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Brain Research Bulletin 70 (2006) 391–405
Neuron theory, the cornerstone of neuroscience, on the centenary
of the Nobel Prize award to Santiago Ramón y Cajal
Francisco López-Muñoz a,∗ , Jesús Boya b , Cecilio Alamo a
a
Neuropsychopharmacology Unit, Department of Pharmacology, Faculty of Medicine,
University of Alcalá, C/Juan Ignacio Luca de Tena 8, 28027 Madrid, Spain
b Department of Cellular Biology, Faculty of Medicine, Complutense University, Ciudad Univeritaria, 28040 Madrid, Spain
Received 7 June 2006; accepted 14 July 2006
Available online 14 August 2006
Abstract
Exactly 100 years ago, the Nobel Prize for Physiology and Medicine was awarded to Santiago Ramón y Cajal, “in recognition of his meritorious
work on the structure of the nervous system”. Cajal’s great contribution to the history of science is undoubtedly the postulate of neuron theory.
The present work makes a historical analysis of the circumstances in which Cajal formulated his theory, considering the authors and works that
influenced his postulate, the difficulties he encountered for its dissemination, and the way it finally became established. At the time when Cajal
began his neurohistological studies, in 1887, Gerlach’s reticular theory (a diffuse protoplasmic network of the grey matter of the nerve centres),
also defended by Golgi, prevailed among the scientific community. In the first issue of the Revista Trimestral de Histologı́a Normal y Patológica
(May, 1888), Cajal presented the definitive evidence underpinning neuron theory, thanks to staining of the axon of the small, star-shaped cells of
the molecular layer of the cerebellum of birds, whose collaterals end up surrounding the Purkinje cell bodies, in the form of baskets or nests. He
thus demonstrated once and for all that the relationship between nerve cells was not one of continuity, but rather of contiguity. Neuron theory is
one of the principal scientific conquests of the 20th century, and which has withstood, with scarcely any modifications, the passage of more than
a 100 years, being reaffirmed by new technologies, as the electron microscopy. Today, no neuroscientific discipline could be understood without
recourse to the concept of neuronal individuality and nervous transmission at a synaptic level, as basic units of the nervous system.
© 2006 Elsevier Inc. All rights reserved.
Keywords: History of neuroscience; Neuron theory; Reticular theory; Cajal
“The facts remain and theories pass away”
Santiago Ramón y Cajal, 1894
1. Introduction
The neuron doctrine constitutes the cornerstone on which,
throughout the 20th century, all the neuroscientific disciplines
were constructed. This year sees the centenary of the award of the
Nobel Prize for Physiology and Medicine to Santiago Ramón y
Cajal (1852–1934), the great ideologue and driving force behind
this theory, for his meritorious work on the structure of the nervous system. One hundred years later, the majority of Cajal’s
postulates, laid out in his lecture to the Swedish Academy, continue to be of remarkable scientific currency, and have made
∗
Corresponding author. Tel.: +34 91 7248210; fax: +34 91 7248205.
E-mail address: [email protected] (F. López-Muñoz).
0361-9230/$ – see front matter © 2006 Elsevier Inc. All rights reserved.
doi:10.1016/j.brainresbull.2006.07.010
Cajal the most cited classical scientist in history. It is in recognition and honour of this notable achievement that we examine in
this work the circumstances in which the neuron doctrine came
into being.
Neuron theory should be considered, from the historical perspective, as the final and definitive link in the development of
cell theory, the doctrine that consolidated itself in the second
half of the 19th century as a result of extensive progress in
the anatomical disciplines, the coming of age of physiological knowledge, the definitive adoption of proper experimental
methods, improvements in optical technology and advances in
micrographic techniques. The first cell theory began to emerge
in the 1830s, thanks to the work of researchers such as Samuel
Thomas von Sömmerring (1755–1830), Karl Asmund Rudolphi (1771–1832), Johann Evangelista Purkinje (1787–1869),
Ernst Heinrich Weber (1795–1878), Johannes Peter Müller
(1801–1858), Friedrich Gustav Jackob Henle (1809–1885) or
Gabriel Gustav Valentin (1810–1863), though it was the con-
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F. López-Muñoz et al. / Brain Research Bulletin 70 (2006) 391–405
tributions of Matthias Jacob Schleiden (1804–1881) and of
Theodor Schwann (1810–1882) that made possible the definitive formulation of cell theory [2]. Schwann produced one of
the first descriptions of cell structure (Zellentheorie) in animal
tissue (cytoblasteme, cell membrane, cell nucleus and nucleoli),
in which he considered the cell as a structure made up of several superimposed layers. Schwann concludes his work with the
proposal of a cell theory (Theorie der Zellen) focusing not such
much on anatomical as on physiological aspects: “In general we
should attribute autonomous life to cells” [61].
Rudolph Albert von Kölliker (1817–1905), in his Handbuch
der Gewebelehre des Menschen, first published in 1852 [45],
brought together and gave substance to all the disperse knowledge that could contribute to cell theory. For Kölliker, the cell
would be made up of a container vesicle (cell membrane) and
a content composed of a liquid surrounding different particles
and a special round body (cell nucleus), which contained in turn,
another liquid and another smaller corpuscle (nucleus corpuscle or nucleolus). Eleven years later, in the 4th German edition
of his Handbuch, von Kölliker stated that cells “should be conceived as the essential formal units of the body”. However, and
with no disrespect to all the other authors mentioned, it is fair to
acknowledge Rudolf Ludwig Carl Virchow (1821–1902) as the
great ideologue of the cellular theory. A disciple of Müller and
Professor at Würzburg and Berlin, in 1855 he pronounced his
famous sentence: “omnis cellula e cellula” [68], that is, cells can
only multiply from themselves. For Virchow, life is essentially
cellular activity, so that the life of organisms is the sum of the
life of each of its cells.
The enormous technical advances in the field of cytohistology during the second half of the 19th century provided
scientists with more and more precise knowledge about the true
structure of cells. It suffices to mention, by way of illustration,
the comprehensive progress in the construction of microscopes
(Leitz), the development of immersion lenses in 1850 (Amici),
the introduction of the microscope condenser system in 1873
(Zeiss and Abbe) and the apochromatic lens in 1887 (Abbe), the
introduction and refinement of the microtome in 1886 (Minot),
techniques of paraffin embedding in 1869 (Klebs) and celloidin
embedding in 1879 (Duval), techniques for fixing sections in
chromic acid in 1850 (Corti), or in chromic-acetic mixtures
in 1854 (Remak), the new colouring methods, such as Gerlach’s carmine (1847), Schultze’s osmic acid (1865), Perkin and
Ehrlich’s aniline derivatives (1854–1870), and Böhmer’s hematoxylin (1865), von Recklinghausen’s metallic impregnations
(1863) and the subsequent discovery of silver bichromate by
Golgi (1883) or silver nitrate by Cajal (1903), the microphotographic methods developed by Gerlach (1863) and Koch (1877),
and so on. Thus, cell theory became definitively consolidated in
the 1890s, precisely the same decade that saw the birth of the
neuron doctrine, thanks largely to the work of Cajal.
In the words of the great science historian Pedro Laı́n Entralgo
(1908-2001), “the work of Cajal thus constitutes the definitive
vindication of cell theory, making Schleiden, Schwann, Virchow
and Cajal the four principal figures in the history of the theory”
[48]. A similar view was expressed by one of Cajal’s disciples,
the Uruguayan Clemente Estable (1894–1976), for whom his
Table 1
Historical milestones in the development of the neuron doctrine
1836
1838
1855
1862
1865
1871
1873
1886/1887
1888
1889
1891
1892
1897
1903
1904
1906
1913
1921
1933
1954/1955
First microscopic image of a nerve cell (Valentin)
First visualization of axons (Remak)
Consolidation of cell theory (Virchow)
First description of the neuromuscular junction (Kühne)
Description of types of processes of nerve cells (Deiters)
Definitive postulate of the reticular hypothesis in the
structure of the nervous system (Gerlach)
Introduction of silver-chromate technique as staining
procedure (Golgi)
First discrepancies with reticular theory (His, Nansen,
Forel)
Birth of the neuron doctrine: the nervous system is made up
of independent cells (Cajal)
Dissemination of neuron theory at the German Anatomical
Society congress (Cajal)
The term “neuron” is coined (Waldeyer). Defence of the
physiological significance of the “diffuse nerve network”
theory (Golgi)
Laws of dynamic polarization of neurons (Cajal)
Concept of synapse (Sherrington)
Introduction of silver nitrate as staining technique (Cajal)
Textura del Sistema Nervioso del Hombre y de los
Vertebrados: coming-of-age of the neuron theory (Cajal)
Nobel Prize: Cajal and Golgi. The integrative action of
nervous system: modern neurophysiology is born
(Sherrington)
Studies on degeneration and regeneration of the nervous
system (Cajal, Tello, Perroncito)
Nobel Prize: Sherrington
Neuronismo o reticularismo: Cajal’s scientific testament
Ultrastructural confirmation of synapses (Palay, Palade, De
Robertis, Bennett)
mentor was “the final illustrious architect of cell theory, the most
transcendent of all the biological theories” [29]. In the present
work we undertake a historical analysis of the circumstances
in which Cajal formulated the neuron doctrine, considering the
authors and works that influenced his postulate, the difficulties
involved in its dissemination within science, and how it finally
became established. Table 1 shows some of the major milestones
in the development of neuron theory.
