Mini-columns
The smallest columnar structure in the neocortex is the mini-column. Mini-columns are about 30um in diameter and contain 80-100 neurons across all five cellular layers. The entire neocortex is composed of mini-columns. You can visualize them as tiny pieces of spaghetti stacked side by side. There are tiny gaps with few cells between the mini-columns sometimes making them visible in stained images.
On the left is a stained image that shows neuron cell bodies in part of a neocortical slice. The vertical structure of mini-columns is evident in this image. On the right is a conceptual drawing of a mini-column (from Peters and Yilmez). In reality is skinnier than this. Note there are multiple neurons in each layer in the column. All the neurons in a mini-column will respond to similar inputs. For example, in the drawing of a section of V1 shown previously, a mini-column will contain cells that respond to lines of a particular orientation with a particular ocular dominance preference. The cells in an adjacent mini-column might respond to a slightly different line orientation or different ocular dominance preference.
Inhibitory neurons play an essential role is defining mini-columns. They are not visible in the image or drawing but inhibitory neurons send axons in a straight path between mini-columns partially giving them their physical separation. The inhibitory neurons are also believed to help force all the cells in the mini-column to respond to similar inputs.
The mini-column is the prototype for the column used in the HTM cortical learning algorithm.
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