Formation of Dendrites and Development of Synaptic Connections

Abstract

Five cardinal theories of the organization of nervous systems originated in the second half of the 19th century and formed the basis of the neuron theory on which modem neuroscience research programs are constructed. The most important advance in our understanding of the historical development of the neuron theory is that it did not originate in the 1880s and 1890s as a single theory but was constructed over a much longer period, starting in the 1840s, by convergence of at least five different cardinal theories and several other auxiliary theories. Those cardinal theories were, firstly, that the nerve cell and its fibers are parts of the same unit (Wagner, 1847; Koelliker, 1850; Remak, 1853, 1855); secondly, that nerve fibers are protoplasmic outgrowths of nerve cells (Bidder and Kupffer, 1857; His, 1886); thirdly, that dendrites and axons are fundamentally different types of nerve fibers (Wagner, 1851; Remak, 1854; Deiters, 1865; Golgi, 1873, 1882-1885); fourthly, that nerve conduction occurs in only one directionfrom dendrites to axons in the same nerve cell, and from axons to dendrites between different nerve cells (van Gehuchten, 1891; Lenhossek, 1893; Ramon y Cajal, 1895); fifthly, that nerve cells are connected by surface contact and not by cytoplasmic continuity (Koelliker, 1879, 1883, 1886; His, 1886; Forel, 1887).

So long as one only substitutes one theory for another, in the absence of direct proof, science gains nothing; one old theory deseroes another.

Claude Bernard (1813–1878), Lecons sur la physiologie et la pathologie du sqeteme neroeux, Vol. 1, p. 4, 1858

I find the fundamental features of a theory of the central ganglion cells in the observation of Remak, that every cell makes connection exclusively with only one motor nerve cell root, and that this is a fiber chemically and physiologically different from all other central processes.... The body of the cell is continuous, without interruption, with a more or less large number of processes which branch frequently.... These processes which ... must not be considered as the source of axis cylinders, or as having a nerve fiber growing from them...will hereafter be called protoplasmic processes.

Otto Friedrich Karl Deiters (1834–1863), Untersuchungen über Gehirn und Rückenmark des Menschen und der Saugethiere, 1865