dog's olfactory bulb. The olfactory bulb is a structure located in the forebrain that receives neural input about odors detected by cells in the nasal cavity. 

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3. Cajal’s Evidence for Individual Neurons (1887–1891)

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3. Cajal’s Evidence for Individual Neurons (1887–1891)

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Spanish neuroscientist Santiago Ramon and Cajal refined Golgi's staining technique and applied it to his studies of the nervous system. During the years 1887-1891 Cajal produced detailed observations and drawings that provided solid evidence for the existence of individual neurons. His work demonstrated that neurons are discrete cells that communicate through specialized contact points which challenged the idea of a continuous neural network.

Overall, Cajal used Golgi’s staining technique, but came to the conclusion that neurons are distinct cells that communicate at contact points (Cajal, 1891).

Cajal's meticulous studies revealed the directional flow of information within neurons, which is from dendrites to axons, supporting the concept of functional polarization (Shepherd, 1991). His microscopic observations and also his neuron drawings provided the first clear evidence of the individual neurons (Public Domain Review, 2017). His findings supported the neuron doctrine which would change our understanding of brain structure and function and lead to endless scientific developments still present today.

Sources:

Cajal, S. R. (1891). Textura del sistema nervioso del hombre y de los vertebrados. Madrid, Spain.

Shepherd, G. M. (1991). Foundations of the Neuron Doctrine. Oxford University Press.

Public Domain Review. (2017). Illustrations of the Nervous System. Retrieved from https://publicdomainreview.org/collection/illustrations-of-the-nervous-system-golgi-and-cajal

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Early Illustrations of the Nervous System by Camillo Golgi and Santiago Ramón y Cajal

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Beautiful diagrams exploring the nervous system from two pioneers in the field.

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In the first drawings of neural networks and later the synapse, the drawings were seemingly defaced with a blue stamp. The blue stamps on Cajal’s neuron drawings were used to prevent theft and confirm authenticity. At the time, stamping the center of documents was a common practice in libraries and universities to stop people from trimming away ownership marks. While it may seem like defacement the stamps were actually meant to protect Cajal’s work from being stolen or altered as there were cases where people took Cajal's drawings home to never be seen again when he first drew them.

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4. Charles Sherrington and the Concept of the Synapse (1897)

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4. Charles Sherrington and the Concept of the Synapse (1897)

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After the findings relating to the Neuron Doctrine, the British physiologist Charles Sherrington introduced the term "synapse" in 1897 to now officially describe the junctions where neurons communicate (Sherrington, 1897). Through extensive research on reflexes and neural pathways, Sherrington illustrated that neural transmission involves discrete interactions between neurons at these synaptic points.

He identified that synapses could either be excitatory or inhibitory which means that they could enhance or suppress neural signals (Sherrington, 1906). This discovery was pivotal in understanding the integrative actions of the nervous system, explaining how complex behaviors and processes arise from simple neural interactions. His findings are also important when trying to understand mental disorders (Silvestri et al., 2019).

Sources:

Sherrington, C. S. (1897). On the reflex action of the spinal cord. Journal of Physiology, 21(3), 233–258.

Sherrington, C. S. (1906). The Integrative Action of the Nervous System. Cambridge University Press.

Silvestri, L., Costantini, I., Sacconi, L., & Pavone, F. S. (2019). Brain structural and functional connectivity: A review of light microscopy and diffusion-based imaging techniques. Frontiers in Cellular Neuroscience, 13, 66. https://www.frontiersin.org/articles/10.3389/fncel.2019.00066/full

Why the First Drawings of Neurons Were Defaced | Quanta Magazine

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Heinrich Wilhelm Gottfried Von Waldeyer is recognized as the founder of the Neuron Doctrine. He coined the term "neuron" in a collection of brief articles discussing recent discoveries about brain function which were published in a German medical journal. His legacy is reflected in the very neural structures he helped to define in the above drawing.

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5. Wilhelm Waldeyer Coins the Term "Neuron" (1891)

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5. Wilhelm Waldeyer Coins the Term "Neuron" (1891)

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In 1891, German anatomist Wilhelm Waldeyer synthesized existing research and formally introduced the term "neuron" to describe the individual nerve cells that constitute the nervous system (Waldeyer, 1891). His work consolidated the findings of contemporaries like Cajal and provided a term that helped further research and communication within the scientific community.

Wilhelm Waldeyer's acceptance and usage of the Neuron Doctrine and his efforts in standardizing the new official vocabulary were instrumental in the widespread acceptance of the concept that the nervous system is composed of discrete cellular units (Silvestri et al., 2019). Coining new terms noy only makes communication within a community of scientists easier, but teaching neuroscience to the future general population, as well.

Sources:

Waldeyer, W. (1891). Über einige neuere Forschungen im Gebiete der Anatomie des Centralnervensystems.

