Elimination of Synapses During the Development of the Central Nervous System

The development of the nervous system, after the setting out of neuronal somas, leads to the formation of a network of synaptic connections that in the adult exhibits both precision and diversity.

Abstract

During the development of the nervous system, after the setting out of neuronal somas, the growth of their processes (axons and dendrites) leads to the formation of a network of synaptic connections that in the adult exhibits both precision and diversity. Synaptic contacts are generally established between partners belonging to well-defined and homogeneous categories of cells. These classes of synapses represent a first level of order within the system; the specificity of recognition between appropriate classes of cell is strict but probably not absolute: functional "heterologous synapses" might be formed between cells which, under normal conditions, never form synapses. The accuracy of synaptic connections within a given class of synapses represents another level of complexity. Without doubt, the connections between subsets of neurons are very precisely established, although it is not clear to what extent the connections have to be precise to generate a given behavior. Several mechanisms have been proposed for the creation of such order and diversity but many of them are not mutually exclusive, because the acquisition of such a synaptic accuracy is likely to be not a single step process but a progressive and sequential one.