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-dendritic 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.
