Extending the neuron doctrine: Carl Ludwig Schleich (1859-1922) and his reflections on neuroglia at the inception of the neural-network concept in 1894

Generating Brain Waves, the Power of Astrocytes

Synchronization of neuronal activity in the brain underlies the emergence of neuronal oscillations termed “brain waves”, which serve various physiological functions and correlate with different behavioral states. It has been postulated that at least ten distinct mechanisms are involved in the formulation of these brain waves, including variations in the concentration of extracellular neurotransmitters and ions, as well as changes in cellular excitability. In this mini review we highlight the contribution of astrocytes, a subtype of glia, in the formation and modulation of brain waves mainly due to their close association with synapses that allows their bidirectional interaction with neurons, and their syncytium-like activity via gap junctions that facilitate communication to distal brain regions through Ca²⁺ waves. These capabilities allow astrocytes to regulate neuronal excitability via glutamate uptake, gliotransmission and tight control of the extracellular K⁺ levels via a process termed K⁺ clearance. Spatio-temporal synchrony of activity across neuronal and astrocytic networks, both locally and distributed across cortical regions, underpins brain states and thereby behavioral states, and it is becoming apparent that astrocytes play an important role in the development and maintenance of neural activity underlying these complex behavioral states.

At the Origin of the History of Glia

The history of brain science is dominated by the study of neurons. However, there are as many glial cells as neurons in the human brain, their complexity increases during evolution, and glial cells play important roles in brain function, behavior, and neurological disorders. Although neurons and glial cells were first described at the same time in the early 19th century, why did the physiological study of glial cells only begin in the 1950s? What are the scientific breakthroughs and conceptual shifts that determined the history of glial cells in relation to that of neurons? What is the impact of the history of glia on the evolution of neuroscience? In order to answer these questions, we reconstructed the history of glial cells, from their first description until the mid-20th century, by examining the relative role of technical developments and scientific interpretations.

Functions of astrocytes and their potential as therapeutic targets

Astrocytes are often referred to, and historically have been regarded as, support cells of the mammalian CNS. Work over the last decade suggests otherwise-that astrocytes may in fact play a more active role in higher neural processing than previously recognized. Because astrocytes can potentially serve as novel therapeutic targets, it is critical to understand how astrocytes execute their diverse supportive tasks while maintaining neuronal health. To that end, this review focuses on the supportive roles of astrocytes, a line of study relevant to essentially all acute and chronic neurological diseases, and critically re-evaluates our concepts of the functional properties of astrocytes and relates these functions and properties to the intricate morphology of these cells.

Cerebral cortex astroglia and the brain of a genius: a propos of A. Einstein's

The glial fibrillary acidic protein immunoreactive astroglial layout of the cerebral cortex from Albert Einstein and other four age-matched human cases lacking any known neurological disease was analyzed using quantification of geometrical features mathematically defined. Several parameters (parallelism, relative depth, tortuosity) describing the primate-specific interlaminar glial processes did not show individually distinctive characteristics in any of the samples analyzed. However, A. Einstein's astrocytic processes showed larger sizes and higher numbers of interlaminar terminal masses, reaching sizes of 15 microm in diameter. These bulbous endings are of unknown significance and they have been described occurring in Alzheimer's disease. These observations are placed in the context of the general discussion regarding the proposal--by other authors--that structural, postmortem characteristics of the aged brain of Albert Einstein may serve as markers of his cognitive performance, a proposal to which the authors of this paper do not subscribe, and argue against.

Interlaminar astroglia of the cerebral cortex: a marker of the primate brain

Evidence for "cable-like" processes stemming from astroglial cells in the supragranular cerebral cortex has been recently presented. In addition to what could be called the "general mammalian-like" astroglial architecture (the so-called "panglial syncytium") of the cerebral cortex, composed of typical stellate astrocytes (intralaminar astrocytes), the anthropoid species, mostly catarrhines, show a manifest vertical, radial distribution of long (interlaminar) astroglial processes. It can be tentatively proposed that evolutionary pressures resulted in the progressive appearance, in primates, of a new type of glial cell. Its soma has a superficial location and unusually long cellular processes that invade, in a predominant radial fashion, the supragranular region of the cerebral cortex. Their existence has been ignored for more than a century. On the neuronal side, modular (columnar) organization of the cerebral cortex may represent an evolutionary acquisition that could optimize communication and information processing, with the least volume compromise in terms of wiring. Yet, for such columns to be functionally operative, adequate isolation from neighboring units would be required. A "mass" operation of the astroglial architecture would tend to compromise spatial definition and the degrees of freedom of such columnar modules. It is proposed that the presence of a "palisade" of interlaminar glial processes represents a relatively recent evolutionary event, instrumental for the optimization of the modular (columnar) organization of the cerebral cortex. It is interesting that the supragranular cortical region has undergone the largest growth among mammalian species during brain evolution, and has been associated with a crucial role in cortico-cortical interactions.

