英語聽力文摘763 大腦為何有那么多溝壑?
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: 一項(xiàng)新的研究顯示，我們的大腦之所以布滿褶皺并具有核桃似的形狀，是因?yàn)榇竽X皮層即灰質(zhì)的快速生長受到了白質(zhì)的限制。研究人員發(fā)現(xiàn)，大腦皮層凹陷的溝和隆起的回取決于兩個(gè)簡單的幾何參數(shù)———灰質(zhì)的生長速度及其厚度。發(fā)表在美國《國家科學(xué)院學(xué)報(bào)》月刊上的研究顯示，可以在實(shí)驗(yàn)室利用雙層凝膠模擬大腦褶皺的形成。研究人員指出，除了這些限制，基因也對決定大腦形狀有所影響，因?yàn)榛驅(qū)ι窠?jīng)細(xì)胞如何增殖并移動(dòng)到它們的目的地起調(diào)控作用。所有哺乳類物種都有大腦皮層，但只有較大型的哺乳類才具有褶皺的大腦皮層。舉例而言，老鼠大腦的表面光滑，而比這大得多的大腦，比如人類的大腦則具有大量溝回。褶皺的大腦皮層擁有更大的表面面積，這就意味著具備更強(qiáng)大的信息處理能力，但尚未完全清楚是什么因素決定了人類大腦的標(biāo)志性溝回形狀。掌握大腦是如何形成褶皺的，可能有助于科學(xué)家更好地解釋多小腦回、巨腦回、無腦回等先天性病癥。歷史上，關(guān)于腦溝和腦回是如何形成的有三種思路：一種看法是，大腦皮層的某些區(qū)域生長較快進(jìn)而比其他區(qū)域凸出，形成了腦回。另一個(gè)看法是，相互之間高度關(guān)聯(lián)的神經(jīng)細(xì)胞群被構(gòu)成白質(zhì)的絲狀軸突機(jī)械地拉近距離。然而，證據(jù)表明上述兩種觀點(diǎn)都不正確。第三種看法是，灰質(zhì)的生長超過白質(zhì)，形成了“褶皺”，令大腦皮層具備這種形狀。但研究人員稱，模擬這種褶皺的早期嘗試并未獲得成功。在此前的研究中，研究人員假設(shè)灰質(zhì)是薄且堅(jiān)硬的，在厚且柔軟的白質(zhì)上生長，但根據(jù)這種假設(shè)所產(chǎn)生的褶皺與真實(shí)的人類大腦上的褶皺并不相同。在新的研究中，研究人員假設(shè)灰質(zhì)和白質(zhì)具有相似的堅(jiān)硬程度，但生長速度不同。利用數(shù)學(xué)模擬的方法，根據(jù)大腦大小的不同，他們的模擬產(chǎn)生了不同的大腦皮層形狀。舉例而言，直徑不足1.3厘米的小型大腦預(yù)計(jì)會(huì)產(chǎn)生光滑的大腦皮層。中等大小的大腦預(yù)計(jì)會(huì)在灰質(zhì)中產(chǎn)生一些腦溝，而體積較大的大腦則會(huì)變得十分褶皺，會(huì)有腦溝陷入白質(zhì)。科學(xué)家們還利用雙層膨脹凝膠材料復(fù)制了大腦形成褶皺的現(xiàn)象，并表明，只有這兩層具備相似的柔軟程度時(shí)，形成的褶皺看上去才會(huì)與人類大腦的腦溝和腦回相似。研究人員指出，雖然他們的模擬可用于最基本的腦溝和腦回，但這并不能解釋大腦更加復(fù)雜的特征，比如，將左右腦分開的大腦縱裂，以及區(qū)分大腦主要腦葉的其他大的腦溝。
