Notes
Fish, amphibians reptiles have 3-layer cortex, mammals developed neocortex with 6-layers. Mammals have hippocampus and olfactory cortex which are still 3-layer - left relatively untouched by evolution.
Early work provided evidence that 3-layer contains basic microcircuit core that has been elaborated in 6-layer.
Table 2: Summary of elements common to cortical circuits (Shepherd, 1988).
- Both three-layer and six-layer cortex are built on pyramidal cells with apical and basal dendritic trees.
- Strong excitatory afferents are received in the spines of the branches of the apical and basal dendrites.
- The spines and local branches create local sites with varying degrees of local information processing properties.
- The spines, local branches, and main stems contain different combinations of Na, K, and Ca ionic channels, which create local sites of integration and boosting of input signals to reach the sites of action potential output in the cell body and axon hillock.
- The pyramidal cells have well developed recurrent axon collaterals.
- The axon collaterals give rise to two main types of intrinsic circuit.
- One type is direct feedback and lateral recurrent excitation.
- This excitatory recurrent system has long lateral extensions, which enable widespread recombination of intrinsic excitation with the excitatory afferent input.
- The other type is feedback and lateral inhibition through inhibitory interneurons.
- Inhibitory interneurons are subdivided into multiple types which target different sites and levels of the soma-dendritic extent of the pyramidal cells.
- Cortical information processing therefore involves a continual balance between excitatory and inhibitory circuits.
- In three-layer cortex, these intrinsic circuits are organized around a single layer of pyramidal and pyramidal like neurons.
- Five- and six-layer neocortex appears an expansion of the three-layer microcircuit into closely integrated superficial and deep layers.
- Cortico-cortical afferents make synapses at different levels of the pyramidal cell soma-dendirtic axis to excite, inhibit, or modulate the transfer of synaptic inputs and extent of backpropagating action potentials in the dendritic trees.
- Brainstem systems provide differential modulation in different layers.
"... there is an underlying logic to the construction of cortical local circuits and microcircuits that provides a common modular framework that is adapted to generate the special properties of the neocortex."
Cortical circuits can be seen to be poised on the knife edge of exciation restrained by inhbition.
In 3-layer inputs make contacts on apical branches. In neocortex inputs come up through the layers and can make contacts to any layer.
Olfactory cortex acts as "content-addressable memory" (Haberly 1985): each site in system contains information about the entire input. "Odor images" (Xu et al 2000). Cortex processes spatial maps - similarities between olfactory (odor images) and hippocampus (2-D spatial location).
Superficial pyramidal cells in neocortex (L2/3) are massively recurrently connected - winner take-all/pattern completion. Deep layers have little recurrent connections, input dominated by superficial pyramids.
Most turtle pyramids are characterized by multiple apical dendrites and no basal dendrites. Pyramids in mammal olfactory are of 2 varieties: one has multiple apical trees and no basal, called "semilunar cell". Superficial pyramids in mammals also only have apical trees.
Broad calcium spikes not seen in turtle pyramids, but seen in neocortical pyramids. These can lead to burst-mode - bursting results in combination of feed-forward basal tree activation and feed-back apacial tuft activation. Different type of signal - more confident, more potent. Could be learning signal, used for back-prop.
Lots of stuff about the dendrites doing processing. Review more of Larkum's work.
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