Why is the cerebral cortex folded?

The folds and fissures of the cerebral cortex are typically explained as a way to fit all that surface area (a couple of square feet, I’ve heard) inside a nice, compact skull.

But why is surface area a necessary feature? How does this help brain performance? Why would a cerebral cortex of the same mass but without all the fissures necessarily be inferior?

I think you answered your own question - the key is surface area. Why?

The neurones (brain cells) for voluntary movement, language, perception, . . . , what makes us human (or at least a ‘higher’ species) are found on the surface of the brain.

The brain is (very roughly) structured as neurons on the surface (grey matter) with their connections (white matter) deeper down.

ETA: Just to be clear, there are also pockets of neurons (grey matter) found deeper (e.g. much of our motor fine tuning, gait, coordination, . . . ) as well but they are a minority.

Is this sort of structure (surface neurons connecting with a substrate) somehow necessary for mental activity, or is it just the way mammals happened to evolve?

Long-held belief is that the brain is static. It has been scientifically established that the opposite is true–it dynamically grows new neurology and in ways that answer challenges we place upon ourselves or that come at us regularly. The cultural hang-over of “beliefs” related to the static brain are still ingrained in our elites who casually write off vast wealths of human potential and sentence individuals to be stigmatized when it’s not a matter of intelligence but readiness at a particular time. This also calls into question the general assumption that you can regimen education into a smorgasbord of studies. That model places the cart of learning before the cart of motivation and the ability to establish meaning relevance. The brain’s grows new neurology daily. But it’s mass does not change much. Evidence has been found that mass increases in some spots based upon what what does and pushes folds to accommodate that growth. The significance of neuroplacticity is monumental, but it is taken by an arrogant society as just some reaction that goes into effect only to repair loss of brain tissue through trauma et al. It must become the central criterion of radical reform and I’m sure that will take a great work to lead people to rebel against orthodoxy.

The way mammals happened to evolve. The cerebrum in simpler vertebrates is just a small smooth lump. Once vertebrates started needing more brain power folding evolved to increase the “surface” area. It’s basically an example of jury-rigged adaptation; if you only knew exactly how brains worked it would probably be possible to intelligently design a better architecture.

A neuron has a thinkin’ end (where the nucleus is), and a business/power end (the axon and myelin). Axons from individual neurons can be several inches long in the brain (multiple *feet *long in the spinal cord and reaching out therefrom).

Painting with a broad brush, the neuron cell bodies are gray matter, and the axons extend through white matter.

The brain, by having all the longish myelin sheaths together, uses resources efficiently. The white matter under the gray matter is the fatty myelin sheath stuff that protects all the axons. It’s like a flower bouquet, with the exciting blossoms spraying out at the top, and all the thin stems together. Except even moreso. So the stem portion gets pretty thick, and the blooms need to get all wrinkly to fit them all in the outer top portion. And the stems need to be soaking in some fatty stuff, which is why that thick mass of stems lays all together.

Still, the closest thing to brains we have engineered, i.e. integrated circuits, are also basically two-dimensional.

Is that determined by blood flow? That is, getting blood to where it’s needed most?

That’s an interesting thought.

In what are called ‘watershed infarctions’, i.e. brain death in areas of tenuous blood supply*, it is typically the deep white matter that’s most affected. This suggests that cortex (surface) has the most robust blood supply. Cause or effect, I’m not sure.

(*occurring, for example, when the blood flow to the brain is globally reduced as opposed to lack of blood flow through a specific artery)

Which is also a manufacturing constraint. Certainly multi-layer, or fully three-dimensional ICs are conceived of. But they are hard to impossible to make with current tech.

It’s a product of how the cells are created. Most neurons are pyramidal cells. The brain is kind of like a rainforest canopy. The leaves are the gray matter/cell bodies, and the trunks are the white matter/axons. These aren’t separate structures but two ends of the same cells. When the brain is developing, the cells “migrate” to the correct place and form layers on the cortex. The order of this is very much structured.

The latter. Birds for example evolved with their higher functions concentrated in various spots inside the brain rather than on the surface. So even smart birds have smooth brains rather than folded ones.

Bird (AKA dinosaur) brains are designed on a totally different physical architecture. The individual types of neurons are substantially the same but the spatial distribution of (loosely speaking) wiring versus logic is quite different.

Arguably the bird/dino approach is better and more scalable than what mammals got.

Gives the insult “bird brain!” a whole new meaning.

This is a timely question. It just so happens that there is brand-new research on the matter.

Another, more detailed article:

Okay, that puts me further away from grokking the term “blood-brain barrier.” I had come to think of it as meaning that the brain is partitioned off from the blood supply, and that the only fluid in direct physical contact with the brain was of the cerebrospinal variety.

Some cogitation on the subject eventually leads me to notice that in such a situation getting oxygen to the brain becomes a problem. So blood MUST be involved, somehow.

What, exactly, DOES “blood-brain barrier” mean?

You’re essentially right, but the blood is still needed to deliver oxygen and nutrients. There’s still an entire network of capillaries, it’s just that each little capillary is surrounded by a barrier that lets oxygen through, but not bad things like bacteria.

Courk is right.

Some tissues (brain and testes, specifically) want to be a bit “insulated” from the blood (for different reasons.

The blood-brain barrier allows gas molecules, sugars, and electrolytes to diffuse, but not most larger molecules, especially proteins. This is often described as a means of protecting the brain from bacteria and viruses, but I doubt this is the main point. The purpose would seem to focus on molecules rather than pathogens, based on the “size” of the filter, the fact that we really don’t like infections anywhere in our body and we have means to deal with them, and the fact that some molecules are selectively passed.

(The blood-testis barrier is more about hiding sperm from the immune system. Given the existence of auto-immune diseases of the brain, esp. multiple sclerosis, the blood-brain barrier may have the same point.)

But the brain isn’t “foreign” in the same way that sperm are: Sperm, being haploid, have a different genome than other bodily cells, but the brain should have the same genome. If autoimmune disorders of the brain are more common than elsewhere in the body, I would expect that to be an effect, not a cause, of the brain-blood barrier.

A six layer neocortex is a distinguishing feature of mammals.

To play along with Ignotus and leahcim, it’s as if our DNA has developed the tech to build partially 3D integrated circuits with 6 layers, but either technical challenges or inherent limitations to the design prevent us from “taking it further”. So while mammals are generally ahead of the competition, when primates wanted to go further (and turn brainpower up to eleven! :slight_smile: ), a strategy of folding or wrinkling had to be employed. Kind of (sort of) like adding extra chips to a circuit board, running out of room, flipping it over and thinking, “I can add more chips here”!

Lots of biologic activities occur at surfaces, and living creatures have developed many strategies to increase surface area at various places and at various anatomic levels. So LIFE (DNA) is quite good at employing this general strategy.

And yet auto-immune diseases of the brain and many other parts of the body exist.