A quantitative analysis of the local connectivity between pyramidal neurons in layers 2/3 of the rat visual cortex

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 microm) 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 microm 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.

Conditions for Propagating Synchronous Spiking and Asynchronous Firing Rates in a Cortical Network Model

“…K exc and K inh were drawn independently from a normal distribution. The local connection probability of two pyramidal neurons in rat visual cortex has been described as a function of their cell body distance (Hellwig, 2000) using Gaussian profiles. This kind of distance dependent connectivity description holds within a cortical layer; the connectivity across layers follows more complicated rules and was not incorporated in this study.…”

Effects of the anesthetic agent propofol on neural populations

“…We point out that the spatial interaction range of axonal connections are specific to the species, the brain area, the layer in the neural tissue and the number of connections. For instance, in layer 2 and 3 in rat visual cortex (Hellwig 2000), the average size of axonal branches is *18 mm and the probabiflity of axonal connections between a pyramidal neuron and a dendritic tree falls off to its half at a distance of about 200 lm. Moreover, in the monkey visual cortex area V4 (Yoshioka et al 1992), layer 3 shows axonal extensions up to 1 mm.…”

Role of Persistent Sodium Current in Bursting Activity of Mouse Neocortical Networks In Vitro

“…The neural network in the computational model representing neocortex includes excitatory and inhibitory cell populations with a multicompartmental representation of each cell type. The network connectivity is based on histological and physiological work (DeFelipe et al 2002;Feldmeyer and Sakman 2000;Hellwig 2000;Krimer and Goldman-Rakic 2001;Mountcastle 1997;Nieuwenhuys 1994) and includes the essence of neocortical microcircuitry.…”