The Problem of Specificity in the Formation of Nerve Connections

For thirty years or so the word’ specificity’ has been much used but under-defined in relation to the developing nervous system, and this has led to considerable confusion. Thus we may talk of specificity of nerve connections and of neuronal specificity; and these two usages have quite different meanings. By’ specificity of nerve connections’ we mean that nerve fibres make predictably particular and selective connections. The phrase, ‘neuronal specificity’, however, has other connotations. It is used to denote the hypothesis, put forward by Sperry (1943; 1944; 1945; 1951; 1963; 1965) to account for the formation of specific or particular nerve connections. I intend to discuss this hypothesis, and consider various of its ramifications, later; here it is merely necessary to point out that, whereas the mechanism proposed in the hypothesis of neuronal specificity is one way to establish specific nerve connections, it is by no means the only way. Specific nerve connections could well be established by a variety of other mechanisms, some of which will also be discussed below. In this chapter I will outline the possibilities as I see them at present, and attempt to assess these in terms of probability.