2. The pre-Cajalian period: first approaches to
knowledge of the microscopic anatomy of the nervous
system
Although some authors, such as Antoni van Leeuwenhoek
(1632–1723), had already, in the early 18th century, carried out
some microscopic observations (Fig. 1A), the first descriptions
of nerve cells are attributed, almost simultaneously, in the period
1833–1837, to Christian Gottfried Ehrenberg (1795–1876), who
analyzed the nervous system of the leech [28], and to Purkinje, Professor at the Universities of Breslau and Prague, who
described, in the mammalian cerebellum, some large cells
(Fig. 1C) that are now known by his name [60]. It is precisely to one of Purkinje’s students, Valentin, that history
attributes the publication of the first microscopic image of a
nerve cell (Kugeln) [66], with its nucleus and its nucleolus,
in 1836 (Fig. 1B). Two years later, in 1838, Robert Remak
(1815–1865) quite likely saw axis-cylinders (“Remak bands”)
F. López-Muñoz et al. / Brain Research Bulletin 70 (2006) 391–405
393
Fig. 1. First illustrations of nerve cells: (A) drawing of longitudinal and transversal sections of a peripheral nerve, by Leeuwenhoek, and found in a letter sent to a
friend in 1719 [62]; (B) supposed first drawing of a nerve cell (cortex of the human cerebellum), with its nucleus and nucleolus, from an 1836 publication by Valentin
[66]; (C) cerebellum cells drawn by Purkinje for the Congress of Physicians and Scientists Conference in Prague, in 1837 [60]. These were published in 1838 and
the cells subsequently called Purkinje cells in his honour; (D) detail of a drawing of nerve cells from the spinal cord, showing axons and dendrites, by Deiters, and
published posthumously, in 1865 [25].
and non-myelinized axons (“Remak fibres”) [31,62]. In 1852,
Kölliker, showed, in the acoustic nerve of the ox, a “nerve cell
with the origin of a fibre” [45], in 1862, Wilhelm Friedrich
Kühne (1837–1900) demonstrated, in the frog, how the motor
nerves terminated in the muscle cells (calling this complex “endplate” [46]), and in 1865, in a posthumous publication, Otto
Friedrich Karl Deiters (1834–1863) described, in human samples, how, of all the processes of the nerve cells only one, a
longer one, remained without dividing, which he called the axiscylinder (Achsencylinderfortsatz), while the rest branched out
extensively (“protoplasmic extensions”) [26] (Fig. 1D).
2.1. The reticular theory of von Gerlach and Golgi
Among the first to describe the intimate structure of the
nervous elements was Virchow himself, who, anticipating the
reticular proposal, spoke of Grundmasse, or fundamental mass,
which would connect and unite all the nerve centres, and would
later be termed “neuroglia” [67]. Several authors used this proposal as the basis for further work during the 19th century,
including Edward Rindfleisch (1836–1908), who postulated his
theory of “diffuse nervous substance”, or Deiters, who defended
the hypothesis of an anastomosis of the fine nerve fibres and the
protoplasmic extensions [25].
However, it would be the German Josef von Gerlach
(1820–1896) who put forward the theory most prevalent until
the emergence of Cajal’s neuron doctrine in 1888: the “reticular
theory” that bears his name (a diffuse protoplasmic network of
the grey matter of the nerve centres). Gerlach proposed in 1871,
in a chapter of Salomon S. Stricker’s (1834–1898) work Handbuch der Lehre von den Geweben des Menschen und der Thiere,
entitled Von dem Rückenmarke, that the grey matter of the nerve
centres was made up of a dense mesh of thin filaments (Nervenfasernetz), shown by staining with the gold chloride method
designed by Gerlach himself [69]. According to his proposal,
the filaments emerging from this mesh would join up to form
nerve fibres that led to the white matter, reaching the spinal cord.
On this view, the origin of the nerve fibres could be a dual one: a
direct origin, from Deiters’ “axis-cylinders”, and an indirect one,
from the mesh of fine filaments or interstitial protoplasmic network. Gerlach concluded his theoretical postulate by affirming
that the central nerve endings would not end freely, but would
rather continue with protoplasmic processes.
This theory was universally accepted by the scientific
community, even by scientists of the calibre of Kölliker, later a
fervent advocate of neuron theory, who proposed that perhaps
only the dendrites were fused, or Camillo Golgi (1844–1926)
(Fig. 2A), who continued to defend the theory when the neuron
doctrine was widely accepted, albeit with some conceptual
modifications (a purely axonic network). Trained in Giulio Bizzozero’s (1846–1901) Laboratory of Experimental Pathology
in Turin, Golgi took up the chair of Histology at the University
of Pavia in 1880, devoting himself from that moment on to the
histological study of the nervous system, on which he would
become one of the major authorities of his time. Already in the
previous decade he had invented a histological method of stain-
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F. López-Muñoz et al. / Brain Research Bulletin 70 (2006) 391–405
Fig. 2. Camillo Golgi in his laboratory at the University of Pavia (A) towards the end of his scientific career, and diagram presented in his Nobel Lecture [38] to
justify the theory of the “rete nervosa diffusa” (B): “the fibres of this bundle, on approaching the reticular layer, which has been mentioned above, subdivide in a most
complicated way and interlace with branches of nerve processes of cells to form with them the reticular zone. One therefore gets the impression that the reticular
layer is interposed as a common region between the nerve processes of one side and the nerve fibres of the other”.
ing that revolutionized the study of the nervous system, and that
would subsequently make possible the work leading up to the
postulation of neuron theory [57]: the silver-chromate technique
[36]. The brain sections were fixed for several days in a mixture
of solutions of potassium bichromate and osmic acid, and subsequently stained through immersion in silver nitrate solution,
producing a silver-chromate precipitate that made it possible to
see nerve cells appearing as black (the famous reazione nera).
By means of this technique, Golgi classified the nerve cells of the
cerebral cortex in two categories: type I, characterized by having
a single process that would contribute to forming the nerve
fibres, to which Golgi eventually attributed a motor function;
type II, which would have a large number of shorter and more
arborescent processes, which Golgi considered to be of a sensory
nature.
Between 1883 and 1886, Golgi published a series of works,
using his staining system, in which he endorsed, at least partially, Gerlach’s theory. In these works it was proposed that the
protoplasmic processes end in a free manner, and that some
axis-cylinders of the nerve cells lose their individuality before
reaching the white matter, becoming fused in a terminal arborization, from which the sensory fibres emerged (rete nervosa diffusa) (Fig. 2B). Moreover, from the functional perspective, Golgi
ascribed to the prolongations of the nerve cells purely trophic
or nutritive functions, and never functions related to the phenomenon of conduction of the nervous impulse [37]. Thus, for
Golgi, the function of the brain is approached in an eminently
holistic way [30]. In relation to this, Cajal states in 1889: “. . .
still influenced by reticular theory (Golgi) admits the existence
of another interstitial nervous network, composed only of ramifications of axons, terminals or collaterals; a fibrilose mesh
that surrounds the nerve elements, without the participation of
protoplasmic processes” [8]. According to Cajal, Golgi’s work
was tremendously fruitful with regard to the development of
a method, but he failed completely in the morphological and
physiological interpretation of his findings. “It is a sad truth that
almost nobody can totally extricate themselves from the tradition and spirit of their times”, recalls Cajal in relation to this
matter [22,23].
