Functional maps of neocortical local circuitry - PubMed (original) (raw)
Functional maps of neocortical local circuitry
Alex M Thomson et al. Front Neurosci. 2007.
Abstract
This review aims to summarize data obtained with different techniques to provide a functional map of the local circuit connections made by neocortical neurones, a reference for those interested in cortical circuitry and the numerical information required by those wishing to model the circuit. A brief description of the main techniques used to study circuitry is followed by outline descriptions of the major classes of neocortical excitatory and inhibitory neurones and the connections that each layer makes with other cortical and subcortical regions. Maps summarizing the projection patterns of each class of neurone within the local circuit and tables of the properties of these local circuit connections are provided.This review relies primarily on anatomical studies that have identified the classes of neurones and their local and long distance connections and on paired intracellular and whole-cell recordings which have documented the properties of the connections between them. A large number of different types of synaptic connections have been described, but for some there are only a few published examples and for others the details that can only be obtained with paired recordings and dye-filling are lacking. A further complication is provided by the range of species, technical approaches and age groups used in these studies. Wherever possible the range of available data are summarised and compared. To fill some of the more obvious gaps for the less well-documented cases, data obtained with other methods are also summarized.
Keywords: EPSP/C (excitatory postsynaptic potential/current); IPSP/C (inhibitory postsynaptic potential/current); cortex; interneuron(e); pyramidal cell; synapse.
Figures
Figure 1
Layer 6 pyramidal cells. The major subclasses of layer 6 pyramidal cells are summarized in this cartoon (blue). On the left are the two types of CT pyramids projecting to different regions of the thalamus. Both are upright pyramids with apical dendritic tufts and narrow, vertically projecting axonal arbours in layer 4 and upper layer 5, respectively. Three dendritic arbours typical of CC pyramidal cells, short upright, inverted and bipolar, are shown and a claustrum projecting cell (far right). Only one horizontally oriented axonal arbour confined to the deep layers is indicated for simplicity, since all CC cells and claustrum projecting cells appear to have similar axonal arbours. The major longer distance inputs to the 6 layers are indicated to the left. Spiny, excitatory postsynaptic targets that have been demonstrated for each group of layer 6 cells with paired intracellular recordings are shown in red, the paler cells being those that receive sparse and weak inputs and the white cells those that have been tested but not to date shown to be significant targets for these layer 6 pyramidal axons.
Figure 2
Layer 5 pyramidal cells. The two major subclasses of layer 5 pyramidal cells are summarized in this cartoon (blue). Large pyramids with a pronounced apical dendritic tuft in layers 1 and 2 project to several subcortical regions, while small, shorter pyramids include CC cells. The major inputs to each layer are indicated to the left. Spiny, excitatory postsynaptic targets that have been demonstrated with paired intracellular recordings are shown in red, the paler cells being those that receive sparse and weak inputs and the white cells those that have been tested but not to date shown to be significant targets for these axons. Where a class of spiny excitatory cell is not indicated in a particular layer, it has not been tested sufficiently often with layer 5 pyramidal cells in paired recordings to determine whether the connection exists.
Figure 3
Layer 4 spiny excitatory cells. The two major subclasses of layer 4 spiny excitatory cells, pyramidal cells and spiny stellates are summarized in this cartoon (blue). The major inputs to each layer are indicated to the left. Spiny, excitatory postsynaptic targets that have been demonstrated with paired intracellular recordings are shown in red, the purple cell indicates a connection that has been demonstrated, but for which there are too few examples to identify target cell class(es) fully. Where a class of spiny excitatory cell is not indicated in a particular layer, it has not been tested sufficiently often with layer 4 spiny cells in paired recordings to determine whether the connection exists.
Figure 4
Layer 2/3 pyramidal cells. Layer 3 pyramidal cells are summarized in this cartoon (blue). The major inputs to each layer are indicated to the left. Spiny, excitatory postsynaptic targets that have been demonstrated with paired intracellular recordings are shown in red and the white cells indicate those that have been tested but not to date shown to be significant targets for these axons. Where a class of spiny excitatory cell is not indicated in a particular layer, it has not been tested sufficiently often with layer 3 pyramidal cells in paired recordings to determine whether or to what extent the connection exists. To date the majority of paired recordings have involved layer 3 pyramids. The outputs of layer 2 pyramids remain to be studied in detail.
