Structure and Function
In 1879, Cajal is appointed as a member of the Faculty of Medicine at Zaragoza Medical School. On his small salary, he purchases a microscope and begins his foray into neuroscience with a paper titled Microscopic Observations on Terrestrials: Nerve Connections in the Voluntary Muscles. After a brief bout studying the cholera microbe, Cajal returns to studying the nervous system through histology (1). In his lifetime, Cajal would produce roughly 4,500 histological slides and a little under 2,000 illustrations (4). Through his work, he would determine the correct structure of neuronal connectivity, turning long-held beliefs on their head.
"The most vital and profound problems of the nervous machine loomed like inaccessible peaks. Everything that refers to the arduous question of the origin and termination of the nerve fibers within the centers, and the no less fundamental and pressing question of intimate intercellular connections, eluded our feverish curiosity."
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-Ramón y Cajal (1)
Camillo Golgi
"Our preparations of the brain, cerebellum, spinal cord, etc., fully confirmed the discoveries of the famous Pavia histologist; but no new fact of importance appeared in them. That's why my faith in the method did not abandon me. I was fully convinced that, to seriously advance the structural knowledge of the nervous centers, it was absolutely necessary to use procedures capable of showing, vigorously and selectively stained on a light background, the faintest nerve rootlets." (1)
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Camillo Golgi was a histologist with the University of Pavia in Italy. He pioneered the Golgi staining method in histology, capable of, as Cajal writes, illuminating 'the faintest of nerve rootlets.' Although a majority of Golgi's histological preparations are lost, his discoveries of neurobiological and cellular biological structures are some of the most important in history: perineuronal nets, the Golgi apparatus, and the Golgi tendon organ (5). Cajal used the Golgi method for over 800 histological preparations in his own research, though the two were locked in a bitter feud over the connectivity of neurons (4, 6).
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The image on the right shows a photo of Golgi, included in Cajal's autobiography. (1)
The Neuron Doctrine
Reticularists
vs.
Neuronists
In the late 19th and early 20th centuries, a battle of theories was occurring in the field of neuroanatomy (6). On one side of the ring stood Golgi and other reticularists, believers in the Reticular Theory, proposed that neurons existed in an interconnected webbing with no breaks between them.On the other side of the ring was Cajal and the neuronists, believers in the Neuron Doctrine, saw neurons as independently operating units.
Cajal frequently called the reticularists evidence just plain bad histology, but this evidence was compiled when microscopes had lower imaging power compared to the modern day.
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So how did Cajal correctly discern that neurons are "contiguous, not continuous" with one another (8)? The proof is in the pudding.
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Alfred von Kölliker, Reticularist
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Kölliker produced this drawing of the reticular view of how motor and sensory neurons connect with one another in the spinal cord. He clearly aligns his illustration with the prevailing reticular theory of the time, depicting a continuous structure.
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The reticular theory prevailed because it explained how information could be transmitted. To researchers at the time, a break in connectivity meant a break in the flow of information.
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Image from (6, Figure 1).
Ramón y Cajal, Neuronist
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In this image, Cajal depicts the synaptic boutons of neurons terminating on the cochlear nucleus. He is clear about his belief in the neuron doctrine in this image. At several points in this illustration, he depicts the gap between the end of one neuron and the dendrites of the next cell.
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It wasn't until usage of the electron microscope in the 1950s. that Cajal's drawings of gaps could be confirmed.
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Image from (6, Figure 5).
The question still stands:
How did he know?
At the time, microscopes were not powerful enough to accurately detail the gaps that synapses make. One must question, then, how Cajal knew that they existed. In his autobiography, he writes that it was a "revelation," a sudden spark that culminated from two years of research (1). Science and art historians instead argue that the answer comes through in Cajal's artistic process.
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Cajal didn't draw what he saw through the microscope in a one-to-one recreation, contrary to that of his contemporaries (8). Microscopes cannot visualize all elements of a neuron at once, both because the field of is limited and because the depth of field cannot capture all parts at once. Translating a 3D cell to 2D slices of that cell adding up to a 3D structure to 2D reproductions on paper requires artistic prowess, technical skill, and spatial understanding (9), something the reticularists may not have had a grasp on. Cajal may not have been able to see all 3 dimensions making up the cell he was investigating, but he nonetheless understood they existed.