Silvestri, L. et al. (2019). Brain structural and functional connectivity: A review. Frontiers in Cellular Neuroscience, 13, 66. https://www.frontiersin.org/articles/10.3389/fncel.2019.00066/full

https://www.frontiersin.org/articles/10.3389/fncel.2019.00066/full

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The above image is an astounding map of the brain’s white matter fiber pathways from the Human Connectome Project. The fibers are color-coded by direction: red = left-right, green = anterior-posterior, blue = ascending-descending (RGB=XYZ). The Human Connectome Project builds on the Neuron Doctrine by mapping how neurons connect to form brain networks. Cajal illustrated that neurons are separate cells that communicate at synapses and the HCP uses fMRI to trace neural pathways. This modern research validates and expands Cajal’s work, showing how billions of neurons interact to create our thoughts and behaviors. Our thoughts, behaviors, and general processing is still not fully understood, but detailed brain mapping progress is significant.

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Contemporary: Modern Neuroscience Imaging and the Human Connectome Project

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Contemporary: Modern Neuroscience Imaging and the Human Connectome Project

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The Human Connectome Project (HCP) is a modern research initiative aimed at mapping the complex network of neural connections in the human brain. Utilizing advanced imaging technologies such as functional Magnetic Resonance Imaging (fMRI) and Diffusion Tensor Imaging, the HCP provides detailed insights into the brain's structural and functional connectivity. These imaging techniques are used for a variety of other things as well for anything involving mapping brain activity by detecting changes in blood flow. It helps researchers and doctors' study which brain regions are active during tasks like thinking, feeling emotions, or moving.

This modern imaging technology allow scientists to visualize and analyze neural pathways with true precision to validate and expand on the base of knowledge established by the concept of the Neuron Doctrine. The Human Connectome Project's findings have significant options to study for understanding brain function, diagnosing neurological disorders, and even for developing targeted treatments.

Sources:

Van Essen, D. C., et al. (2013). The Human Connectome Project: Progress and Prospects. Neuron, 80(3), 607–620. https://www.frontiersin.org/articles/10.3389/fncel.2019.00066/full

Human Connectome Project |  Mapping the human brain connectivity

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A photo of Eric Richard Kandel. He studied the fields of psychiatry, psychoanalysis and neuroscience at Harvard, went to NYU medical school, and went on to work in College of Physicians and Surgeons at Columbia University. He has been awarded a total 6 notable awards including a Nobel peace prize in 2000. Finally, he has also written and contributed to a variety of books on neuroscience.

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Eric Kandel is a renowned neuroscientist whose research has profoundly influenced our understanding of memory and learning. Born in 1929, Kandel's work focuses on the molecular mechanisms that underlie synaptic plasticity. Synaptic plasticity is the ability of synapses to strengthen or weaken over time. This is believed to be fundamental to learning and memory.

By studying the sea slug aplysia californica, Kandel uncovered how changes at the synaptic level contribute to the formation of memories. Kandel's work provided crucial evidence supporting the neuron doctrine. By explaining the mechanisms of synaptic changes, Kandel bridged the gap between the structural principles of the neuron doctrine and some of the functional processes of behavior and thinking. His contributions were recognized in 2000 when he was awarded the Nobel Prize in Physiology or Medicine, alongside Arvid Carlsson and Paul Greengard, for their collective discoveries concerning signal transduction in the nervous system.

He is the perfect docent to not only cover the basics of neurons and the information found relating to the concept of the neuron doctrine, but a Nobel Peace Prize winning neuroscientist who could explain the details we know about learning and memory in modern day.

Sources:

Eric Kandel | Biography, Nobel Prize, & Facts | Britannica

Eric Kandel - Alchetron, The Free Social Encyclopedia

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Eric Kandel (born November 7, 1929, Vienna, Austria) is an Austrian-born American neurobiologist who, with Arvid Carlsson and Paul Greengard, was awarded the Nobel Prize for Physiology or Medicine in 2000 for discovering the central role synapses play in memory and learning.

Kandel received a medical degree from New York University's School of Medicine in 1956. Following residency in psychiatry and employment at Harvard University, he served as associate professor at New York University (1965-74). Beginning in 1974, Kandel held a series of professorships at Columbia University, where he also directed its Center for Neurobiology and Behavior until 1983. In 1984 he became an investigator at the Howard Hughes Medical Institute.

Kandel's award-winning research centred on the sea slug Apylsia, which has relatively few nerve cells, many of them very large and easy to study. The sea slug also has a protective reflex to guard its gills, which Kandel used to study the basic learning mechanisms. These experiments, combined with his later research on mice, established that memory is centred on the synapses, as changes in synaptic function form different types of memory. Kandel showed that weak stimuli give rise to certain chemical changes in synapses; these changes are the basis for short-term memory, which lasts minutes to hours. Stronger stimuli cause different synaptic changes, which result in a form of long-term memory that can remain for weeks.

Kandel's books included The Age of Insight: The Quest to Understand the Unconscious in Art, Mind, and Brain: From Vienna 1900 to the Present (2012) and The Disordered Mind: What Unusual Brains Tell Us About Ourselves (2018). In Search of Memory: The Emergence of a New Science of Mind (2006) was an autobiography.

The Editors of Encyclopaedia Britannica Encyclopaedia Britannica.

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Eric Kandel | Biography, Nobel Prize, & Facts | Britannica

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Eric Kandel, Austrian-born American neurobiologist who, with Arvid Carlsson and Paul Greengard, was awarded the Nobel Prize for Physiology or Medicine in 2000 for discovering the central role synapses play in memory and learning. Learn more about Kandel's life and career.