In sum, the conceptions of Gerlach and Golgi, a kind of
continuation of Galen’s synekheia, constituted, at the historical
moment when they were proposed and defended, a clear exception to the prevailing and widely accepted cell theory, since nerve
cells would lack the independence with which the rest of the
organism’s cells were equipped, and the nervous system would
be reduced to a mere syncitium on a large scale.
2.2. First discrepancies in the reticular theory pave the way
for the neuron doctrine
Among the few researchers of the time who disagreed with
reticular theory—both that of Gerlach and that of Golgi—were
Wilhelm His (1831–1904), Fridtjof Nansen (1861–1930) and
Auguste Henri Forel (1848–1931) (Fig. 3). His (Fig. 3A), in
1886, using embryological material, confirmed that the single
processes of nerve cells established no relation of continuity
with any adjacent or distant element, but rather ended in a free
and independent manner, though he affirmed that “. . . other connections of the fibres are either solely indirect or have originated
in a secondary fashion” [40]. It was 3 years later when His proposed definitively (though he would never verify it in reality) the
independence of nerve cells in the adult nervous system: “. . . it
cannot be seen because, later, this form of growth (of the nerve
fibres in the embryo) would have to change. Moreover, we are
aware of a whole series of nerve endings that are free, such as
those of the cornea, of the skin, of the Pacini corpuscles, of the
motor plates, and so on. All of these endings are verified, either
by an independent fibre or by a non-anastomosized arborization.
F. López-Muñoz et al. / Brain Research Bulletin 70 (2006) 391–405
395
Fig. 3. First “anti-reticularists” and defenders of a hypothetical neuronism: Wilhelm His (A), Fridtjof Nansen (B) and Auguste Forel (C).
It would seem, therefore, really quite irrational to make such a
major distinction between central and peripheral nerve endings”
[41].
In 1887, the Norwegian researcher Fridtjof Nansen (Fig. 3B),
future explorer of the North Pole and Nobel Peace laureate in
1922, using Golgi’s staining technique in the study of the nervous
system of invertebrates, also appears to anticipate neuronal discontinuity, when, in his doctoral thesis, he remarks that “direct
anastomoses of protoplasmic processes (of ganglion cells) do
not exist. . . the endings of the nerve processes do not anastomose” [55].
For his part, Forel (Fig. 3C), an entomologist and psychiatrist
(and for 19 years the director of the prestigious Burghölzli Psychiatric Hospital in Zurich), also distances himself from reticular
theories in a work published in 1887: “I think all the systems of
fibres and the so-called networks of fibres of the nervous system are no more than nervous processes of cells that ramify to
a greater or lesser distance in the form of ramifications within
other ramifications, but not anastomosized” [32]. However, these
assertions, like those of His, could not be based on objective
confirmation through visualization of the mentioned independence, so that they remained in the realm of pure—even though
correct—speculation. Subsequently, in 1891, experimental studies, on specific lesions of the nervous system (retrograde degenerations induced by von Gudden’s method), lent support to
Forel’s proposals, when it became impossible to explain how
the area of the lesion was circumscribed to a specific region
if there was generalized reticular continuity of the nerve fibres
[33]. Likewise, this Swiss author was responsible for the socalled “Forel Contact Theory”, in which he rejects the hypothesis
of anastomosis of the nerve fibres with those of the muscles at
the level of the neuromuscular junction, and postulates that the
transmission of the information from the nervous system can
take place at this stage through pure contact [24].
With regard to the contributions of these two researchers,
Cajal recalls: “This conception of His and Forel, based on both
direct observations and on highly ingenious and credible gener-
alizations, represented a vigorous reaction against the reticular
theories of Gerlach and of Golgi, which at the time dominated
neurological science” [22]. The merit of His’s and Forel’s contributions is clear, and this has led some authors to consider them as
“cofounders of neuron theory” [1,42]. In our view, as we shall
remark below, the contribution of Cajal in this regard was so
overwhelmingly immense, compared to that of the other two,
that it would be a historical injustice to limit, albeit partially, his
paternity of that theory [51].
3. Neuron theory: Cajal’s great contribution to the
history of neuroscience
3.1. The formulation of the “neuron doctrine” (1888–1889)
In early January of 1884, Cajal (Fig. 4), having won the
corresponding competitive exam, moved to Valencia to take up
his post as Professor of Anatomy at the city’s University. His
time in Valencia (1884–1887) coincided with a period of great
development of medical scientific activity there [49]. It is while
Cajal was in Valencia that he first came into contact with the
intimate texture of the nervous system, “that masterwork of life”
[21]. His future success in this field would be due, as he himself
acknowledges, to the influence of Golgi’s silver-chromate
procedure, to which he was introduced by the Valencian
psychiatrist and neurologist Luis Simarro (1851–1921) on his
return from a trip to Italy in 1887, and to Cajal’s own initial plan
to apply it to lower animals or to the early stages of ontogenic
evolution [21]. The culmination of this plan would occur after
his move to Barcelona. Cajal, appointed Professor of Normal
Histology and Pathological Anatomy at the University of
Barcelona in November 1887, admits that his most important
scientific contributions were made during his first years in that
city, in which he spent 5 years (1887–1892). The year 1888
he considers as “my crowning year, my fortunate year” [21].
That year saw the discoveries that made it possible to postulate
neuron theory [51], it being demonstrated that the relationship
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F. López-Muñoz et al. / Brain Research Bulletin 70 (2006) 391–405
Fig. 4. Santiago Ramón y Cajal in his laboratory at the University of Valencia, in
1887, around the time when he became interested in the structure of the nervous
system.
between nerve cells was not one of continuity, but rather of
contiguity.
When Cajal began his neurohistological studies, in 1887,
Gerlach’s reticular theory prevailed among the scientific community, with very few dissenters, among them His and Forel.
However, the scientific grounds for divergence from Gerlach’s
theory (a work of embryology—1886—and one of experimental pathology—1887, published by His and Forel, respectively)
[40,32] lacked sufficient objective basis to bring down the ruling
doctrine, remaining at the level of mere hypotheses, albeit bold
and brilliant (“inspired inklings”, as Cajal put it [21]). In the
opinion of the Spaniard, these contributions “were hidebound
by two serious deficiencies: the clear, indisputable demonstration of the way the axons terminated in the centres, and the true
relationships between the nerve ramifications and the neurons”
[22,23]. Thus, the works of His and Forel do not seem to offer
the evidence to justify claims for their authorship of neuron theory, as apparently defended by some authors, such as Jacobson
[42].
Cajal, on the other hand, would be the first to provide indisputable morphological evidence in relation to the theory of
the free termination of neurons, thanks basically to the combination of two crucial developments: the double impregnation
technique, resulting from the improvement of Golgi’s silver-
chromate technique, and the ontogenic method, based on the use
of embryological material. Thus, and precisely in the first issue
of the Revista Trimestral de Histologı́a Normal y Patológica (1
May 1888), edited and financed by Cajal himself, he reports the
historic event responsible for providing this evidence [6], which
resulted from dyeing the axon of the small, star-shaped cells of
the molecular layer of the cerebellum of birds, whose collaterals ended up surrounding the Purkinje cell bodies, like baskets
or nests (see Fig. 5A). In this same work, Cajal also describes
the relationship between the mossy fibres and the digitiform
arborizations of the dendrites of the granular cells, commenting that the empty space between them is the mould for rosettes
(Fig. 5B). “Each nerve cell is a totally autonomous physiological
canton”, states Cajal in this paper. Likewise, it is in this work
that we find the first description of the dendritic spines, structures which today are of enormous biological significance, due
to their plastic capacity and their role as the location of excitatory synapses of the cerebral cortex. In the second issue of
the same journal (1 August 1888), reticular theory finally collapses under the weight of two new findings [7]: the parallel
fibres, originating in the axon of the cerebellar cortex grains,
terminated freely, and the climbing fibres, proceeding from the
ganglion cells of the protuberance, reached the Purkinje cells,
establishing a relationship of contiguity and not of continuity
(Cajal’s “law of pericellular contact”): “. . . having arrived at the
level of the first arms of the mentioned dendritic stem, they split
up into snaking parallel plexuses that ascend all along the protoplasmic branches, hugging their form, like ivy or lianas that
cling to tree trunks”. Cajal published 11 original works in this
journal (the third and final issue appeared in March, 1889), some
of vital importance for the future of the neurosciences, presenting his discoveries on the cerebellum of birds and mammals,
and the retina, spinal cord and optic lobe of birds. Fig. 6 shows a
diagram from one of these publications, comparing the Golgian
reticular theory with Cajal’s neuron postulate.