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Cajal was often criticized (or applauded, depending on who is asked) for the artistic liberties he took when illustrating his findings. Comparisons between his original slides and the drawings produced show that Cajal emphasized certain details, left out others, and sometimes inferred structures (4). This created clear imagery that made his illustrations part of his argument, not just a supplement to the words he was writing.
These images show one of Cajal's studies on the motor neuron of a cat, specifically aiming to show the spindly neurofilaments within them (4). The image on the left shows Cajal's histology using the Reduced Silver Nitrate Method. The image on the right shows Cajal's illustration of this slide.
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Note that Cajal has let the background fade away in his illustration. While the histological slide shows artifacting from other cells present on the slide, Cajal chose not to include this in his reproduction. Through this choice, Cajal can emphasize what he wants readers to look at: the complex inner workings of the neuronal cytoskeleton.
These images show neurons of the cerebral cortex (5). The leftmost image shows Golgi's original slide, stained using the Golgi Method. The middle image shows his illustration of the slide. The right image shows Cajal's drawings of cerebral cortex neurons (D through J on image).
Like Cajal, Golgi also leaves out the artifacts left behind by his staining to clearly show the neurons in view. Contrary to the reticular theory Golgi defended, his illustration does not clearly show if the axons, drawn in red, of one neuron and the dendrites of the next are connected to one another. While it shows they overlap, there is no clear connectivity. In his original paper including this drawing, Golgi notes that some axons could not be followed to the next because they became "lost in the diffuse net" of neuronal connections (5).
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Golgi's last comment is telling of his staunch belief in reticular theory. Rather than observing the evidence presented to him under the microscope, he instead writes explanations for his evidence such that they conform to his beliefs.
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Cajal's illustration does suggest one must be forgiving toward Golgi's mistakes. The branching, overlapping, intertwining dendrites and neurons are difficult to distinguish from one another. Combined with the presence of other cell types around the cortical neurons, structures are difficult to discern.
The most clear part of all 3 images is the cell body, but when trying to determine connectivity and flow of information, its prominent structure is useless. It is no wonder that it took until the invention of new technology that the reticular theory never trulwent away; it required creativity and a mind that worked outside of the box, like Cajal's, to infer the real structure of neurons.
Despite their differences, Cajal and Golgi shared the 1906 Nobel Prize in Medicine or Physiology "in recognition of their work on the structure of the nervous system" (10).
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Golgi's half of the prize was awarded because of his development of the Golgi staining method, while Cajal's was awarded for the immense number of neuroanatomical works he produced (10). By 1906, Cajal's Neuron Doctrine was widely accepted. Still, Golgi used his Nobel Lecture in attempt to garner support for the dying-out Reticular Theory.
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The image to the left is Cajal's Nobel Diploma, included in his autobiography (1).
"The Father of Neuroscience"
Cajal is often called "The Father of Neuroscience" thanks to his contributions to the field. Since his death in 1934, nearly 9 decades have passed, and more than a centuries worth of research has occurred since he purchased his first microscope.
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Cajal is often taught about in modern scientific textbooks (12). His work is included in Neuroscience by Purves et al., considered one of the premier textbooks in the neurobiological field. Through discussions of past and ongoing research in the field, Neuroscience is useful to explore how ideas of the nervous system have changed since Cajal put forth his revolutionary research.
If there was one Thing Cajal and Golgi could agree on, it was the shape of individual neurons.
They may not have been able to agree on neuronal connectivity, but they could agree on neuron morphology. Even modern day depictions of the neurons tend to agree with, or elaborate upon, illustrations created by Cajal. Take, for example, the Purkinje cell of the cerebellum:
The left image shows Golgi’s drawing of a Purkinje cell from 1883 (5). The middle shows Cajal’s drawing from 1899 (13). The right image shows the diagram of a Purkinje cell in Neuroscience (12). All 3 illustrations emphasize the extensive arborization of the Purkinje cell’s dendrites, the bulbous cell body, and sparsely branching axon. The extensive processes give a hint as to the function of Purkinje cells: to receive copious amounts of information, integrate at the cell body, and 'make decisions' regarding coordination of motor control.
