History Of The Neuron

Most types of cell do not have a history.

Peters, Palay, and Webster (1991)

The challenge to neuroscientists imposed by the size and complexity of neuronal form is illustrated by the colorful and somewhat contentious history of the neuron concept. Once the concept of the cell as the unit of structure and function in all living things was established by Schleiden and Schwann in the early nineteenth century, scientists rapidly accepted that various tissues were made up of individual cells. Not so with nervous tissue, where nearly another 100 years would pass before the fact that neurons were distinct cellular entities, also known as the Neuron Doctrine,

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Figure 1 Neurons stained by Golgi impregnation. (A) Pyramidal neuron from the rat cerebral cortex. The plexus of fine fibers visible in the background is likely axon collaterals. (B) Three pyramidal neurons from the rat hippocampus. (C) Higher magnification view of a spiny dendrite showing the spiny protuberances from the dendritic shaft and varicose axons in the background.

Figure 1 Neurons stained by Golgi impregnation. (A) Pyramidal neuron from the rat cerebral cortex. The plexus of fine fibers visible in the background is likely axon collaterals. (B) Three pyramidal neurons from the rat hippocampus. (C) Higher magnification view of a spiny dendrite showing the spiny protuberances from the dendritic shaft and varicose axons in the background.

would become universally accepted. Competing with the adherents of the Neuron Doctrine were the reticularists, who maintained that the nervous system comprises a syncitia of anastomosing filamentous processes in protoplasmic continuity with one another, much like a network of blood vessels. A leader of the reticularist school was the Italian neuroscientist Ca-millo Golgi, also famous for introducing the eponymous stain that was instrumental in elucidating the structure of the neuron, the Golgi reaction. Until the development of the Golgi reaction, most stains revealed only a complex jumble of fibrillar structures intertwined with corpuscular globules. The Golgi stain had the advantage of staining only a small number of neurons but revealing their entire form in glorious detail, including their far-reaching processes (Fig. 1). Ironically, the man most instrumental in putting forth the Neuron Doctrine, Santiago Ramon y Cajal, based most of his observations on material stained with the

Golgi method developed by his competitor. The two scientists shared the Nobel prize in 1906.

With the development of biological electron microscopy in the 1950s, the limiting membrane of nerve cells and the separation between cells at points of contact were clearly visualized. However, whereas these early images from electron microscopy failed to reveal evidence for cytoplasmic connections between neurons at these arborized sites, more recently developed methods have revealed filamentous structures bridging the synaptic gap. This system, dubbed the transcellular filament system, may represent the constituents stained by the Golgi method, which Golgi interpreted as a network. Indeed, in the end, Golgi may have been partially right! Clearly it was correct to extend the cell doctrine to the nervous system and define the neuron. It appears that it was also correct to posit that a continuum of filamentous structures provides a reticular fabric for the nervous system and that this is transcellular, especially at the synapses, which were at the heart of this controversy.

The establishment of the neuron doctrine did not mark the end of the evolving conception of the nerve cell. As our understanding of nerve cell physiology, biochemistry, and molecular biology has progressed, neuroscientists have had to create and modify models of information processing in the nervous system. For example, simple views of nerve cell processes as cables that either actively or passively propagated information in the form of electrical signals have given way to an increased emphasis on chemical signaling pathways and compartmentalization. The emphasis on molecular interactions has been fueled by advances in molecular genetics and methods for identifying and localizing the chemical constituents of nerve cells by using techniques such as immunocytochemistry. Thus, any review of the neuron represents a snapshot in time with an emphasis that is highly dependent upon the prevailing winds of scientific inquiry. However, regardless of the prevailing wind, any discussion of the neuron necessarily begins with its morphology.

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