Golgi, Cajal and the Neuron Doctrine

Geometric constraints on human brain function

The anatomy of the brain necessarily constrains its function, but precisely how remains unclear. The classical and dominant paradigm in neuroscience is that neuronal dynamics are driven by interactions between discrete, functionally specialized cell populations connected by a complex array of axonal fibres1-3. However, predictions from neural field theory, an established mathematical framework for modelling large-scale brain activity4-6, suggest that the geometry of the brain may represent a more fundamental constraint on dynamics than complex interregional connectivity7,8. Here, we confirm these theoretical predictions by analysing human magnetic resonance imaging data acquired under spontaneous and diverse task-evoked conditions. Specifically, we show that cortical and subcortical activity can be parsimoniously understood as resulting from excitations of fundamental, resonant modes of the brain's geometry (that is, its shape) rather than from modes of complex interregional connectivity, as classically assumed. We then use these geometric modes to show that task-evoked activations across over 10,000 brain maps are not confined to focal areas, as widely believed, but instead excite brain-wide modes with wavelengths spanning over 60 mm. Finally, we confirm predictions that the close link between geometry and function is explained by a dominant role for wave-like activity, showing that wave dynamics can reproduce numerous canonical spatiotemporal properties of spontaneous and evoked recordings. Our findings challenge prevailing views and identify a previously underappreciated role of geometry in shaping function, as predicted by a unifying and physically principled model of brain-wide dynamics.

Lampreys and spinal cord regeneration: "a very special claim on the interest of zoologists," 1830s-present

Employing history of science methods, including analyses of the scientific literature, archival documents, and interviews with scientists, this paper presents a history of lampreys in neurobiology from the 1830s to the present. We emphasize the lamprey's roles in helping to elucidate spinal cord regeneration mechanisms. Two attributes have long perpetuated studies of lampreys in neurobiology. First, they possess large neurons, including multiple classes of stereotypically located, 'identified' giant neurons in the brain, which project their large axons into the spinal cord. These giant neurons and their axonal fibers have facilitated electrophysiological recordings and imaging across biological scales, ranging from molecular to circuit-level analyses of nervous system structures and functions and including their roles in behavioral output. Second, lampreys have long been considered amongst the most basal extant vertebrates on the planet, so they have facilitated comparative studies pointing to conserved and derived characteristics of vertebrate nervous systems. These features attracted neurologists and zoologists to studies of lampreys between the 1830s and 1930s. But, the same two attributes also facilitated the rise of the lamprey in neural regeneration research after 1959, when biologists first wrote about the spontaneous, robust regeneration of some identified CNS axons in larvae after spinal cord injuries, coupled with recovery of normal swimming. Not only did large neurons promote fresh insights in the field, enabling studies incorporating multiple scales with existing and new technologies. But investigators also were able to attach a broad scope of relevance to their studies, interpreting them as suggesting conserved features of successful, and sometimes even unsuccessful, CNS regeneration. Lamprey research demonstrated that functional recovery takes place without the reformation of the original neuronal connections, for instance, by way of imperfect axonal regrowth and compensatory plasticity. Moreover, research performed in the lamprey model revealed that factors intrinsic to neurons are integral in promoting or hindering regeneration. As this work has helped illuminate why basal vertebrates accomplish CNS regeneration so well, whereas mammals do it so poorly, this history presents a case study in how biological and medical value have been, and could continue to be, gleaned from a non-traditional model organism for which molecular tools have been developed only relatively recently.

Tracking Development of Connectivity in the Human Brain: Axons and Dendrites

The neuron doctrine laid the foundation for our current thinking about the structural and functional organization of the human brain. With the basic units of the nervous system-neurons-being physically separate, their connectivity relies on the conduction of action potentials in axons and their transmission across the synaptic cleft to the dendrites of other neurons. This study reviews available ex vivo data about the cellular composition of the human cerebral cortex, focusing on axons and dendrites, to conceptualize biological sources of signals detected in vivo with magnetic resonance imaging. To bridge the gap between ex vivo and in vivo observations, I then explain the basic principles of virtual histology, an approach that integrates spatially cell- or process-specific transcriptomic data with magnetic resonance signals to facilitate their neurobiological interpretation. Finally, I provide an overview of the initial insights gained in this manner in studies of brain development and maturation, in both health and disease.

