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Why The Brain Has So Many Folds

"Dear A Moment of Science, I know the human brain is an incredible thing, the most complex object in the known universe. But, honestly, it’s not much to look at. Just a big, gray, foldy lump. And what’s with all those folds? Why does the brain look a lot like a scrunched up piece of paper?"

Our brains, and the brains of some other animals, have a lot more in common with a balled up piece of paper than you might think.

First, though, it’s worth mentioning that not all brains look the same. Some animals, including rats and mice, have smooth brains. No folds or wrinkles. Humans and other animals, such as pigs, have brains with many folds.

Scientists at the University of Rio de Janeiro have been trying to figure out what accounts for the folds. One idea postulated that as brains evolve and the more neurons they have, the more folds are required to allow neurons to form connections. But elephants have nearly twice as many folds in their brains yet have fewer neurons than we do, so that theory seems improbable.

The researchers have, however, determined that how human brains fold follows the same physical principles guiding folds and creases resulting from crumpling a sheet of paper. That’s interesting because it enabled the scientists to reason that the two main factors determining a brain’s shape are the surface area and thickness of the outer layer cortex. The larger the cortex surface area, the more folds a brain will have. But greater thickness results in fewer folds.

Same goes for a piece of paper.

ps: 一项新的研究显示，我们的大脑之所以布满褶皱并具有核桃似的形状，是因为大脑皮层即灰质的快速生长受到了白质的限制。研究人员发现，大脑皮层凹陷的沟和隆起的回取决于两个简单的几何参数———灰质的生长速度及其厚度。发表在美国《国家科学院学报》月刊上的研究显示，可以在实验室利用双层凝胶模拟大脑褶皱的形成。研究人员指出，除了这些限制，基因也对决定大脑形状有所影响，因为基因对神经细胞如何增殖并移动到它们的目的地起调控作用。所有哺乳类物种都有大脑皮层，但只有较大型的哺乳类才具有褶皱的大脑皮层。举例而言，老鼠大脑的表面光滑，而比这大得多的大脑，比如人类的大脑则具有大量沟回。褶皱的大脑皮层拥有更大的表面面积，这就意味着具备更强大的信息处理能力，但尚未完全清楚是什么因素决定了人类大脑的标志性沟回形状。掌握大脑是如何形成褶皱的，可能有助于科学家更好地解释多小脑回、巨脑回、无脑回等先天性病症。历史上，关于脑沟和脑回是如何形成的有三种思路：一种看法是，大脑皮层的某些区域生长较快进而比其他区域凸出，形成了脑回。另一个看法是，相互之间高度关联的神经细胞群被构成白质的丝状轴突机械地拉近距离。然而，证据表明上述两种观点都不正确。第三种看法是，灰质的生长超过白质，形成了“褶皱”，令大脑皮层具备这种形状。但研究人员称，模拟这种褶皱的早期尝试并未获得成功。在此前的研究中，研究人员假设灰质是薄且坚硬的，在厚且柔软的白质上生长，但根据这种假设所产生的褶皱与真实的人类大脑上的褶皱并不相同。在新的研究中，研究人员假设灰质和白质具有相似的坚硬程度，但生长速度不同。利用数学模拟的方法，根据大脑大小的不同，他们的模拟产生了不同的大脑皮层形状。举例而言，直径不足1.3厘米的小型大脑预计会产生光滑的大脑皮层。中等大小的大脑预计会在灰质中产生一些脑沟，而体积较大的大脑则会变得十分褶皱，会有脑沟陷入白质?？蒲Ъ颐腔估盟闩蛘湍翰牧细粗屏舜竽孕纬神拗宓南窒螅⒈砻?，只有这两层具备相似的柔软程度时，形成的褶皱看上去才会与人类大脑的脑沟和脑回相似。研究人员指出，虽然他们的模拟可用于最基本的脑沟和脑回，但这并不能解释大脑更加复杂的特征，比如，将左右脑分开的大脑纵裂，以及区分大脑主要脑叶的其他大的脑沟。