Microcircuitry of the Neocortex

Markram, H. (2010) Microcircuitry of the Neocortex. Handbook of Brain Microcircuits. 

Markram is a really famous neuroscientist - he was one of the first people to identify spike-timing dependent plasticity. I'll talk about STDP in more detail later. Now he is working on the Blue Brain Project - which is supported by IBM, and one of the more promising groups that are out there trying to build a brain.

Notes:
Neocortex is six layer sheet of neurons. Has a functional module called a cortical column, which has diameter approximately the size of basal dendrites of L5 pyramdial neurons.

86% of synapses in column exitatory, 14% inhibitory. Excitatory synapses - 1/3 from local neurons, 1/3 from neighboring columns, 1/3 from other distal regions (cortical, opposite hemisphere, subcortical). Most inhibition is local.

Principal Neurons:

• LII/III pyramids.
• LIV - two types, classical and star. In sensory areas also spiny stellate cells (targeted by thalamus).
• LV - thin untufted: project to opposite hemisphere. Thick tufted: project subcortically.
• LVI - at least 4 types depending on projection: cortico-cortico, cortico-thalamic, cortico-callosal, cortical-claustral.
• Connectivity directional tendency within column: 4 -> 2/3 -> infragranular (5 and 6).

Interneurons:

• Tons of possible interneurons. Morphologically:
• 4 types in L1
• 9 types in L2-6: large basket, nest basket, small basket, bitufted, bipolar, neurogliaform, Martinotti, Double bouqet, chandelier.
• Each anatomical type can have different electrical behaviors. Combinatorically there could be 200 types of interneurons, also including layer differences.
• Can also be subclassified according to molecular expression.

So, you can see how incredibly complex neocortex is. One of the big mysteries is why are there so many interneurons. There's clearly some combinatorics, but we should try and think of what the different functions of all of these interneurons could be and unify them into a single theory. Neocortex just seems too complicated to make sense of the microcircuitry and figure out what kind of computation it is doing. But we know that neocortex evolved from 3-layer cortex, and so maybe we can get some insights by studying 3-layer cortex more closely.