In October 1889 Cajal published, in the journal La Medicina Práctica, a review on the subjects of the cerebellum, the
optic nerve and the spinal cord of birds, in which he summarized all his findings of the previous 2 years. In this article he
states categorically that “. . . it is high time to release histology
from all physiological obligations and adopt the only opinion
that is in harmony with the facts, namely: that the nerve cells are
independent elements that are never anastomosed . . . and that
nervous propagation is verified by contacts at the level of certain
apparatus or cogs devices” [8]. The importance of these facts is
reflected in the words of the author himself, on describing “the
last ramifications of the central axis-cylinders, seen by no-one”
[21]. But he goes on to point out, in a clear allusion to the work
of His and Forel, that “vaguely accepting the fact of immediate transmission or interneuronal articulation, without indicating
precisely between which cellular appendices it is produced, is as
just as comfortably dangerous as the well-worn reticular theory”
[21].
Cajal’s findings in his studies on the grey matter of the
cerebellum, published in 1888 and 1889, are summarized in
four laws that revolutionized the conception of the time on
the histological structure of the nervous system, and laid the
F. López-Muñoz et al. / Brain Research Bulletin 70 (2006) 391–405
397
Fig. 5. Original drawings from the first work by Santiago Ramón y Cajal in which he postulates neuron independence, published in 1888 [6]: (A) “cross-section
of a lamina of cerebellum. A and B, and star-shaped cells from the molecular layer (cells in baskets), whose axon (a) generates terminal nests around the Purkinje
cells (C); b, axon of these corpuscles”; (B) “lengthwise section of a cerebellar circumvolution. A, molecular layer; B, layer of Purkinje cells; C, grain layer; D, white
matter; a, rosettes of the moss fibres; b, soma of the Purkinje cells; c, parallel filaments; d, grains with their ascendent axon; e, division of this axon (semi-schematic
figure)”.
bases for the new neuron doctrine. It is worth recalling them
[21]:
“1. The collateral and terminal ramifications of all axis-cylinders
end in the grey matter, not by means of a diffuse network, as
argued by Gerlach, Golgi and the majority of neurologists,
but by means of free arborizations, disposed in a variety of
forms (pericellular baskets or nests, climbing branches, etc.).
2. These ramifications apply themselves intimately to the body
and dendrites of the nerve cells, establishing a contact or
articulation between the receptor protoplasm and the final,
tiny axonic branches.
3. Given that the body and dendrites of neurons are applied
narrowly (to) the final tiny roots of the axis-cylinders, it is
necessary to accept that the body and the protoplasmic processes participate in the chain of conduction, that is, that they
receive and propagate the nervous impulse, in contrast to the
opinion of Golgi, for whom these cell segments would play
a merely nutritive role.
4. The exclusion of substantial continuity between cell and cell
leaves the way open for the opinion that the nervous impulse
is transmitted by contact, as in the articulations of electrical
conductors, or by a kind of induction, as with induction coils.”
directed by Karl Heinrich von Bardeleben (1849–1919). The first
paper describes the “tassel” or “brush” terminations in relation
to the Purkinje cells of birds (pigeons and hens) and mammals,
whilst the second confirms, contrary to the anastomotic proposals of Alexander S. Dogiel (1852–1922), discussed below,
the independence of the cells of bird retina. However, it was
by virtue of his participation in the German Anatomical Society’s annual congress, in Berlin in October, 1889, that Cajal
managed to achieve international recognition of his findings.
Among the large audience at the presentation by this “modest
Spanish anatomist”, was Kölliker (“the venerable patriarch of
3.2. The dissemination of neuron theory (1889–1891)
With the intention of his contributions reaching as wide an
international audience as possible, in 1889 Cajal managed to
secure publication (in French translation) of the results of his
works mentioned above confirming neuron independence in two
prestigious German journals, the Internationalen Monatschrift
für Anatomie und Physiologie [10], edited by Professor Wilhelm Krause (1833–1910), and the Anatomischer Anzeiger [9],
published by the German Anatomical Society and founded and
Fig. 6. Diagram drawn by Cajal, in one of his publications from 1889 on the
structure of the spinal cord, in which he sets out the differences between the
reticularist conception of Golgi (I) and his own postulates on neuron independence (II). Illustration of sensory-motor communications of the spinal cord.
Taken from Cajal [21].
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Fig. 7. The disputes between defenders and detractors of neuron theory marked the development of “neurology” in the 1890s. Among the most prestigious defenders
could be mentioned the German neurohistologists Rudolph Albert von Kölliker (A) and Heinrich Wilhelm von Waldeyer (B), whilst among the most fervent detractors
were Alexander S. Dogiel (C) and Albrecht Bethe (D).
German Histology”) (Fig. 7A) who was so enthused by Cajal’s
demonstrations that, in successive years, he helped the extensive dissemination of the Spaniard’s findings, thanks partly to
his being director of the prestigious journal Zeitschrift für wissenschafliche Zoologie.
It was precisely at this same conference that Cajal made
contact with numerous and prestigious scientists, in addition to
Kölliker, such as Heinrich Wilhelm von Waldeyer (1836–1921)
(Fig. 7B), Magnus Gustav Retzius (1842–1919), Gustav Albert
Schwalbe (1844–1916), or His himself, who was one of those
most interested in learning about the Spanish histologist’s con-
tributions. The diffusion of Cajal’s ideas in this scientific forum
appear to confirm that the results of his research were known to
Forel when, in 1891, year of the second edition of his book Der
Hypnotismus, he claimed priority in relation to the neuron doctrine [34]. Indeed, it was Cajal who was unaware of the previous
works of these German authors when, in early 1888, he wrote
his first neurohistological work (“only later—1890—did we read
the embryological papers by His, which pre-dated my research,
and only thanks to the kindness of the illustrious histologist of
Leipzig”, notes Cajal in his work Neuronismo o reticularismo
[22]).
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399
Fig. 8. Different neurohistological drawings by Cajal: (A) diagram of the optic lobe of birds a few days old, “where we see the connection between the arborizations
of the optical fibres and some elements with archiform axons”. This diagram, published in 1891, shows for the first time the famous arrows that “indicate the direction
of the nervous impulse”; (B) diagram from an 1896 publication on some elements of the retina of birds (a, centrifugue fibre; b, amacrine cell; c, horizontal axon)
“with the probable direction of currents”. Taken from Cajal [21].
The years 1890 and 1891 were “periods of hard work and
pleasing satisfactions” [21], in which Cajal devoted himself to
scientific work “with genuine fervour . . . work was giving me
pleasure. It was a delicious inebriation, an irresistible spell” [21].
Proof of such dedication were the 20 articles he published in
1890, six of them in French, in different European journals [52].
The result of his work in these years served to propagate internationally the concepts of neuron theory, as well as the virtues
of the Golgi method. Nevertheless, Cajal’s merit lies not only
in being the first scientist to provide morphological proof of
the independence of neurons, but also in the fact that, as López
Piñero points out, he drew up a cellular model of the texture
of the nervous system “that would serve as the foundation of
neurophysiology, neuropathology and the clinical approach to
nervous illness” [50].
3.3. The consolidation of neuron theory (1891–1906)
The results reported by Cajal generated throughout Europe
a veritable bibliographical avalanche, led by the most prestigious histologists of the period (His, Forel, Waldeyer, Kölliker,
Edinger, Lugaro, van Gehuchten, von Lenhossék, Retzius,
Azoulay, Dejerine, Duval, etc.), who, according to Cajal [22,23],
“confirmed our findings and illustrated and notably extended
His’s conception, enriching it with a wealth of invaluable evidence”. However, this enthusiasm aroused by the incipient neuron doctrine was not shared by all researchers of the time, with
a small group of reputable scientists continuing for some time
to defend certain neo-reticularist positions (Golgi, Dogiel, von
Apathy, Bethe, Held, etc.).
