Why are wrinkly brains an evolutionary advantage? Would it not take away extra potential brain volume? Why is more brain surface area better rather than brain volume.
Let’s consider just the cerebrum for the purpose of discussion, and disregard the cerebellum and brainstem. In the cerebrum, nearly all of your information processing ability—sensation, muscle control, thought, memory, consciousness, etc.—occurs in a surface layer just 2 to 3 mm thick, called the cerebral cortex.
Now look at this slice of brain tissue. The darker tan tissue on the surface is the gray matter of the cerebral cortex (the colors are altered by the preservative). In sectional view, you can easily see the infoldings of the cortex. In several places, I’ve marked surface and infolded gray matter with arrows.
As you can see, there would be a lot less cortex if the brain were smooth, lacking these infoldings. The extensive folding enables the cranium to contain about three times as much of this information-processing tissue as it could if the brain were smooth.
Most of the brain volume is “white matter” beneath the surface. This is like the cables under the floor of a computer lab—just connections from one place to another in the brain without any information processing or storage capacity. Increasing brain volume could give you more “cables” but not more “brain power.” For that, you need the wrinkles.
Compare a cat brain, below left, with a human brain, right, and you can see how the amount of folding of the cerebral cortex is correlated with relative intelligence.
Here’s another way to look at it. Consider a standard 8 1/2 x 11 inch sheet of paper. It has a surface area (total of both sides) of 187 sq.in. (~ 1,206 sq.cm.). Wad it into a ball, and you could fit all that into a much smaller space, but the surface area would be unchanged. Similarly, a large area of cerebral cortex “wadded into a ball” can fit into a relatively small cranial cavity with no loss of surface area.
Compared to 1,206 sq.cm. in the paper wad below, what do you think the surface area would be in a smooth paper sphere of the same diameter? Just now, I wadded up an 8 1/2 x 11 inch sheet of paper into a paper wad; it came to about the same size as in this photo and I measured it at 4.5 cm. dia. A sphere of that diameter has an easy-to-calculate surface area of 63.6 sq.cm. That paper ball, though, has a surface area of 1,206 sq.cm. (per side). So you see, complex folding fits 19 times as much surface into the same volume. The same principle applies to a convoluted brain surface contained within the volume of the cranium.
Post a Comment