An Important Step in Neuroscience: Camillo Golgi and His Discoveries

Camillo Golgi (Figure 1) is one of the most prestigious personalities of modern medicine [...].

"You've Got to Work on This Axon": J. Z. Young and Squid Giant Axon Preparations in 20th -Century Neurobiology

Employing and extending Hans-Jörg Rheinberger's analytical concept of epistemic things, this essay proposes one reason why squid giant axons, unusually large invertebrate nerve fibers, had such great impacts on twentieth-century neurobiology. The 1930s characterizations of these axons by John Zachary Young reshaped prevailing assumptions about nerve cells as epistemic things, I argue. Specifically, Young's preparations of these axons, which consisted of fibers attached to laboratory technologies, highlighted similarities between giant axons and more familiar ones via lines of comparative study common to aquatic biology. Young's work convinced other biologists that the squid giant fibers were, in fact, axons, despite their unusual fused (syncytial) structures, thereby promoting further studies, such as intracellular measurements, made possible by the fiber's size. Tracing direct relations between preparations of squid axons and broader interpretations of neurons as epistemic things, this paper renders an actors' category, "preparations," into an analytical one. In turn, it offers glimpses into how aquatic organisms shaped twentieth-century neurobiology and how local experiments can drive broader, disciplinary changes.

Two classes of functional connectivity in dynamical processes in networks

The relationship between network structure and dynamics is one of the most extensively investigated problems in the theory of complex systems of recent years. Understanding this relationship is of relevance to a range of disciplines-from neuroscience to geomorphology. A major strategy of investigating this relationship is the quantitative comparison of a representation of network architecture (structural connectivity, SC) with a (network) representation of the dynamics (functional connectivity, FC). Here, we show that one can distinguish two classes of functional connectivity-one based on simultaneous activity (co-activity) of nodes, the other based on sequential activity of nodes. We delineate these two classes in different categories of dynamical processes-excitations, regular and chaotic oscillators-and provide examples for SC/FC correlations of both classes in each of these models. We expand the theoretical view of the SC/FC relationships, with conceptual instances of the SC and the two classes of FC for various application scenarios in geomorphology, ecology, systems biology, neuroscience and socio-ecological systems. Seeing the organisation of dynamical processes in a network either as governed by co-activity or by sequential activity allows us to bring some order in the myriad of observations relating structure and function of complex networks.

Camillo Golgi (1843-1926): scientist extraordinaire and pioneer figure of modern neurology

Camillo Golgi was an extraordinary scientist whose contributions in the domain of neuroanatomy proved to be critical for emergence of neuroscience as a sovereign scientific discipline. Golgi's invention of the Black Reaction (La reazione nera) was a watershed event as it allowed remarkable visualization of the organizational pattern of elements of nervous system among complex puzzle of close knit interconnections. Till this time thin filamentary extensions of neural cells (axon and dendrites) could not be visualized with available staining techniques because of their slender and transparent nature. However invention of Black Reaction and its subsequent application demystified the basic architecture of brain tissue which was now visible to the scholars in all its complexity in microscopic studies. Golgi is also credited with the discovery of two types of sensory receptors in muscle tendons: Golgi tendon organ and Golgi-Mazzoni corpuscles. Golgi was the first to be successful in staining myelin component of axon, which he used to discover the myelin annular apparatus. He identified the complete life cycle of Plasmodium (malarial parasite) in human erythrocytes. His research on histological details of human kidney highlighted the existence of juxtaglomerular apparatus. Later on Spanish scientist Santiago Ramón y Cajal, based on the use of Golgi's Staining (Black Reaction) documented the morphologic details of nervous system in a more refined manner, which eventually led to the emergence of Neuron Doctrine. In recognition of their exemplary contributions in neuroscience Golgi and Cajal were jointly awarded the Nobel Prize for Physiology or Medicine in 1906.