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Note that Neuroscience's diagram of this cell is a direct tracing of Cajal's. This suggests a perfection to Cajal’s work, that no changes were needed in the century since Cajal produced this illustration. Cajal's 1899 drawing perfectly captured the complexity of a Purkinje cell's projections. This makes it a worthy inclusion in Neuroscience, highlighting the uniqueness in structure that neurons can take that nonetheless still abide by the rules of Cajal’s neuron doctrine.
Cajal's Impact on Neuroscience must be considered in Context Of What Is Being Taught.
Neuroscience aims to show the diversity of neuron structure. So does Cajal.
In the top image, Neuroscience depicts the various morphologies that neurons can have (12). Between the cortical pyramidal cell's spindly extensions, the shortened dendrites and axons of retinal bipolar cells, the reaching dendrites of the retinal ganglion cell, and the axon-less amacrine cell, readers are imparted with knowledge that neurons can come in all shapes and sizes. In the bottom images, illustrations from Cajal also highlight the vast array of structures that neurons can take.
Pyramidal neurons are both pictured with winding extensions that cover a vast surface area. Cajal is sure to highlight the pyramidal cell's prominent longest dendrite in the center of the illustration, extending skyward. The shorter secondary dendrites extend to the side. Both Neuroscience and Cajal communicate the prominence of the apical dendrite and the equally as important basal dendrites extending from the cell body.
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In the next two images, Cajal illustrates the cellular circuit in the retina. In the left image, retinal bipolar cells are shown labeled as (a) and (b). Compared to the retinal ganglion cell, labeled (d) in the same image, the surface area of the dendrites are nearly identical. This is in contrast to Neuroscience's depiction, showing bipolar cells have much smaller dendrite extensions than ganglion cells. However, Cajal and Neuroscience can agree on axon length. While bipolar cells are both illustrated with stunted axons, ganglion cell axons extend much longer. Perhaps this disparity between Cajal's and Neuroscience's depiction of bipolar cell dendrites is yet another example of the diversity of structures observed, even between the same cell types.
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In the middle image, Cajal illustrates retinal amacrine cells as short, stocky, and prominent. The ball of black ink against the page depicting the cell body draws attention to these axon-less neurons. The solid black cell body contrasts with the hashed or stippled patterns on other cell bodies. In this way, Cajal makes both their coloration and structure more visually striking.
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In the second-to-last image, Cajal illustrates neurons in the mesencephalic trigeminal nucleus. Both Cajal's and Neuroscience's illustrations are reminiscent of snakes slithering through grass or garden eels jutting out of the sand. With prominent cell bodies and winding axons, both show once again the unique morphologies these cells can take on.
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The first 4 images are from (1). The rightmost image is from (13).
Theory and Reality
In 1897, while Cajal was busy publishing his revolutionary findings and being elected a member of the Royal Academy of Sciences in Madrid, a German scientist named Paul Ehrlich proposed the Side-Chain Hypothesis of Antibody Formation (1, 14). In his papers, he hypothesized that, in response to a toxin, the body produces antibodies to protect the body from future invading toxins. While his hypothesis turned out to be more complicated than he made it, Ehrlich's diagrams of his model still hold up, such as the image to the side (14).
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Interestingly, Ehrlich had no knowledge as to the structure of these antibodies and purely theorized about them. However, their vague Y-shape gave rise to the way in which antibodies are envisioned today. As art historian James Elkins writes, "The pictures became the theory... The diagrams initiated and partly guided the subsequent experimental practice" (15).
What does this have to do with Cajal?
EverythinG.
Cajal's illustrations were a number of things: a recreation of histological slides, evidence and arguments for a theory, a reaction to and debunking of a competing theory, and the basis of neuroscience being done today. His work continues to be used to this day to teach, to study, and to inspire. Cajal's illustrations had become the reality of neuroscience, not only revolutionizing the field at the turn of the century but encouraging new research well into the modern day as well. Because of his work, theory became reality.