3.3.1. Defenders or “neuronists”
Among the most passionate defenders of the new theory was
Waldeyer (Fig. 7B), who coined, in 1891, the term “neuron”
to refer to nerve cells [70]. Nevertheless, in his publication
he did not present a single contribution of his own, confining
himself to discussing the contributions previously published by
different authors such as His, Cajal, Retzius or Kölliker [62,24].
In spite of this, acceptance of the term neuron was extremely
widespread, and it became common to find in the scientific literature of the time the erroneous expression “Waldeyer’s neuron
theory” (Neuronenlehre). Waldeyer stated in 1891: “The nervous system is constituted by numerous nervous units (neurons)
without anatomical or genetic connection. Each nervous unit
comprises three parts: the nerve cell, the nerve fibre and the terminal arborizations. The physiological process of conduction
can take place in the direction from cell to arborizations or in
the opposite direction. Motor conduction takes place only in the
direction of cells to terminal arborizations, whilst sensory conduction can take place in one direction or the other” [70]. The
basic morphological characteristic of neurons, their processes,
also acquired their definitive name at that time; His proposed the
term “dendrites” for Deiters’ “protoplasmic processes” in 1889
[41], while it was Kölliker who, in 1896, relabelled Deiters’
“axis cylinders” as “axons”.
In 1892, having taken up the chair of Histology at the Central
University of Madrid, Cajal set out another of his most important
contributions to neuron theory, the “laws of dynamic polarization” of neurons. Already, in 1889, studying the nerve cells of
the retina and olfactory bulb, he had observed that the dendrites were oriented towards the external environment, whilst
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the axons were oriented towards central nerve centres [8,9].
Following up these studies, he set out his physiological analysis of neuronal functioning in terms of circuits of transmission
of information between neurons, so that this information would
be transported from the dendrites to the soma, and from there
to the axon (“axipetic conduction”), which would transmit the
electrical information to the following neuron (“somatofugue
or dendrifugue conduction”) [11]. Moreover, from this point on
Cajal marked all his drawings and diagrams with his famous
arrows (Fig. 8), indicating the direction of nervous conduction,
and which subsequently made such an impression on Charles
Scott Sherrington (1858–1952) in the conceptualization of his
synaptic hypothesis. This Cajalian approach served as the initial
basis, as Jones [43] notes, for structuring our current knowledge
of the functioning of the nervous system.
During the 1890s, the results of numerous histological studies
began to underpin the new neuron doctrine. We might mention
some of them by way of example: the calyxes of Held in the
nucleus of the trapezoid body and the basket-like connections
of the ventral ganglion of the cochlear (1891, Held), the visual
arborizations in the external geniculate body (1891, Ramón),
the peripheral terminations of the cochlear nerve in the Organ
of Corti and of the vestibular nerve (1892, Retzius), the axodendritic connections by cog devices of Ammon’s horn (1893,
Cajal) and the axo-somatic connections in the form of “poor
nests” in the layer of internal grain of the retina (1895, Cajal),
the connections between diffuse nervous plexuses and the bodies
and stems of the brain pyramids (1894, Cajal), the bulbous terminations (Endfüssen) of the collaterals of the spinal cord in the
grey matter (1897, Held), the climbing fibres of the sympathetic
ganglions and the “bramble” terminations (1905, Cajal), etc.
The importance of Cajal’s contributions became evident in
the mid-1890s, when Mathias Duval (1844–1907), a Professor
at the Faculty of Medicine in Paris, in his Foreword to the French
edition of Cajal’s work Les nouvelles ideés sur la structure du
système nerveux chez lhomme et chez les vertebrales—published
in 1895 and considered as the precursor of his masterwork (Textura del Sistema Nervioso del Hombre y los Vertebrados), noted
that “while the name of Golgi will continue to be associated with
the preparatory procedure, that of Ramón y Cajal will endure as
a symbol of the beginning of a new era with more rational concepts about the constitution of the nervous system” [12].
In 1904, and coinciding with the publication of his most
important work, Textura del Sistema Nervioso del Hombre y
los Vertebrados, Cajal categorically defended neuron theory and
definitively consigned to history of all the reticularist conceptions: “The histological demonstration of the free termination
of axonic processes and of protoplasmic processes is now concluded. Its consequence, neuron theory, that is, the theory of the
unity and independence of the nerve cell, including all its appendices, is now established, on the basis of an excessive number
of positive facts, of certain observations” [16].
All the contributions mentioned gave rise, then, to the definitive profile of the so-called “neuron theory”, that is, the constitution of the nervous system by independent cells, which
represent genetic, anatomical, functional and trophic units. The
construction of this monumental work, like that of a cathe-
dral, saw the participation of brilliant and outstanding artisans,
who contributed the necessary skill for decorating the edifice,
but one cannot but attribute to Cajal the role of great architect
who designed such a masterpiece and endowed it with sufficient
consistency to rest permanently solid on its foundations. Cajal
himself recalls in relation to his activity in this period: “The
years 1905 and 1906 coincide with the zenith of my scientific
career. During those years fortune smiled on me to the extent
that I achieved the highest honours to which a man of science
can aspire and at that time, apart from less significant communications, I made decisive observations for the consolidation of
the neuron doctrine” [21].
3.3.2. Detractors or “neo-reticularists”
During the final decade of the 19th century and the first
decade of the 20th, some authors, such as Golgi himself and
a few of his disciples, continued to defend a watered-down
version of the reticular theory, labelled “diffuse” (rete nervosa
diffusa) (Fig. 2B) [37]. Even in 1906, in his Nobel Prize acceptance lecture to the Swedish Academy, Golgi persisted with
his neo-reticularist postulates, in opposition to neuron theory:
“At this point, while I shall come back to this question later,
I must declare that when the neuron theory made, by almost
unanimous approval, its triumphant entrance on the scientific
scene, I found myself unable to follow the current of opinion,
because I was confronted by one concrete anatomical fact; this
was the existence of the formation which I have called the diffuse nerve network. I attached much more importance to this
network, which I did not hesitate to call a nerve organ, because
the very manner in which it is composed clearly indicated its significance to me. In fact, although in various ways and to varying
extent, every nerve element of the central nervous system contributes towards its formation” [38]. Golgi concluded that the
nervous network concept he defended explained “the anatomical and functional continuity between nerve cells,” and went on,
“And this is why I have been unable to accept the idea of the
independence of each nerve cell” [38].
In addition to the members of the Italian school, the neoreticular postulates, vehemently refuted by Cajal, were taken up
by other prestigious authors of the time, such as Alexander S.
Dogiel (Fig. 7C), Stephan von Apathy (1863–1923), Albrecht
Bethe (1872–1954) (Fig. 7D) and Hans Held (1866–1942), in
the last two cases in an attempt to revive the classical catenary or
polygenic theories of Augustus Waller (1816–1870) and Louis
Antoine Ranvier (1835–1922) [2]. Dogiel was the most radical
defender of the reticularism proposed by Gerlach, thanks to his
studies of the retina, using methylene blue staining. With this
method, Dogiel described, in 1891, two types of network: one
formed by anastomosis of the protoplasmic processes of nerve
cells, and another formed by the union of the terminal ramifications of these same cells; Dogiel even went so far as to describe
dense inter-dendritic connections at the level of the retina [27].
These findings were later refuted by Cajal himself, using the
same staining method [15], though he had previously expressed
the opinion that some of Dogiel’s observations, such as the anastomosis between dendrites, were no more than superpositions of
the ends of the fibres from different bipolar cells [8].
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Through von Apathy’s use of gold chloride as a staining
method in his preparations of invertebrates [3], and Bethe’s use
of toluidin blue in vertebrates [5], these authors were able to
observe the “neurofilaments” within the interior of nerve cells,
and attributed to them the organization of the neuron networks in
the grey matter, in addition to the connections between the protoplasmic processes of these cells and the terminal arborizations of
the sensory nerves. It was Cajal who refuted these proposals with
the reduced silver nitrate method, which he himself conceived,
demonstrating that these neurofilaments formed authentic networks, but circumscribed to the cell soma, and never linking up
different neuron processes [14]. The errors in these approaches
were also demonstrated by other authors, such as Jean Nageotte
(1866–1948) [54].