Network architectures supporting learnability

Human learners acquire complex interconnected networks of relational knowledge. The capacity for such learning naturally depends on two factors: the architecture (or informational structure) of the knowledge network itself and the architecture of the computational unit-the brain-that encodes and processes the information. That is, learning is reliant on integrated network architectures at two levels: the epistemic and the computational, or the conceptual and the neural. Motivated by a wish to understand conventional human knowledge, here, we discuss emerging work assessing network constraints on the learnability of relational knowledge, and theories from statistical physics that instantiate the principles of thermodynamics and information theory to offer an explanatory model for such constraints. We then highlight similarities between those constraints on the learnability of relational networks, at one level, and the physical constraints on the development of interconnected patterns in neural systems, at another level, both leading to hierarchically modular networks. To support our discussion of these similarities, we employ an operational distinction between the modeller (e.g. the human brain), the model (e.g. a single human's knowledge) and the modelled (e.g. the information present in our experiences). We then turn to a philosophical discussion of whether and how we can extend our observations to a claim regarding explanation and mechanism for knowledge acquisition. What relation between hierarchical networks, at the conceptual and neural levels, best facilitate learning? Are the architectures of optimally learnable networks a topological reflection of the architectures of comparably developed neural networks? Finally, we contribute to a unified approach to hierarchies and levels in biological networks by proposing several epistemological norms for analysing the computational brain and social epistemes, and for developing pedagogical principles conducive to curious thought. This article is part of the theme issue 'Unifying the essential concepts of biological networks: biological insights and philosophical foundations'.

On the nature and use of models in network neuroscience

Network theory provides an intuitively appealing framework for studying relationships among interconnected brain mechanisms and their relevance to behaviour. As the space of its applications grows, so does the diversity of meanings of the term network model. This diversity can cause confusion, complicate efforts to assess model validity and efficacy, and hamper interdisciplinary collaboration. In this Review, we examine the field of network neuroscience, focusing on organizing principles that can help overcome these challenges. First, we describe the fundamental goals in constructing network models. Second, we review the most common forms of network models, which can be described parsimoniously along the following three primary dimensions: from data representations to first-principles theory; from biophysical realism to functional phenomenology; and from elementary descriptions to coarse-grained approximations. Third, we draw on biology, philosophy and other disciplines to establish validation principles for these models. We close with a discussion of opportunities to bridge model types and point to exciting frontiers for future pursuits.

A novel histochemical method of simultaneous detection by a single- or double-immunofluorescence and Bielschowsky's silver staining in teased rat sciatic nerves

BACKGROUND

The Golgi silver method has been widely used in neuroscience for the study of normal and pathological morphology of neurons. The method has been steadily improved and Bielschowsky's silver staining method (BSSM) is widely used in various pathological conditions, like Alzheimer's disease.

NEW METHOD

In this work, teased sciatic nerves were silver impregnated using BSSM. We also developed simultaneous staining by silver impregnation and single- or double-immunofluorescence of the same section in teased nerve preparations. We immunostained against non-myelinating Schwann cells and different myelinating Schwann cell domains.

RESULTS

BSSM teased nerves show a strong staining of axons (black) and a gold-brown staining of myelinating and non-myelinating Schwann cells. We were also able to stain by immunofluorescence these BSSM teased nerves with specific molecular markers against non-myelinating Schwann cells, also against non-compact myelin such as the Schmidt-Lanterman incisures or paranodal regions and compact myelin, but not axons.

COMPARISON WITH EXISTING METHODS AND CONCLUSIONS

In peripheral nerves, several silver impregnation methods have been used to stain nerves in paraffin sections, but not in teased nerves to enable the assessment of isolated nerve fibers. In conclusion, BSSM gives accurate information of nerve morphology and combining the procedure with immunofluorescence it would be very useful to study the molecular nerve domain organization of the nerve fibers, and to study the molecular pathology of axon degeneration, or myelin disorders, or of any peripheral neuropathy, also to study demyelination diseases in the central nervous system.

