Clusters of synaptic inputs on dendrites of layer 5 pyramidal cells in mouse visual cortex

TLDR

The spatial organization of synaptic inputs on cortical dendrites is crucial for dendritic integration and neural computation, yet little is known about it. The study aims to mathematically characterize the organization of synaptic inputs by developing a combinatorial analysis of synapse arrangement likelihoods. The authors mapped glutamatergic synapses between layer 5 pyramidal cells in mouse neocortical slices using optogenetics and two‑photon calcium imaging, then applied the combinatorial analysis to characterize input organization. Synapses of intralaminar inputs cluster on basal dendrites of layer 5 pyramidal cells, with 4–14 synapses within ≤30 µm, suggesting non‑linear dendritic integration during synchronous activation.

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.