A cortical minicolumn is a vertical column through the cortical layers of the brain, comprising perhaps 80–120 neurons, except in the primate primary visual cortex (V1), where there are typically more than twice the number. There are about 2×108 minicolumns in humans. From calculations, the diameter of a minicolumn is about 28–40 µm. Minicolumns grow from progenitor cells within the embryo and contain neurons within multiple layers (2 - 6) of the cortex.
Contents
Many sources support the existence of minicolumns, especially Mountcastle, with strong evidence reviewed by Buxhoeveden and Casanova who conclude "... the minicolumn must be considered a strong model for cortical organization" and "[the minicolumn is] the most basic and consistent template by which the neocortex organizes its neurones, pathways, and intrinsic circuits". See also Calvin's Handbook on cortical columns.
Cortical minicolumns can also be called cortical microcolumns. Cells in 50 µm minicolumn all have the same receptive field; adjacent minicolumns may have very different fields (Jones, 2000).
Number of neurons in a minicolumn
Estimates of number of neurons in a minicolumn range from 80-100 neurons.
Jones describes a variety of observations that may be interpreted as mini- or micro-columns and gives example numbers from 11 to 142 neurons per minicolumn.
Number of minicolumns
Estimates of the number of neurons in cortex or in neocortex are on the order of 2x1010. Most (perhaps 90%) cortical neurons are neocortical neurons.
Johansson and Lansner use an estimate of 2x1010 neurons in neocortex and an estimate of 100 neurons per minicolumn, yielding an estimate of 2x108 minicolumns.
Sporns et al. give an estimate of 2x107-2x108 minicolumns with no derivation.
Size
The minicolumn measures of the order of 40–50 µm in transverse diameter (Mountcastle 1997, Buxhoeveden 2000, 2001); 35–60 µm (Schlaug, 1995, Buxhoeveden 1996, 2000, 2001); 50 µm with 80 µm spacing (Buldyrev, 2000), or 30 µm with 50 µm (Buxhoeveden, 2000). Larger sizes may not be of human minicolumns, for example macaque monkey V1 minicolumns are 31 µm diameter, with 142 pyramidal cells (Peters, 1994) — 1270 columns per mm2. Similarly, the cat V1 has much bigger minicolumns, ~56 µm (Peters 1991, 1993) .
The size can also be calculated from area considerations. If cortex (both hemispheres) is 1.27×1011 µm2 then if there are 2×108 minicolumns in the neocortex then each is 635 µm2, giving a diameter of 28 µm (if the cortex area were doubled to the commonly quoted value, this would rise to 40 µm). Johansson and Lansner do a similar calculation and arrive at 36 µm (p51, last para).
Downwards projecting axons in minicolumns are ≈10 µm in diameter, periodicity and density similar to those within the cortex, but not necessarily coincident (DePhilipe, 1990).