The neuron theory

In 1740 the Swedish scientist and philosopher Emmanuel Swedenborg described what is the first known anticipation of the neuron (a nerve cell with its processes). One hundred years later Ehrenberg, Remak and Purkinje recognized the nerve cell as the important element of the nervous system and provided its first accurate description. Vilhelm von Waldeyer in 1891 proposed to call the unit 'neuron' from the Greek word for 'sinew'. The 'neuron theory' or 'neuron doctrine', which emerged at the end of the 19th century, asserts that nerve tissue is composed of individual cells, which are genetic, anatomic, functional and trophic units. The pioneers of the neuron doctrine included neuroscientists, physicians, a polar explorer and three Nobel Laureates. The classic neuron doctrine has served well as the theoretical basis for the great advances in our current understanding of the cellular basis of nervous system functions.

General physiology, experimental psychology, and evolutionism. Unicellular organisms as objects of psychophysiological research, 1877-1918

This essay aims to shed new light on the relations between physiology and psychology in the late nineteenth and early twentieth centuries by focusing on the use of unicellular organisms as research objects during that period. Within the frameworks of evolutionism and monism advocated by Ernst Haeckel, protozoa were perceived as objects situated at the borders between organism and cell and individual and society. Scholars such as Max Verworn, Alfred Binet, and Herbert Spencer Jennings were provoked by these organisms to undertake experimental investigations situated between general physiology and psychology that differed from the physiological psychology advocated by Wilhelm Wundt. Some of these investigations sought to locate psychological properties in the molecular structure of protoplasm; others stressed the existence of organic and psychological individuality in protozoa. In the following decades, leading philosophers such as Friedrich Nietzsche, Charles Sanders Peirce, and Henri Bergson, as well as psychological researchers like Sigmund Freud, integrated the results of these investigations into their reflections on such problems as the nature of the will, the structure of the ego, and the holistic nature of the reactions of organisms to their environment.

Acutely isolated astrocytes as models to probe astrocyte functions

Neuroscientists have become increasingly aware and accepting of the concept that astrocytes likely have many important functions in the CNS. One limitation in establishing these functions is the usual problem of what constitutes suitable experimental approaches. A major experimental step for functional studies of astrocytes has been the widespread use of primary astrocyte cultures, an approach that Leif Hertz pioneered. However, it is now becoming clear that, building on this work, an experimental paradigm shift is now needed. Namely, to increasingly study preparations corresponding to in situ conditions, such as slices. An alternative experimental system where the cells have some of the technical advantages of primary astrocyte cultures is freshly isolated astrocytes. Recent experiments from our laboratory have shown metabotropic glutamate receptor expression by such cells. Examples are given of how functional receptor studies and channel activity measured by patch clamp electrophysiology can be combined with single cell RT-PCR to define further the receptor or channel type are described to illustrate the uses of such preparations.

Bain on neural networks

In his book Mind and body (1873), Bain set out an account in which he related the processes of associative memory to the distribution of activity in neural groupings--or neural networks as they are now termed. In the course of this account, Bain anticipated certain aspects of connectionist ideas that are normally attributed to 20th-century authors--most notably Hebb (1949). In this paper we reproduce Bain's arguments relating neural activity to the workings of associative memory which include an early version of the principles enshrined in Hebb's neurophysiological postulate. Nonetheless, despite their prescience, these specific contributions to the connectionist case have been almost entirely ignored. Eventually, Bain came to doubt the practicality of his own arguments and, in so doing, he seems to have ensured that his ideas concerning neural groupings exerted little or no influence on the subsequent course of theorizing in this area.

New views on synapse-glia interactions

Although glial cells ensheath synapses throughout the nervous system, the functional consequences of this relationship are uncertain. Recent studies suggest that glial cells may promote the formation of synapses and help to maintain their function by providing nerve terminals with energy substrates and glutamate precursors.