A quantitative analysis of the local connectivity between pyramidal neurons in layers 2/3 of the rat visual cortex |
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Authors: | Bernhard Hellwig |
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Institution: | (1) Max-Planck-Institut für biologische Kybernetik, Spemannstr. 38, D-72076 Tübingen, Germany, DE |
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Abstract: | This study provides a detailed quantitative estimate for local synaptic connectivity between neocortical pyramidal neurons.
A new way of obtaining such an estimate is presented. In acute slices of the rat visual cortex, four layer 2 and four layer
3 pyramidal neurons were intracellularly injected with biocytin. Axonal and dendritic arborizations were three-dimensionally
reconstructed with the aid of a computer-based camera lucida system. In a computer experiment, pairs of pre- and postsynaptic
neurons were formed and potential synaptic contacts were calculated. For each pair, the calculations were carried out for
a whole range of distances (0 to 500 μm) between the presynaptic and the postsynaptic neuron, in order to estimate cortical
connectivity as a function of the spatial separation of neurons. It was also differentiated whether neurons were situated
in the same or in different cortical layers. The data thus obtained was used to compute connection probabilities, the average
number of contacts between neurons, the frequency of specific numbers of contacts and the total number of contacts a dendritic
tree receives from the surrounding cortical volume. Connection probabilities ranged from 50% to 80% for directly adjacent
neurons and from 0% to 15% for neurons 500 μm apart. In many cases, connections were mediated by one contact only. However,
close neighbors made on average up to 3 contacts with each other. The question as to whether the method employed in this study
yields a realistic estimate of synaptic connectivity is discussed. It is argued that the results can be used as a detailed
blueprint for building artificial neural networks with a cortex-like architecture.
Received: 30 March 1999 / Accepted in revised form: 5 August 1999 |
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