Finally, Held defended the theory Cajal referred to as encrustation, consisting in the intimate union of the axon terminations
on the body or the dendrites of other neurons, even possibly penetrating them. Subsequently, Held even maintained that from the
terminal processes of the nerve fibres there emerged some fine
filaments that connected with the neurofilaments of the adjacent
neurons [39].
3.4. The definitive triumph of neuron theory: the award of
the Nobel Prize (1906)
Cajal’s enormous contributions to neuroscience were
acknowledged in 1906, by the Committee of the Karolinska
Institutet in Stockholm, with the award (together with Golgi)
of the Nobel Prize for Physiology and Medicine, “in recognition of meritorious work on the structure of the nervous system”.
On 12th December, 1906, just one day after Golgi’s lecture (The
neuron doctrine. Theory and facts [38]), in which the Italian continued to defend his reticularist position, Cajal gave his Nobel
Lecture, entitled The structure and connexion of neurons [18].
The content of Cajal’s lecture was an earnest defence of neuron
doctrine, backed up by a detailed presentation of his own and
others’ experimental development that led to the postulate. He
began in categorical fashion: “The nerve cells are morphological
entities, neurons, to use the word brought into use by the authority of Professor Waldeyer”. He went on by recalling that “. . .we
applied Golgi’s method, firstly in the cerebellum and then in
the spinal cord, the cerebrum, the olfactory bulb, the optic lobe,
the retina and so on of embryos and young animals, and our
observations revealed, in my opinion, the terminal arrangement
of the nerve fibres. . . the nerve elements possess reciprocal relationships in contiguity but not in continuity. It is confirmed also
that those more or less intimate contacts are always established,
not between the nerve arborizations alone, but between these
ramifications on the one hand, and the body and protoplasmic
processes on the other” [18].
The neo-reticularist hypotheses prevailing in a few European
scientific circles did not escape some well-worded criticism from
Cajal in his Nobel Lecture: “The irresistible suggestion of the
reticular conception, of which I have spoken to you (and the form
of which changes every five or six years), has led several physiologists and zoologists to object to the doctrine of the propagation
of nerve currents by contact or at a distance. All their allegations
401
are based on the findings by incomplete methods showing far
less than those which have served to build the imposing edifice
of the neuronal conception. If the said intercellular unions are
not the result of an illusion, they represent accidental dispositions, perhaps deformities whose value would be almost nil in
the face of the nearly infinite quantity of the perfectly observed
facts of free ending” [18].
During the following 10 years, diverse experimental procedures, as well as observations of a histopathological nature,
helped to consolidate the neuron doctrine. First of all, embryological studies confirmed that adult neurons, with their different
types of processes, proceeded from the development of “germinal cells”, which become “neuroblasts”, which in turn produce
“growth cones” that terminate freely [17]. The experimental
work of Jorge Francisco Tello (1880–1958) [64] also contributed
clarifying data, insofar as they confirmed that after the section of
a nerve there is axonal regeneration, through the “shoots from
the central stub”, which pass through the scar and reach the
peripheral terminations, both sensory and motor; these findings
were confirmed in the work of Edoardo Bellarmino Perroncito
(1847–1936) [58]. All the experiments on the degeneration and
regeneration of the nervous system carried out by Cajal’s group
were compiled and published in a large volume, in 1913 [20],
which even includes discussion of aspects of tremendous relevance today, such as neuron plasticity, a term Cajal took from
the Rumanian researcher Ioan Minea (1878–1941) [44]. Finally,
some histopathological studies also threw light on the theory of
neuron discontinuity. An example is constituted by the survival
of the baskets and the star-shaped cells of the molecular layer
of the cerebellum after the disappearance of the Purkinje cells
in general paralysis or after experimental section of the axons
of these cells in the grain layer [19]. For Cajal, this subsistence
of neurons that have lost their principal connections constitutes
“a vehement indication of the anatomical discontinuity of nerve
cells” [22,23].
However, and despite the international significance of the
Nobel Prize award, the development of the neuron doctrine did
not progress satisfactorily in the years after 1913, due in large
part to the disastrous effects of World War One (1914–1918).
Cajal himself notes that “the horrendous European war of 1914
dealt a bitter blow to my scientific activity”. As he says in his
autobiography “for six years I was cut off from foreign laboratories, and reduced to a monologue in which apathy and
discouragement were the basic norm” [21]. In fact, the majority
of the European scientists who initially supported and helped to
propagate Cajal’s theories died during the war (van Gehuchten,
Waldeyer, Ehrlich, Nissl, Krause, Obersteiner, Dejerine, Brodmann, Alzheimer, Edinger, Retzius, Holmgren, Rossi, etc.).
Fig. 9 shows a photograph of Cajal and one of his famous drawings from this period.
The magnitude of Cajal’s scientific production and the
nature of his contributions were highlighted by Ernesto Lugaro
(1870–1940), the eminent Italian neurologist and Professor of
Psychiatry at the University of Turin, in an obituary published in
1935 and reproduced by Tello: “This man [Cajal] has managed
. . . to construct a colossal work of science, as harmonious as a
work of art, solid enough to defy the centuries . . . Especially in
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Fig. 9. Cajal in his private laboratory in the early 1920s (A) and detail of a section of the visual cortex of the cat, showing “the grain layer, the layer of the large
solitary pyramids and the region of the large polymorphous corpuscules” (B). This corresponds to one of the works from Cajal’s final stage as a researcher, in 1921,
when neuron theory was absolutely recognized by the entire scientific community. Taken from Cajal [21].
the field of nervous morphology, it can be said that Cajal, all by
himself, has produced more than all the other neurologists put
together” [65].
4. The culmination of neuron theory: introduction to
the concept of synapse
The mid-19th century also saw the emergence of arguments
on the way neurons enter into contact with one another or with
other structures. Kühne, using histological methods of fixation
and staining, described, in the 1870s, the way the nerve endings
terminate on a formation of the muscular membrane (subsequently the motor end-plate), labelling the whole structure “the
neuromuscular junction”. For Kühne, this neuromuscular junction constituted, from the physiological perspective, a form of
transmission of the impulse of the motor nerves to the muscle
[47].
However, the limited power of optic microscopes at the time
made it impossible to see the contacts between neurons in the
central nervous system, so that these contacts were for some
time mere speculation, though quite accurate speculation, as in
the case of Cajal himself. In his first neurohistological works, in
addition to postulating the neuron doctrine, he actually anticipated the concept of the synapse: “. . . nervous propagation
is verified by contacts at the level of certain apparatus or dispositions of mechanisms, whose objective is to establish the
connection, multiplying considerably the surfaces of influence”
[8]. In this same line, during the 1890s, Held managed to verify, in embryological material, how certain axons terminated in a
broadening on other cells, and Cajal, in his Nobel Lecture, noted
that “a granular cement, or special conducting substance would
serve to keep the neuron surfaces very intimately in contact”
[18].
Cajal’s anatomical drawings, presented in the Croonian Lecture of 1894, above all the arrows he used for indicating the
direction of nervous conduction [13], influenced Sherrington
(Fig. 10) in producing his outline of the dynamic of nervous
transmission, which he developed through his studies on the
reflex arc and Wallerian degeneration [63]. For Sherrington, the
functioning of the nervous system was based on the existence
of pre-established anatomical circuits, which would constitute
the basis of reflex responses to sensory stimulation. This would
permit the execution of its principal function: the integration of
information.
It was when Sherrington was asked to write a chapter on
the nervous system for the Textbook of Physiology (1897), by
Michael Foster (1837–1907) [35], that the concept of synapse
emerged: “So far as our present knowledge goes, we are led to
think that the tip of a twig of the arborescence is not continuous
with but merely in contact with the substance of the dendrite
or cell-body on which it impinges. Such a special connection of
one nerve cell with another might be called a ‘synapse’. The lack
of continuity between the material of the arborization of one cell
and that of the dendrite (or body) of the other offers the opportunity for some change in the nature of the nervous impulse as
it passes from one cell to the other.” With this interpretation,
Sherrington provided a functional explanation of the structural
postulates of Cajal, combining the anatomical and physiological
concepts in a single unit. This was the basis of what would subsequently become known as the neurotransmission mechanism.
From a semantic point of view, the term “synapse” was not the
one initially chosen by Sherrington, who proposed for denoting
the internal contact zone the term syndesm. But Foster, at the
request of Sherrington himself, consulted a doctoral student at
Trinity College named Verrall, who suggested “synapse”, which
literally means “joining together” [4,53].
