Rearrangement of synapses during development

Synaptic rearrangements in early postnatal life are widespread. These rearrangements probably reflect the operation of the few fundamental rules that govern quantitative apportionment of innervation. It seems likely that axons and target cells in various regions of the nervous system obey the same rules, although this obedience may sometimes be obscured by the anatomy of the particular situation (as in the comparison of muscle fibers and autonomic ganglion cells). A number of influences are evidently at work in this process, including those aspects of target cells that provide for qualitative recognition of appropriate synaptic partners. Competition between axons, however, seems a fundamental theme as well as a driving force in establishing patterns of innervation. The object of terminal competition may be a trophic factor supplied by the target cell, although the arguments for this view remain circumstantial. The outcome of competition between terminals innervating the same target cell evidently depends on the geometry of postsynaptic cells and the pattern of neural activity among the competitors. The nature of the rearrangements that are observed in most systems that can be carefully studied strongly suggests a link between the positive feedback supplied by the postsynaptic cells and patterns of activity. Some linkage of this sort is required to explain the identification of a set of synapses which arises from a single axon and the unitary capture of individual postsynaptic cells. The immense range of convergent synaptic strength and neural unit configuration in the nervous system is presumably generated by varying a mix of these several influences.