BMC Neuroscience

, 10:P58 | Cite as

Random axon outgrowth and synaptic competition generate realistic connection lengths and filling fractions

  • Marcus Kaiser
  • Claus C Hilgetag
  • Arjen van Ooyen
Open Access
Poster presentation


Axon Outgrowth Individual Neuron Dendritic Tree Testable Prediction Filling Fraction 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.


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 [1] 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, [2]) establish synapses with spatially neighboring neurons.


Investigating local connectivity between individual neurons [3], we show that simple, random outgrowth of axons can reproduce distance-dependent connectivity as found in many neural systems. Experimentally observed filling fractions can also be generated by competition for free space at the dendritic tree; a model markedly different from previous explanations. In our simple model, which relies on fewer factors than previous approaches, the filling fraction can be determined by the ratio between axon collaterals and free target sites which we call competition factor. The modeled filling fraction decays exponentially with the competition factor. We derive experimentally testable predictions for the relation between filling fraction, average axonal length, and competition. Figure 1.
Figure 1

Synaptic competition for dendritic space (A) leads to a decay in filling fraction with neuron density (B). Both with and without competition the connection length distribution (C) is similar to experimental studies.


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.


  1. 1.
    Kaiser M, Hilgetag CC: Modelling the Development of Cortical Networks. Neurocomp. 2004, 58–60: 297-302. 10.1016/j.neucom.2004.01.059.CrossRefGoogle Scholar
  2. 2.
    Stepanyants A, Hof PR, Chklovskii DB: Geometry and structural plasticity of synaptic connectivity. Neuron. 2002, 34: 275-88. 10.1016/S0896-6273(02)00652-9.PubMedCrossRefGoogle Scholar
  3. 3.
    van Ooyen A: Modeling Neural Development. 2003, MIT PressGoogle Scholar

Copyright information

© Kaiser et al; licensee BioMed Central Ltd. 2009

This article is published under license to BioMed Central Ltd.

Authors and Affiliations

  • Marcus Kaiser
    • 1
    • 2
  • Claus C Hilgetag
    • 3
    • 4
  • Arjen van Ooyen
    • 5
  1. 1.School of Computing ScienceNewcastle UniversityNewcastle upon TyneUK
  2. 2.Institute of Neuroscience, Newcastle UniversityNewcastle upon TyneUK
  3. 3.School of Engineering and ScienceJacobs University BremenBremenGermany
  4. 4.Department of Health SciencesBoston University, Sargent CollegeBostonUSA
  5. 5.Department of Integrative NeurophysiologyVU University AmsterdamAmsterdamThe Netherlands

Personalised recommendations