Figure 5
Local circuit excitatory spiny cell targets of pyramidal and spiny stellate cells. The major classes of spiny excitatory neocortical neurones and their local circuit connections with other excitatory neurones. This figures combines Figures 1–4. Red cells are those for which significant inputs from each of the classes of presynaptic neurones indicated (blue) have been demonstrated with paired intracellular recordings. Pale red indicates cells that appear to receive sparse and/or weak inputs and the purple cell indicates that such an input has been recorded, but too few targets identified for the cells type(s) to be fully identified. White cells indicate types of cells that have been tested but which appear not to be significant targets for that type of presynaptic axon.
Figure 6
Interneuronal targets of spiny cells axons. The major classes of spiny excitatory neocortical neurones (blue) and their local circuit connections onto inhibitory interneurones. Red cells are those for which significant inputs from each of the classes of presynaptic neurones indicated have been demonstrated with paired intracellular recordings. Purple cells are the most likely candidates for postsynaptic targets where connections have been recorded, but the cell class(es) involved have not been studied in detail. White cells indicate types of interneurones that have been tested but which appear not to be significant targets for that type of presynaptic axon. These include layer 6 interneurones which receive little input from layer 6 CC pyramidal cells (but a strong input from CT cells) and many layer 3 interneurones not apparently innervated by layer 4 spiny cells. Only rarely have inputs from layer 4 spiny cells been recorded in layer 3 interneurones and these involved only the larger basket-type cells. Three broad groups of interneurones are included. Small, medium and large multipolar, fast spiking cells are indicated by round somata with radially extending dendrites. Many of these cells are PV-immunopositive basket cells, but in the cartoon these symbols also include (for simplicity) large adapting CCK basket cells and small VIP/CCK basket cells. The three major groups of bitufted, typically SOM-immunopositive cells are indicated by the larger fusiform somata with vertically oriented bitufted dendritic arbours. Small fusiform bipolar cells are also illustrated. An attempt is made to indicate the overall dimensions of the interneuronal classes, but for the larger cells these groups also include larger cells with longer dendrites.
Figure 7
Regions innervated by the axons of inhibitory interneurones. The major classes of inhibitory interneurones (as outlined for Figure 6) are shown with approximate denditic arbour dimensions (dense color) and typical axonal arbours (transparent color). Proximally targeting small medium and large basket cells are in red (left). Some of the large basket cells in layers 3 and 4 have long horizontal axon branches that form discrete bouton clusters. These larger cells may also have major axons that descend, e.g., to layer 5 from layer 3 or to layer 6 from layer 4. Similarly, large layer 5 basket cells can have two large axonal ramifications, one in layer 5 and one in layer 3. All layers also contain small and medium sized basket cells whose axons and dendrites may be confined to the layer of origin. Dendrite-preferring interneurones are summarized on the right. Larger bitufted interneurones (typically SOM-immunopositive, in blue) include Martinotti cells in layers 2–6 whose fine dense axonal arbours course toward the pial surface and extend horizontally in layer 1, double bouquet cells in layers 3 and 4 with a dense axonal arbour near their origin and a long narrow “mares tail” of descending axons and other bitufted interneurones whose less dense axonal abours extend above and below the soma, but rarely reach layer 1. Bipolar interneurones most commonly found in the upper layers, some of which have long narrow, vertically oriented axonal arbours, are indicated in green. Finally a small multipolar burst firing interneurone is shown close to the layer 1/2 border (light green).
References
- Ali A. B., Bannister A. P., Thomson A. M. (2007) Robust correlations between action potential duration and the properties of synaptic connections in layer 4 interneurones in neocortical slices from juvenile rats and adult rat and cat. J. Physiol. 580, 149–169 10.1113/jphysiol.2006.124214 - DOI - PMC - PubMed
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