F. López-Muñoz et al. / Brain Research Bulletin 70 (2006) 391–405
403
Fig. 10. Sir Charles Scott Sherrington, introducer of the concept of synapse, and cover of his work The Integrative Action of the Nervous System (1906), in a later
edition, by Cambridge University Press, 1947.
The physiological work of Sherrington, compiled in one
of the classic texts of modern neurophysiology, The Integrative Action of the Nervous System (Fig. 10), first published in
1906—the same year as Cajal’s Nobel Prize award, provides the
first data on the role in the integration or modulation of nervous
transmission of certain higher structures, such as the brainstem
and the cerebellum [63]. Likewise, the first data on the existence
of excitatory and inhibitory synapses are due to this “father of
neurophysiology”, whose work on the functioning of the nervous
system won him the Nobel Prize for Physiology and Medicine
in 1921.
Finally, the definitive confirmation of neuron theory and of
the concept of synapse would come in the mid-1950s, with
the development of electron microscopy techniques. In 1954,
Fig. 11. Sanford L. Palay (A); George E. Palade (B) (Nobel Laureate for Physiology and Medicine in 1974); Eduardo De Robertis (C) definitively confirmed the
existence of synapses in the mid-1950s, through the use of electron microscopy. The cycle of neuron theory was thus finally closed.
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Fig. 12. The definitive consolidation of neuron theory came with the electron
microscope. Electron micrography of an axo-somatic synapse of the anterior
horn of the (human) spinal cord, and detail of the synapse at greater magnification. We can see the axon: (a) which constitutes the presynaptic element, with the
typical synaptic vesicles, which make contact with the presynaptic membrane,
as well as the synaptic space (arrow) and (b) the membrane of post-synaptic element. The existence of the synaptic space or cleft clearly demonstrates neuron
independence.
Sanford Louis Palay (1918–2002) (Fig. 11A) and George Emil
Palade (1912) (Fig. 11B) (New York), and in 1955, Eduardo
De Robertis (1913–1988) (Fig. 11C) (Buenos Aires) and Henry
Stanley Bennett (1910–1983) (Seattle), demonstrated ultrastructurally the individuality of the neurons and synaptic discontinuity, reaffirming Cajal’s proposals of half a century earlier
made on the basis of conventional optical microscopy techniques (“physical contact—of the nerve endings—may attain
great intimacy, but in any case there always exists between
the two surfaces of the synapse a separating frontier”, noted
Cajal in 1933). Both groups, almost simultaneously, described
at the synaptic level a local swelling of the neuron membranes, a set of small vesicles close to the broadening of the
presynaptic element 20–60 nm in diameter and, in addition,
an extracellular space between the two swollen membranes of
some 20 nm [56,26]. At last, not only had the mechanisms of
integration and transmission of signals in the nervous system
been revealed, but there was also now physical and tangible
evidence of the key structure of these processes: the synapse
(Fig. 12).
5. Epilogue: the historical relevance of neuron theory
Cajal’s great achievements in defence of neuron theory
throughout 45 years of work are compiled in the so-called “scientific testament of Cajal”, in a work first published in 1933
in the journal Archivos de Neurobiologı́a, entitled Neuronismo
o reticularismo. Las pruebas objetivas de la unidad anatómica
de las células nerviosas [22], and also published, posthumously
(1935), in German, with the title Neuronenlehre, in the Handbuch der Neurologie, by Oswald Bumke (1877–1950) and Otfrid
Foerster (1873–1941), being republished in 1952 by the Spanish
Higher Council for Scientific Research [23]. This work sets out
the six fundamental principles of neuron independence, according to Cajal: morphological unity, genetic unity, functional unity,
regenerative unity, unity of pathological regeneration and unity
of conduction.
Although some of the concepts of the so-called neuron doctrine proposed by Cajal, such as trophic or metabolic unity,
cannot be conceived today just as they were formulated at the
time, neuron theory constitutes one of the scientific doctrines
that has endured longest, retaining its integrity, in the history of
biology, together with Charles R. Darwin’s (1809–1882) theory
of evolution. Neuron theory, one of the principal scientific conquests of the 19th century [59], has not only stood the test of time
(more than a century) with scarcely any modifications, but has
also been confirmed and vindicated with the development of new
technologies, such as electron microscopy, neurophysiology or
immunohistochemistry. And moreover, today no neuroscientific
discipline could be understood without recourse to the concept
of neuronal individuality and nervous transmission at a synaptic
level, as basic units of the nervous system.
By way of conclusion, let us recall the words of Cajal himself,
a century ago in his Nobel Lecture: “While awaiting the work of
the future, let us be calm and confident in the future of our work.
If future science reserves big surprises and wonderful conquests
for us, it must be supposed that she will complete and develop
our knowledge indefinitely, while still starting from the present
facts” [18].
References
[1] K. Akert, A. Forel, Cofounder of the neuron doctrine (1848–1931), Brain
Pathol. 3 (1993) 425–430.
[2] A. Albarracı́n, La teorı́a celular. Historia de un paradigma, Alianza Editorial, Madrid, 1983.
[3] S.V. Apathy, Das Leitende Element der Nervensystems und seine
topographischen Beziehungen zu den Zellen, Mitth. Zool. Stat. Neapel.
12 (1897) 4.
[4] M.R. Bennett, The early history of the synapse: from Plato to Sherrington,
Brain Res. Bull. 50 (1999) 95–118.
[5] A. Bethe, Allgemeine Anatomie und Physiologie des Nervensystems,
Thieme, Leipzig, 1903.
[6] S.R. Cajal, Estructura de los centros nerviosos de las aves, Rev. Trim.
Histol. Norm. Patol. 1 (1888) 1–10.
[7] S.R. Cajal, Sobre las fibras nerviosas de la capa molecular del cerebelo,
Rev. Trim. Histol. Norm. Patol. 1 (1888) 33–49.
[8] S.R. Cajal, Conexión general de los elementos nerviosos, Med. Práct. 2
(1889) 341–346.
[9] S.R. Cajal, Sur le morphologie et les conexions des éléments de la retine
des oiseaux, Anat. Anzeiger. 4 (1889) 111–121.
[10] S.R. Cajal, Sur l’origene et la direction des prolongations nerveuses de la
couche moléculaire du cervelet, Int. Monatschr. Anat. Physiol. 6 (1889).
[11] S.R. Cajal, El nuevo concepto de la histologı́a de los centros nerviosos,
Rev. Cienc. Med. 18 (1892) 457–476.
[12] S.R. Cajal, Les Nouvelles ideés sur la structure du système nerveux chez
l‘Homme et chez les vertebrates, Reinwald, Paris, 1894.
[13] S.R. Cajal, The Croonian lecture: La fine structure des centres nerveux,
Proc. R. Soc. Lond. 55 (1894) 444–468.
F. López-Muñoz et al. / Brain Research Bulletin 70 (2006) 391–405
[14] S.R. Cajal, Consideraciones crı́ticas sobre la teorı́a de Bethe, acerca de la
estructura y conexiones de las células nerviosas, Trab. Lab. Invest. Biol. 2
(1903).
[15] S.R. Cajal, Das Neurofibrillennetz der Retina, Int. Monatschr. Anat. Physiol. 21 (1904) 4–8.
[16] S.R. Cajal, Textura del Sistema Nervioso del Hombre y de los Vertebrados,
Moya, Madrid, 1904.
[17] S.R. Cajal, Génesis de las fibras nerviosas del embrión y observaciones
contrarias a la teorı́a catenaria, Trab. Lab. Invest. Biol. 4 (1906).
[18] S.R. Cajal, Nobel Lecture. The Structure and Connexions of Neurons.
December 12, 1906. URL: http://nobelprize.org/medicine/laureates/1906/
cajal-lecture.html.
[19] S.R. Cajal, Los fenómenos precoces de la regeneración neuronal en el
cerebelo, Trab. Lab. Invest. Biol. 9 (1911) 1–38.
[20] S.R. Cajal, Estudios sobre la degeneración y regeneración del sistema
nervioso, vol. 1. Degeneración y regeneración de los nervios, Moya,
Madrid, 1913.
[21] S.R. Cajal, Recuerdos de mi vida, in: Historia de mi labor cientı́fica, 3rd
ed., Imprenta de Juan Pueyo, Madrid, 1923.
