Why Does The Brain Have A Wrinkled Appearance?

Q: I am in a heated discussion with coworkers, here is the question. The brain looks as though it is “squished” into the head, with wrinkles and such. Is the reason it is like that as opposed to smoothed out due to the fact that it can then hold more memory? – Mike

Dear Mike,

why the brain is wrinkledYou are in a heated discussion over brain anatomy? You may be a bigger geek than I am. (I say that with admiration.)

The reason the brain looks wrinkled is that it soaks up the surrounding cerebrospinal fluid and gets all pruney, like fingers in a bathtub. I thought everyone knew that.

Of course, there are other theories involving far-out notions like “surface area” and “efficiency.” Some people think that folding the outer layers of the brain in on itself provides anatomical advantage. As devastating as it may be to the Prune Finger Theory, they may be onto something.

In order to answer your question, we need to briefly review, and grossly oversimplify, basic brain anatomy.

We’ve all probably heard of grey matter and white matter. The cortex (outer surface of the brain) is made up of grey matter. At about three millimeters thick, this is where the magic happens – it is where we think, speak, remember, analyze, see, hear, and so on. There are pockets of grey matter within the brain, but we’re mainly interested in the outer cortex for your question. Think of it as a giant computer network spread out over a thin surface.

The cortex looks grey in preserved brains because it is packed with neurons and support cells, including my personal favorite: the lowly glia. These little guys clean up the garbage, hold neurons in place, feed the neurons, and generally keep the local environment in good shape so that the fancy-pants neurons can steal all the glory. Even though they greatly outnumber neurons, glial cells are deprived of the press they deserve. Alas, there would be no grey matter without them.

The white matter beneath the cortex is made of axons, which are long arms connecting neurons to each other. Think of white matter as huge bundles of wires. The white coloring comes from the fatty myelin sheath that surrounds each axon, insulating it from other axons. Without it, the brain would short circuit. In fact, the breakdown of myelin has been implicated in Alzheimer’s disease, and it is believed to be cause of disorders like multiple sclerosis, where the immune system attacks myelin.

Mother Nature is Groovy, Baby

So. We have a giant network (grey matter) with bundles of interconnecting wires (white matter). In answer to your question, the brain has a wrinkled appearance because folding the cortex in on itself allows for larger surface area, and therefore a larger network.

According to my anatomy book, a full 2/3 of the human cortex is hidden within grooves and fissures. Folding the cortex inward means that an average 2.5 square feet of cortex can be packed into a relatively small space. To get a rough idea of the brain’s size, hold your fists together, palm-to-palm. Pretty compact, right? Without the folds – or “invaginations” as some anatomists indelicately describe them – we would all be as top heavy as Charlie Brown.

If my high-school geometry still serves, the human brain would need to be roughly 10.8 inches in diameter if it had a smooth surface. That’s almost 2 inches wider than your standard basketball. Add to that a thick skull to contain this orb and you would have quite an impressive melon. “Ten-gallon hat” takes on new meaning.

There is a second advantage to folding the cortex inward: it greatly reduces distance and therefore the amount of interconnecting white matter that would otherwise be needed. That makes the brain more efficient. Were it not for that architectural feature, we would be hard pressed to achieve the evolutionary pinnacle of talking on the cell phone, drinking coffee, shaving, and balancing a Big Mac on our knee while navigating rush hour traffic.

I hope this settled the debate before violence erupted. Someone among you has earned bragging rights for being close enough with the “squished brain holds more memory” theory. (I recommend a rhyming taunt to drive the point home.)