On glioblastoma and the search for a cure: where do we stand?

Although brain tumours have been documented and recorded since the nineteenth century, 2016 marked 90 years since Percival Bailey and Harvey Cushing coined the term "glioblastoma multiforme". Since that time, although extensive developments in diagnosis and treatment have been made, relatively little improvement on prognosis has been achieved. The resilience of GBM thus makes treating this tumour one of the biggest challenges currently faced by neuro-oncology. Aggressive and robust development, coupled with difficulties of complete resection, drug delivery and therapeutic resistance to treatment are some of the main issues that this nemesis presents today. Current treatments are far from satisfactory with poor prognosis, and focus on palliative management rather than curative intervention. However, therapeutic research leading to developments in novel treatment stratagems show promise in combating this disease. Here we present a review on GBM, looking at the history and advances which have shaped neurosurgery over the last century that cumulate to the present day management of GBM, while also exploring future perspectives in treatment options that could lead to new treatments on the road to a cure.

Mapping brain structure and function: cellular resolution, global perspective

A comprehensive understanding of the brain requires analysis, although from a global perspective, with cellular, and even subcellular, resolution. An important step towards this goal involves the establishment of three-dimensional high-resolution brain maps, incorporating brain-wide information about the cells and their connections, as well as the chemical architecture. The progress made in such anatomical brain mapping in recent years has been paralleled by the development of physiological techniques that enable investigators to generate global neural activity maps, also with cellular resolution, while simultaneously recording the organism's behavioral activity. Combination of the high-resolution anatomical and physiological maps, followed by theoretical systems analysis of the deduced network, will offer unprecedented opportunities for a better understanding of how the brain, as a whole, processes sensory information and generates behavior.

From 'Nerve Fiber Regeneration' to 'Functional Changes' in the Human Brain-On the Paradigm-Shifting Work of the Experimental Physiologist Albrecht Bethe (1872-1954) in Frankfurt am Main

Until the beginning 1930's the traditional dogma that the human central nervous system (CNS) did not possess any abilities to adapt functionally to degenerative processes and external injuries loomed large in the field of the brain sciences (Hirnforschung). Cutting-edge neuroanatomists, such as the luminary Wilhelm Waldeyer (1836-1921) in Germany or the Nobel Prize laureate Santiago Ramón y Cajal (1852-1934) in Spain, debated any regenerative and thus "plastic" properties in the human brain. A renewed interest arose in the scientific community to investigate the pathologies and the healing processes in the human CNS after the return of the high number of brain injured war veterans from the fronts during and after the First World War (1914-1918). A leading research center in this area was the "Institute for the Scientific Study of the Effects of Brain Injuries," which the neurologist Ludwig Edinger (1855-1918) had founded shortly before the war. This article specifically deals with the physiological research on nerve fiber plasticity by Albrecht Bethe (1872-1954) at the respective institute of the University of Frankfurt am Main. Bethe conducted here his paradigmatic experimental studies on the pathophysiological and clinical phenomena of peripheral and CNS regeneration.

Santiago Ramón y Cajal and Ivan Petrovic Pavlov: their parallel scientific lives, schools and nobel prizes

Santiago Ramón y Cajal was not only a great scientist but he was also a dedicated teacher who managed to create his own School in Spain. Cajal was active at the end of the XIX and the beginning of the XX century, a period in which Ivan Petrovich Pavlov, another great contemporary scientist, also established a strong School in Russia. While these two acclaimed scientists shared a similar vision on science, a view they also conveyed to their disciples, they applied quite distinct criteria in the way they dealt with their followers. Interestingly, despite the geographic and idiomatic barriers that had to be overcome, the paths of these two great figures of XX century science crossed at least three times. First when they competed for the City of Moscow Prize, second when they both attended the "Congreso Internacional de Medicina de Madrid" (Medicine International Congress in Madrid) in 1903 and finally, they competed on four consecutive occasions for the Nobel Prize in Physiology or Medicine. Here we discuss their scientific vision, their different attitudes in the interaction with disciples and the distinct circumstances in which their paths crossed.

