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Cerebral Folding and Brain Size

2015-07-04

A new article in Science magazine describes a mathematical function which relates cerebral folding to surface area and thickness.   The thinner the cerebral cortex, the more folds it can make and the more surface area it can accommodate.  The article can be found here.

Here is a quote from the abstract:

“Larger brains tend to have more folded cortices, but what makes the cortex fold has remained unknown. We show that the degree of cortical folding scales uniformly across lissencephalic and gyrencephalic species, across individuals, and within individual cortices as a function of the product of cortical surface area and the square root of cortical thickness. ”

This formula can be demonstrated by crumpling different thicknesses of paper; maximum crumpling tolerance depends on the square root of the thickness of the paper and results in different final volumes for the ball of crumpled paper.

Therefore, as lissencephalic brains tend to be found in patients with mental retardation, and gyrencephaly is said to be associated with genius, the surface area of the brain is important to its functional ability.   The average human neocortex contains about 20 billion neurons and roughly 30 billion glial cells (the number of glia varies widely) in a six-layered coating of gray matter on top of large numbers of long axons covered in myelin, which shows up as the fatty white matter underneath.  The more surface area, and the more folding, the more neurons and glia can be included in the neocortex.  It appears that the brain functions by sending signals from an introductory ball of interneurons to a more abstract layer of neocortical cells, and even from the initial neocortical area to another, yet more abstract set of neocortical cells.  The processing done by each set of neurons to signals received from below relates to how the brain responds to a given set of sensory signals.  “Thinking” is really a series of processes carried out by sets of neural cells that react to their inputs by sending an output either to a higher level set of cells or towards a set of cells that initiates motor function.  Even motor function is controlled by several layers of sets of cells; in a sense, function is merely passing along signals from one’s input cells to the corresponding output cells.  Even within a single anatomically localized set of cells, there may be layers: cells that receive input from the distant region, cells that receive local inputs and send only to equally local output cells, and finally cells that receive local input and send their output to a higher distant region.

For example, in the visual system, there are cells in the retina that sense light, a layer of interneurons, and below that, a layer of cells with axons that project in the optic nerve, cross in the chiasm, and end in the lateral geniculate body.  This set of neurons has cells that receive the input of axons from the optic nerve and pass them along to interneurons; then there is an output level that sends its axons along to the occipital area of the neocortex, on the posterior pole of the brain.  From there, another bundle of axons radiates to the frontal or prefrontal cortex, which is a third level of abstraction.

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