Cajal: the story of a will
The zoologist Theodor Schwann (1810-1882) and the botanist Matthias Jacob Schleiden (1804-1881) proposed the cell theory at the end of the 1830s, according to which the cell is the anatomical and functional unit of all living organisms. At the end of the 19th century, only the nervous system challenged this explanation. The mainstream theory until then was the reticular theory supported by Joseph von Gerlach (1820-1896) and Otto Deiters (1834-1863) and, particularly, by the Italian Camillo Golgi (1844-1926). According to this theory the nervous system was a single continuous network in which the axons of some cells were intertwined with the dendrites of others. This was in opposition to the individuality described by the cell theory.
Cajal's dedication to the histology of the nervous system began in 1887 while he was a Professor of Anatomy in Valencia, Spain. Then he learned from the hand of the Spanish neuropsychiatrist Luis Simarro a staining method referred to as the black reaction (la reazione nera) that had been originally described in 1873 by Golgi.
The Golgi method allowed the selective, staining, although randomly, of whole neurons using silver chromate in order to facilitate their microscopic examination. From that moment on, Cajal's attention focused almost exclusively on the microscopic anatomy of the nervous system. Cajal modified the Golgi method by applying a double impregnation technique, in which the nervous tissue was soaked twice in silver chromate for shorter periods of time, instead of a single prolonged exposure as Golgi suggested. He applied the modified technique to the histological study of embryos because of the advantage of easier staining when axons lack myelin sheaths. This technique, together with other staining procedures, such as the reduced silver technique, developed by Cajal and his disciples were part of the legacy that allowed us to deepen our knowledge of the histology of the nervous system.
The peak year of Cajal's work came in 1888, while he was a Professor of Histology at the University of Barcelona. During this year he described the neuron as the basic anatomical and functional unit of the nervous system, a concept that was later referred to as the neuron doctrine. Initially outlined by Wilhem His (1831-1904) and Auguste-Henri Forel (1848-1931), it not only helped to reinforce the cell theory, but it became an absolute milestone in the history of neuroscience. In the following years, Cajal gave new evidence of this functional individuality when he established that the transmission of the nerve impulse from one neuron to another took place by contact and not by contiguity.
By 1889, he formulated the law of dynamic polarization, which stated that nerve impulses are transmitted from the dendrites to the cell body and from there to the axon, to later reach the dendrites of the neighboring neurons. Subsequently, he proposed the neurotrophic theory on embryonic development of nerve fibers.
Cajal studied with special dedication the microscopic anatomy of the cerebellum, retina and hippocampus. Other essential contributions of Cajal were the description of dendritic spines and the discovery of the growth cone. In the same way, his work on the degeneration and regeneration of the nervous system is fundamental.
Many of these findings were collected in his magnus opus “Textura del Sistema Nervioso del Hombre y de los Vertebrados”, published in three volumes in 1897, 1898 and 1904 respectively. The French translation of the original Spanish version contained additional information and was available in 1911 with the title “Histologie du système nerveux de l'homme et des vertébrés”, which is considered to be the most complete compilation of the microscopic anatomy of the nervous system and its illustrations continue to be reproduced in neuroscience textbooks.
Cajal's contributions to the knowledge of the vestibular system were numerous and, even today, they are of great relevance. His description of the layout of the nervous system was enhanced by detailed and precise drawings that helped expand both anatomical and physiological knowledge.
In these great encyclopedic tomes Cajal dived into the functional meaning of structures and relationships between form and function, dedicating an entire chapter to the eighth cranial nerve. He described that the nerve split into two divisions: a vestibular branch and a cochlear branch, and he was able to unravel the path that both nerves followed along the brainstem, where he found the connection to the lateral vestibular nucleus, also referred to as Deiter’s nucleus.
In this same manuscript, he described for the first time the “interstitial nucleus of the vestibular nerve”, the “cerebellum-acoustic".
Inucleus and the direct vestibulocerebellar fibers as well as the interstitial, mesencephalic, commissural and paravestibular nuclei. t is the interstitial nucleus of Cajal with its output to the oculomotor nucleus that is linked to the regulation of rotational nystagmus.
Cajal’s knowledge was so vast, and his research was so important, that researchers from all over the world contacted him asking for help, be it in the form of materials or information. Such was the case of Róbert Bárány (1876-1936), the Austro-Hungarian otologist who received the Nobel Prize in Physiology or Medicine in 1914 for his work on the vestibular system. In 1913, Bárány contacted Cajal asking for slides to add to his presentation on the anatomy and physiology of the semicircular canals for the 85th meeting of German scientific researchers. Bárány told Cajal that he strongly believed the latter’s findings on the trajectory of the vestibular nerve in the brainstem, to be accurate, and he held doubt on the findings reported by other researchers. Bárány sought such a degree of precision that he specifically requested a group of histological slides that showed one of Cajal´s greatest findings at the time: the division of the vestibular nerve within the brainstem, into its ascending and descending portions.
It is impossible, in a small review such as this, to appraise each and every one of Cajal´s contributions to the study of the vestibular system, but it is safe to say that his work remains one of the greatest contributions to the study of neurosciences.
ANGEL BATUECAS-CALETRIO MD, PhD
JUAN MANUEL ESPINOSA-SÁNCHEZ MD
History Committee for the XXXI Barany Society meeting