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Abstract
The spatial organization of synaptic inputs on the dendritic tree of cortical neurons plays a major role for dendritic integration and neural computations, yet, remarkably little is known about it. We mapped the spatial organization of glutamatergic synapses between layer 5 pyramidal cells by combining optogenetics and 2-photon calcium imaging in mouse neocortical slices. To mathematically characterize the organization of inputs we developed an approach based on combinatorial analysis of the likelihoods of specific synapse arrangements. We found that the synapses of intralaminar inputs form clusters on the basal dendrites of layer 5 pyramidal cells. These clusters contain 4 to 14 synapses within ≤30 µm of dendrite. According to the spatiotemporal characteristics of synaptic summation, these numbers suggest that there will be non-linear dendritic integration of synaptic inputs during synchronous activation.
Highlights
Computations on the level of single neurons critically depend on the location and spatial relationship between active synapses (e.g. Branco and Hausser, 2011; Losonczy and Magee, 2006; Polsky et al, 2004)
Synapses between pairs of layer 5 (L5) pyramidal cells are reported to be located to a large extend on the proximal basal dendrite of the postsynaptic cell (Markram et al, 1997; Sjostrom and Hausser, 2006)
We focused on the basal dendrites of L5 pyramidal cells to map their inputs from other L5 pyramidal cells
Summary
Computations on the level of single neurons critically depend on the location and spatial relationship between active synapses (e.g. Branco and Hausser, 2011; Losonczy and Magee, 2006; Polsky et al, 2004). Computations on the level of single neurons critically depend on the location and spatial relationship between active synapses Knowing the connectivity of neurons with single synapse resolution is essential to understand how synaptic integration at the cellular level contributes to neural circuit function. One type of spatial organization is of particular interest These are clusters of synaptic inputs, which can give rise to superlinear summation during synchronous activity We analyze the spatial organization of synapses with a novel method for identifying individual synapse clusters. This allows characterizing cluster parameters, such as dendritic length and number of inputs, in order to put them into the perspective of the rules of dendritic integration and the arithmetic of synaptic summation This allows characterizing cluster parameters, such as dendritic length and number of inputs, in order to put them into the perspective of the rules of dendritic integration and the arithmetic of synaptic summation (e.g. Branco and Hausser, 2011; Losonczy and Magee, 2006; Polsky et al, 2004)
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