Excitatory and inhibitory connections show selectivity in the neocortex - PubMed (original) (raw)

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Excitatory and inhibitory connections show selectivity in the neocortex

Jo Watts et al. J Physiol. 2005.

Abstract

The cerebral cortex is pivotal in information processing and higher brain function and its laminar structure of six distinct layers, each in receipt of a different constellation of inputs, makes it important to identify connectivity patterns and distinctions between excitatory and inhibitory pathways. The 'feedforward' projections from layer 4-3 and from 3-5 target pyramidal cells and to lesser degrees interneurones. 'Feedback' projections from layer 5-3 and from 3-4, on the other hand, mainly target interneurones. Understanding the microcircuitry may give some insight into the computation and information processing performed in this brain region.

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Figures

Figure 1

Figure 1. Two examples of layer 3 to layer 5 pyramid–pyramid connections in rat visual cortex

In these connections the presynaptic soma is always very near, within 50 μm, of the postsynaptic apical dendrite and the two neurones have apical dendritic tufts in the most superficial layers that overlap in space. In these pairs, the postsynaptic layer 5 neurone was always a large, burst firing pyramidal cell, never a small pyramidal cell, with an apical dendrite that terminated in layer 4 or layer 3 (adapted from Fig. 4 of Thomson & Bannister, 1998).

Figure 2

Figure 2. An example of a pyramid to interneurone connection in rat

In this example, the presynaptic layer 6 neurone (yellow soma/dendrites, white axon) was a corticothalamic-like pyramidal cell with an apical dendritic tuft in layer 4. The postsynaptic interneurone (red soma/dendrites, blue axon) was a bitufted, low-threshold spiking, dendrite-preferring interneurone. The interneurone also had an extensive axonal arbour in layer 4. The EPSP elicited by this connection was strongly facilitating with a paired pulse ratio > 3. Despite at least 6 (confirmed ultra-structurally) and possibly up to 12 synapses (visible as close membrane appositions at the microscopic level) mediating this connection, the failure rate in response to the first presynaptic spike of a train was > 30% (adapted from Fig. 4 of Deuchars & Thomson, 1995_a_).

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