Independently Outgrowing Neurons and Geometry-Based Synapse Formation Produce Networks with Realistic Synaptic Connectivity

Abstract

Neuronal signal integration and information processing in cortical networks critically depend on the organization of synaptic connectivity. During development, neurons can form synaptic connections when their axonal and dendritic arborizations come within close proximity of each other. Although many signaling cues are thought to be involved in guiding neuronal extensions, the extent to which accidental appositions between axons and dendrites can already account for synaptic connectivity remains unclear. To investigate this, we generated a local network of cortical L2/3 neurons that grew out independently of each other and that were not guided by any extracellular cues. Synapses were formed when axonal and dendritic branches came by chance within a threshold distance of each other. Despite the absence of guidance cues, we found that the emerging synaptic connectivity showed a good agreement with available experimental data on spatial locations of synapses on dendrites and axons, number of synapses by which neurons are connected, connection probability between neurons, distance between connected neurons, and pattern of synaptic connectivity. The connectivity pattern had a small-world topology but was not scale free. Together, our results suggest that baseline synaptic connectivity in local cortical circuits may largely result from accidentally overlapping axonal and dendritic branches of independently outgrowing neurons.

Emergence of the small-world architecture in neural networks by activity dependent growth

“…These findings can shed light on the study of neural networks growth and development, because hubs could integrate information from the distant regions of neural network. The node distributions are similar to experimental [41,19,42] and theoretical distributions [43]. Scale-free networks (SFNs) have degree distributions with power-law tails P (k) ∼ k −γ .…”

Spag6 Negatively Regulates Neuronal Migration During Mouse Brain Development

“…It has been shown that when the leading process of neurons reach the marginal zone (MZ), intensive branching occurs, giving rise to complex dendritic arborization and synaptic connectivity (Chai et al 2014;van Ooyen et al 2014). However, in the Spag6-transfected neurons at P5, we noticed that the number of dendrites and their length were significantly decreased compared with those in the control (Fig.…”

“…Mammalian cortical development requires several subtle processes, including progenitor cell proliferation (InglisBroadgate et al 2005;Weimer et al 2009), neuronal migration (Frotscher et al 2009;Liu 2011), and establishment and refinement of synaptic connectivity (Chai et al 2014;van Ooyen et al 2014). Disturbance in any single step causes specific neurological disorders with cognitive impairment.…”

Dynamics of Periodic Waves in a Neural Field Model

“…Neuron connectivity is provided by numerous axons whose density decrease approximately exponentially as a function of distance [23,24]. In modelling, the connectivity functions are usually considered to be symmetric, that is, axon connections between points x and y have the same density as between y and x.…”

“…Connectivity functions can be estimated from the data in [23,24]. It exponentially decreases with the rate of decrease in the interval 3-10 times at the at the distance 0.03 cm.…”