Random axon outgrowth and synaptic competition generate realistic connection lengths and filling fractions
KeywordsAxon Outgrowth Individual Neuron Dendritic Tree Testable Prediction Filling Fraction
On various spatial scales, from connectivity between individual neurons in Caenorhabditis elegans and rat visual cortex to connectivity between cortical areas in the mouse, macaque  and human brain, connection length distributions have very similar shapes, with a long flat tail representing the admixture of long-distance connections to mostly short-distance connections. Furthermore, not all potentially possible synapses are formed and only a fraction of axons (called filling fraction, ) establish synapses with spatially neighboring neurons.
Simple models that assume a random axonal outgrowth and competition for target space can account for the experimentally found exponential decay in the connection length distribution and the filling fraction.
We thank the EPSRC (EP/E002331/1) and the Royal Society (RG/2006/R2) for financial support.
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