Axonal Development

Abstract

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.

When you are criticizing the philosophy of an epoch, do not chiefly direct your attention to those intellectual positions whichits exponents feel it necessary explicitly to defend. There will be some fundamental assumptions which adherents ofall the variant systems within the epoch unconsciously presuppose. Such assumptions appear so obvious that people do not know what they are assuming because no otherway of putting things has ever occurred to them.

Alfred North Whitehead (1861–1947), Science and the Modem World, p. 71,1925

If there exists any surface or separation at the nexus between neurone and neurone, much of what is characteristic of the conduction exhibited by the reflex-arc might be more easily explainable.... The characters distinguishing reflex-arc conduction from nerve-trunk conduction may therefore be largely due to intercellular barriers, delicate transverse membranes, in the former.

In view, therefore, of the probable importance physiologically of this mode of nexus between neurone and neurone, it is convenient to have a term for it. The term introduced has been synapse.

Charles Scott Sherrington (1857–1952), The Integrative Action of the Nervous System, 1906