A cortical minicolumn is a vertical column

Cortical column

A cortical column, also called hypercolumn or sometimes cortical module, is a group of neurons in the brain cortex which can be successively penetrated by a probe inserted perpendicularly to the cortical surface, and which have nearly identical receptive fields...

 through the cortical

Cerebral cortex

The cerebral cortex is a sheet of neural tissue that is outermost to the cerebrum of the mammalian brain. It plays a key role in memory, attention, perceptual awareness, thought, language, and consciousness. It is constituted of up to six horizontal layers, each of which has a different...

 layers of the brain, comprising perhaps 80–120 neuron

Neuron

A neuron is an electrically excitable cell that processes and transmits information by electrical and chemical signaling. Chemical signaling occurs via synapses, specialized connections with other cells. Neurons connect to each other to form networks. Neurons are the core components of the nervous...

s, except in the primate

Primate

A primate is a mammal of the order Primates , which contains prosimians and simians. Primates arose from ancestors that lived in the trees of tropical forests; many primate characteristics represent adaptations to life in this challenging three-dimensional environment...

 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.

Many sources support the existence of minicolumns, especially Mountcastle

Vernon Mountcastle

Vernon Benjamin Mountcastle is Professor Emeritus of Neuroscience at Johns Hopkins University.He discovered and characterized the columnar organization of the cerebral cortex in the 1950s...

, 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.

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 cortex 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).

Facts

• Cells in 50 µm minicolumn all have the same receptive field; adjacent minicolumns may have very different fields (Jones, 2000).
• 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).
• Thalamic input (1 axon) reaches 100–300 minicolumns.
• The number of fibres in the corpus callosum is 2–5×10⁸ (Cook 1984, Houzel 1999) — perhaps related to the number of minicolumns.