[22] S.R. Cajal, ¿Neuronismo o reticularismo? Las pruebas objetivas de unidad
anatómica de las células nerviosas, Arch. Neurobiol. 13 (1933) 1–
144.
[23] S.R. Cajal, ¿Neuronismo o reticularismo? Instituto Ramón y Cajal (CSIC),
Madrid, 1952.
[24] E. Clarke, C.D. O’Malley, The Human Brain and Spinal Cord. A Historical
Study by Writting from Antiquity to the Twentieth Century, University of
California Press, Berkeley, 1968.
[25] O. Deiters, Untersuchungen über Gehirn und Rückenmarke des Menschen
und der Säugethiere, Braunschweig, 1865.
[26] E.D.P. De Robertis, H.S. Bennett, Some features of the submicroscopic
morphology of the synapses in frog and earthworm J. Biophys. Biochem.
Cytol. 1 (1955) 47–55.
[27] A.S. Dogiel, Uber die nervösen Elemente in der Retina des Menschen,
Arch. Mikrosk. Anat. 38 (1891).
[28] C.G. Ehrenberg, Beobachtungeiner auffallenden bisher unerkannten Strukfurdes Seelenorgans bei Menschen und Thieren, Königlichen Akademie der
Wissenschchaft, Berlin, 1836.
[29] C. Estable, Don Santiago Ramón y Cajal, Bol. Centro Coop. Cient.
UNESCO 3 (1952) 3–22.
[30] S. Finger, Origins of Neuroscience. A History of Explorations into Brain
Function, Oxford University Press, New York, 1994.
[31] H. Fodstad, The neuron theory, Stereotact. Funct. Neurosurg. 77 (2001)
20–24.
[32] A.H. Forel, Einige hirnanatomische Betrachtungen und Ergebnisse, Arch.
Psychiatr. Nervenkrank. 18 (1887) 162–198.
[33] A.H. Forel, Ueber das Verhältniss der experimentellen Atrophie und Degenerationsmethode zur Anatomie und Histologie des Centralnervensystems,
Müller, Zurich, 1891.
[34] A.H. Forel, Der Hypnotismus; seine psycho-physiologische, medicinische,
Strafechtliche Bedeutung, 2nd ed., Enke, Stuttgart, 1891.
[35] M. Foster, A Textbook of Phisiology. Part three: The Central Nervous System, 7th ed., Macmillan and Co. Ltd., London, 1897.
[36] C. Golgi, Sulla struttura della sostanza grigia del cervello, Gaz. Med. Ital.
6 (1873) 244–246.
[37] C. Golgi, La rete nervosa diffusa degli organi centrali del sistema nervoso:
suo significata fisiologico, Arch. Ital. Biol. 15 (1891) 434–463.
[38] C. Golgi, Nobel Lecture. The Neuron Doctrine—Theory and Facts. December 11, 1906. URL: http://nobelprize.org/medicine/laureates/1906/golgilecture.html.
[39] H. Held, Zur weiteren Kenntniss der Nervenfüssen, Abh. Math. Phys.
Klasse Köning Sächs Ges. Wiss. 71 (1904).
[40] W. His, Zur Geschichte des menschlichen Rückenmarks und der Nervenwurzel, Abh. Math. Phys. Klasse Köning Sächs Ges. Wiss. 13 (1886)
209–477.
405
[41] W. His, Die Neuroblasten und deren Entstehung im embryonalen Mark,
Abh. Math. Phys. Klasse Köning Sächs Ges. Wiss. 15 (1889) 311–372.
[42] M. Jacobson, Foundations in Neuroscience, Plenum Press, New York, 1993.
[43] E.G. Jones, Golgi, Cajal and the neuron doctrine, J. Hist. Neurosci. 8 (1999)
170–178.
[44] E.G. Jones, Plasticity and neuroplasticity, J. Hist. Neurosci. 9 (2000) 37–39.
[45] A. Kölliker, Handbuch der Gewebelehre des Menschen für Aertze und
Studirende, Leipzig, 1852.
[46] W. Kühne, Über dieperipherischen Endorgane der motorischen Nerven,
Engelmann, Leipzig, 1862.
[47] W. Kühne, Nerv- und Muskelfaser, 1869, in: Contenido en: Stricker S.
Handbuch der Lehre von den Geweben, Engelmann, Leipzig, 1871.
[48] P. Laı́n Entralgo, Cajal porsus cuatro costados, in Santiago Ramón y Cajal,
in: Expedientes Administrativos de Grandes Españoles, Servicio de Publicaciones del Ministerio de Educación y Ciencia, Madrid, 1978.
[49] J.M. López Piñero, M.J. Baguena, J.L. Barona, M.L. López Terrada, J.A.
Mico, Cajal y la medicina valenciana de su tiempo, Secretariado de Publicaciones de la Universidad de Valencia, Valencia, 1983.
[50] J.M. López Piñero, Cajal, Salvat Editores S.A., Barcelona, 1988.
[51] F. López-Muñoz, J.L. Calvo, J. Boya, Algunas consideraciones sobre Cajal
y la paternidad de la teorı́a neuronal, Psiquiatr. Biol. 4 (1997) 33–34.
[52] F. López-Muñoz, A.L. Carbonell, J. Boya, Aproximación a la producción
cientı́fica de Cajal desde una perspectiva bibliométrica, Arch. Neurobiol.
61 (1998) 41–66.
[53] L.H. Marshall, H.W. Magoun, Discoveries in the Human Brain. Neuroscience Prehistory, Brain Structure and Function, Humana Press, Totowa,
1998.
[54] J. Nageotte, La structure fine du système nerveux, Maloine, Paris, 1905.
[55] F. Nansen, The Structure and Combination of the Histological Elements
of the Central Nervous System, Bergen Mus Aarsberetning, Bergen, 1887,
pp. 29–214.
[56] G.E. Palade, S.L. Palay, Electron microscope observations of interneuronal
and neuromuscular synapses, Anat. Rec. 118 (1954) 335–336.
[57] E. Pannese, The Golgi stain: invention, diffusion and impact on neurosciences, J. Hist. Neurosci. 8 (1999) 132–140.
[58] A. Perroncito, Die Regeneration der Nerven, Beitr. Path. Anat. Allg. Path.
42 (1907) 355–446.
[59] A. Probst, D. Langui, La théorie du neurone: une des principales conquêtes
scientifiques du XIXe siècle, Schweiz. Rundschau Med. (Praxis) 83 (1994)
462–469.
[60] J.E. Purkinje, Neueste, Untersuchungen aus der Nerven-und Hirnanatomie,
Opera Omnia 3 (1837) 45–49.
[61] T. Schwann, Mikroscopische Untersuchungen über dier Uebereinstimmung
in der Struktur und dem Wachstum der Thiere und Pflanzen, Sandersche
Buchhandlung, Berlin, 1839.
[62] G.M. Shepherd, Foundations of the Neuron Doctrine, Oxford University
Press, New York, 1991.
[63] C.S. Sherrington, The Integrative Action of the Nervous System, Yale University Press, New Haven, 1906.
[64] J.F. Tello, Dégéneration et régéneration du plaques motrices après la section
des nerfs, Trav. Lab. Rech. Biol. 5 (1907) 117–149.
[65] J.F. Tello, Cajal y su labor histológica, Universidad Central (Cátedra
Valdecilla), Madrid, 1935.
[66] G.G. Valentin, Über den Verlauf und die letzten Enden der Nerven, Nova
Acta Phys-Med. Acad. Leopoldina (Breslau) 18 (1836) 51–240.
[67] R. Virchow, Ueber eine im Gehirn und Rückenmark des Menschen aufgefundene Substanz mit der chemsichen Reaction der Cellulose, Virchow’s
Arch. 6 (1854) 135–138.
[68] R. Virchow, Cellular-pathologie, Virchow’s Arch. 8 (1855) 3–39.
[69] J. von Gerlach, Von den Rückenmarke, in: S. Stricker (Ed.), Handbuch der
Lehre von den Geweben, Engelmann, Leipzig, 1871, pp. 665–693.
[70] H.W.G. Waldeyer, Ueber einige neuere Forschungen im Gebiete der
Anatomie des Centralnervensystems, Dtsc. Med. Wschr. 17 (1891),
1213–1218, 1244–1246, 1287–1289, 1331–1332, 1352–1356.