The history of ideas about the forms and functions of neurons shows that the conditions which pennit different scientists to uphold totally opposed hypotheses are, firstly, that the evidence is contradictory and inconclusive and, secondly, that one or both hypotheses are based on erroneous assumptions. One of the major assumptions of the late 19th century was that animal cells lack a cell membrane. The cell surface was believed to be a transition between two phases, without special structure. Therefore, it was assumed that protoplasmic bridges between cells could freely appear and disappear. To recognize the significance of this fundamental assumption is to gain an entirely fresh view of the history of rival theories of formation of nerve connections. Proponents of the neuron theory believed that nerve cells only come into close contact and are never in direct protoplasmic continuity, whereas proponents of the reticular theory believed that nerve cells are directly connected by protoplasmic bridges or networks. The reticular theory is consistent with the fundamental assumption that cells lack membranes; the neuron theory is in conflict with that assumption.
KeywordsNerve Growth Factor Schwann Cell Actin Filament Growth Cone Axonal Transport
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