Physical integration: a causal account for consciousness

The issue of integration in neural networks is intimately connected with that of consciousness. In this paper, integration as an effective level of physical organization is contrasted with a methodological integrative approach. Understanding how consciousness arises out of neural processes requires a model of integration in just causal physical terms. Based on a set of feasible criteria (physical grounding, causal efficacy, no circularity and scaling), a causal account of physical integration for consciousness centered on joint causation is outlined.

The renaissance of the neuron doctrine: Cajal rebuts the Rector of Granada

The Spanish histologist Santiago Ramón y Cajal and the Italian anatomist Camillo Golgi, who were jointly awarded the 1906 Nobel Prize in Physiology or Medicine for their discoveries on the structure of the nervous system, are two of the most notable figures in neuroscience. It was the’ Golgi method’ that enabled Cajal to gather evidence and defend neuronism (the contiguity of neurons as independent cellular units) against his chief rival’s reticularism (the intracellular continuity of the cytoplasm among neurons in a widespread reticulum). Seven months after his Nobel lecture in Stockholm, Cajal wrote a powerful article which he titled’ El renacimiento de la doctrina neuronal’ (the rebirth, revival, or renaissance of the neuron doctrine) as a response to an insurrection of reticularist ideas. This new wave of reticularism was instigated in Spain by the pathologist Eduardo García Solá, Rector of the University of Granada at the time, and stemmed from the interpretation of nerve regeneration experiments conducted by the German physiologist Albrecht von Bethe in Strassburg (today Strasbourg, France) and the Hungarian histologist Stephan von Apáthy in Kolozsvár (today Cluj-Napoca, Romania). Cajal’s article was hosted by four different journals (three in Spain and one in Argentina). It constitutes an important testimony for the history of the neuron theory that has gone unheeded thus far. Therefore, we provide an English translation of Cajal’s Spanish paper, placing it in the context of evolving notions during that first decade of the twentieth century crucial for neurobiology.

Golgi in retrospect: a historiographic examination of contextual influence in tracing the constructs of neuronal organization

The concepts underlying the connectivity of neurons and the dynamics of interaction required to explain information processing have undergone significant change over the past century. A re-examination of the evolution of the modern view in historical context reveals that rules for connectivity have changed in a manner that might be expected from critical analysis enabled by technical advance. A retrospective examination of some germane issues that moved Camillo Golgi to question the widely held dogma of his era reveals network principles that could not have been recognized a century ago. The currently evolving rules of cellular discontinuity and interaction have proven sufficiently complex to justify the arguments of critical skepticism that sustain scientific progress.

The brain on itself: Nobel laureates and the history of fundamental nervous system function

The Nobel Prize in Physiology or Medicine has been given in recognition of work in the neurosciences a number of times. Laureates have been awarded for work on both fundamental and more complex nervous system functions. This review is restricted to contributions by 20th century laureates to the understanding of fundamental nervous system function on the cellular level. In 1906, Camillo Golgi and Ramón y Cajal were awarded for their work on the microscopic structure of the nervous system. Their achievement and those of others within this field, coupled with technological progress, gradually allowed more complex physiological studies. In 1932, the prize was awarded to Charles Sherrington and Edgar Adrian for their discoveries of how neurons function. They were followed in 1944 by Herbert Gasser and Joseph Erlanger who uncovered the highly differentiated functions of single nerve fibers. Alan Hodgkin and Andrew Huxley were awarded for the detection of the ionic mechanism of the action potential and its mathematical explanation in 1963. In 1991, Erwin Neher and Bernd Sakmann were awarded for their work on single ion channels. Although the scientists who proved the hypothesis (Fridjof Nansen, Wilhelm His, and August Forel) were never awarded by the Nobel Committee, their studies gave rise to one of the most fundamental questions in 20th century neuroscience: How is information carried from one neuron to another or to an effector cell? This was first solved in the vegetative nervous system, and, in 1936, Henry Dale and Otto Loewi received the prize for their discoveries relating to chemical transmission of nerve impulses. In 1963, John Eccles was awarded the prize for his work on the physiology of synapses. In 1970, Bernhard Katz received the Nobel Prize for the discovery of quantal release. Katz shared the prize with Julius Axelrod and Ulf von Euler, who were central in finding that transmitters are stored in presynaptic vesicles and that the effect in many synapses is terminated by reuptake. This review does not include 21st century laureates, although the prize has already been given to neuroscientists twice this century; Arvid Carlsson, Paul Greengard, and Eric Kandel received the award in 2000 for their discoveries related to signal transduction, and Richard Axel and Linda Buck received the award in 2004 for their work in the field of odorant receptors and the organization of the olfactory system.

The sensory neuron and the triumph of Camillo Golgi

While Golgi's concept of the sensory neuron provided sound reasons for his rejection of the polarity principles underlying the 'neuron doctrine', it is now apparent that his concern about recovery of function after injury and the vast modern findings of ephemerality of connexin-clustered connections in the cerebral cortex and elsewhere in the central nervous system, and credibly termed 'reticularist', has somewhat eclipsed the polarized neuron doctrine of reflex physiology with the "fixed and immutable" connections championed by Cajal. Although Golgi's view was not the result of incisive reasoning based on subsequently confirmed observation, both principles espoused by these combatant Nobel laureate partners have proven robustly operative in different spheres and time frames of neural activity that have vastly enhanced contemporary understanding of neural connectivity.

Neuroanatomy: Cajal and after Cajal

This essay commences with a consideration of the relative contributions of Cajal and Golgi to the study of the anatomy of the nervous system. It demonstrates the extent to which Cajal depended upon Golgi's work and how his modifications of the Golgi technique permitted a remarkable series of investigations in which the foundations of the neuron doctrine were laid and in which the intrinsic connectivity of virtually every part of the central nervous system was charted. Cajal's readiness to seize on and develop new techniques was one of the many keys to his success. After him, neuroanatomical studies tended to be focused more on long tract connectivity, using techniques such as those of Nissl and Marchi that had been in place before Cajal commenced his studies. Development of degeneration-based techniques of tracing connections in the late 1950s spearheaded a revolution in neuroanatomy while introduction of mixed aldehyde fixation made possible similarly intensive studies of the fine structure of the nervous system. At this time, the Golgi technique experienced a brief resurgence as neuroanatomists made efforts to bridge the gap between light and electron microscopy. Later developments in techniques for tracing connections included anterograde tracing by autoradiography and retrograde tracing by horseradish peroxidase. These were soon superseded by tracing techniques of increasing sensitivity and specificity that rely upon the cellular and molecular biology of neurons. Although neuroanatomy in its traditional form is perhaps no longer fashionable as a discipline, the techniques of neuroanatomy remain preeminent in many, perhaps all areas of neuroscience.

Whither withered Golgi? A retrospective evaluation of reticularist and synaptic constructs

The 100th anniversary of the shared first Nobel prize in neuroscience by Camillo Golgi and Ramon y Cajal invites reappraisal of the merits of the arguments adduced by these two combative scientists in the light of contemporary knowledge. Guided by cogent reasons for reluctance in accepting the inviolable polarity principle of the neuron doctrine and concern for explaining cerebral recovery of function, Golgi joined the 'reticularists' of his generation. Modern observations of axo-axonic and dendro-dendritic synapses, gap-junction interconnections, rules for the direction and mode of analog or impulse conduction, the myriad diversity of ion channels and gating principles and the complexities of synaptic plasticity have eclipsed the polarized neuron doctrine explanations of reflex physiology and the 'fixed and immutable' connections successfully championed by Cajal. Without violating the cell theory, expanded modes of neuronal and glial communication have encompassed reticularist notions and provided insight into the long-term changes underlying synaptic and extra-synaptic neural patterns. Both laureates espoused operative principles that have survived in different modes and distinctive temporal domains. Together, they reflect the roots of our contemporary understanding of neural interaction.

Neuron theory, the cornerstone of neuroscience, on the centenary of the Nobel Prize award to Santiago Ramón y Cajal

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 Histología 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.