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If after a few months' exposure to our David Lynch economy, in which housing markets spontaneously combust, coworkers mysteriously disappear and the stifled moans of dying 401（k） plans can be heard through the floorboards, you have the awful sensation that your body's stress response has taken on a self-replicating and ultimately self-defeating life of its own, congratulations. You are very perceptive. It has.
As though it weren't bad enough that chronic stress has been shown to raise blood pressure, stiffen arteries, suppress the immune system, heighten the risk of diabetes, depression and Alzheimer's disease and make one a very undesirable dinner companion, now researchers have discovered that the sensation of being highly stressed can rewire the brain in ways that promote its sinister persistence.
Reporting earlier this summer in the journal Science, Nuno Sousa of the Life and Health Sciences Research Institute at the University of Minho in Portugal and his colleagues described experiments in which chronically stressed rats lost their elastic rat cunning and instead fell back on familiar routines and rote responses, like compulsively pressing a bar for food pellets they had no intention of eating.
Moreover, the rats' behavioral perturbations were reflected by a pair of complementary changes in their underlying neural circuitry. On the one hand, regions of the brain associated with executive decision-making and goal-directed behaviors had shriveled, while, conversely, brain sectors linked to habit formation had bloomed.
In other words, the rodents were now cognitively predisposed to keep doing the same things over and over, to run laps in the same dead-ended rat race rather than seek a pipeline to greener sewers. "Behaviors become habitual faster in stressed animals than in the controls, and worse, the stressed animals can't shift back to goal-directed behaviors when that would be the better approach," Dr. Sousa said. "I call this a vicious circle."
Robert Sapolsky, a neurobiologist who studies stress at Stanford University School of Medicine, said, "This is a great model for understanding why we end up in a rut, and then dig ourselves deeper and deeper into that rut."
The truth is, Dr. Sapolsky said, "we're lousy at recognizing when our normal coping mechanisms aren't working. Our response is usually to do it five times more, instead of thinking, maybe it's time to try something new."
And though perseverance can be an admirable trait and is essential for all success in life, when taken too far it becomes perseveration - uncontrollable repetition - or simple perversity. "If I were to try to break into the world of modern dance, after the first few rejections the logical response might be, practice even more," said Dr. Sapolsky, the author of "Why Zebras Don't Get Ulcers," among other books. "But after the 12,000th rejection, maybe I should realize this isn't a viable career option."
Happily, the stress-induced changes in behavior and brain appear to be reversible. To rattle the rats to the point where their stress response remained demonstrably hyperactive, the researchers exposed the animals to four weeks of varying stressors: moderate electric shocks, being encaged with dominant rats, prolonged dunks in water. Those chronically stressed animals were then compared with nonstressed peers. The stressed rats had no trouble learning a task like pressing a bar to get a food pellet or a squirt of sugar water, but they had difficulty deciding when to stop pressing the bar, as normal rats easily did.
But with only four weeks' vacation in a supportive setting free of bullies and Tasers, the formerly stressed rats looked just like the controls, able to innovate, discriminate and lay off the bar. Atrophied synaptic connections in the decisive regions of the prefrontal cortex resprouted, while the overgrown dendritic vines of the habit-prone sensorimotor striatum retreated.
According to Bruce S. McEwen, head of the neuroendocrinology laboratory at Rockefeller University, the new findings offer a particularly elegant demonstration of a principle that researchers have just begun to grasp. "The brain is a very resilient and plastic organ," he said. "Dendrites and synapses retract and reform, and reversible remodeling can occur throughout life."
Stress may be most readily associated with the attosecond pace of postindustrial society, but the body's stress response is one of our oldest possessions. Its basic architecture, its linked network of neural and endocrine organs that spit out stimulatory and inhibitory hormones and other factors as needed, looks pretty much the same in a goldfish or a red-spotted newt as it does in us.
The stress response is essential for maneuvering through a dynamic world - for dodging a predator or chasing down prey, swinging through the trees or fighting off disease - and it is itself dynamic. As we go about our days, Dr. McEwen said, the biochemical mediators of the stress response rise and fall, flutter and flare. "Cortisol and adrenaline go up and down," he said. "Our inflammatory cytokines go up and down."
The target organs of stress hormones likewise dance to the beat: blood pressure climbs and drops, the heart races and slows, the intestines constrict and relax. This system of so-called allostasis, of maintaining control through constant change, stands in contrast to the mechanisms of homeostasis that keep the pH level and oxygen concentration in the blood within a narrow and invariant range.
Unfortunately, the dynamism of our stress response makes it vulnerable to disruption, especially when the system is treated too roughly and not according to instructions. In most animals, a serious threat provokes a serious activation of the stimulatory, sympathetic, "fight or flight" side of the stress response. But when the danger has passed, the calming parasympathetic circuitry tamps everything back down to baseline flickering.
In humans, though, the brain can think too much, extracting phantom threats from every staff meeting or high school dance, and over time the constant hyperactivation of the stress response can unbalance the entire feedback loop. Reactions that are desirable in limited, targeted quantities become hazardous in promiscuous excess. You need a spike in blood pressure if you're going to run, to speedily deliver oxygen to your muscles. But chronically elevated blood pressure is a source of multiple medical miseries.
Why should the stressed brain be prone to habit formation? Perhaps to help shunt as many behaviors as possible over to automatic pilot, the better to focus on the crisis at hand. Yet habits can become ruts, and as the novelist Ellen Glasgow observed, "The only difference between a rut and a grave are the dimensions."
It's still August. Time to relax, rewind and remodel the brain.
如果你能在大衛(wèi)林奇經(jīng)濟(jì)狀況下呆上幾個月，這的房屋自發(fā)燃燒，辦公室的人謎一般的消失，充斥著耳膜的是人們面對致人死地的工作計劃時發(fā)出的讓人窒息的呻吟聲，這時候，你身體的壓力反應(yīng)系統(tǒng)已經(jīng)啟動了自我修復(fù)機(jī)制，但最終還是自暴自棄。恭喜你，你很敏感。的確如此。
如果說長期的壓力導(dǎo)致血壓上升，動脈硬化，免疫系統(tǒng)抑制，高糖尿病患病率，抑郁，阿爾茨海默癥和使人變成惡性競爭者還不是太糟的話，現(xiàn)在研究者們發(fā)現(xiàn)長期處在壓力狀態(tài)下能夠使大腦重建來提高它的惡性的承受力。
今年初夏的科學(xué)雜志上報到，波多黎各Minho大學(xué)生命健康研究所的Nuno Sousa 和他的同伴描述了這樣一項實驗：在長期高壓下生活的大鼠失去了他們狡猾的本性，它們寧愿拒絕吃東西，也不會嘗試回到熟悉的路線和回憶曾經(jīng)通過死記硬背掌握的反應(yīng)，例如強(qiáng)制性的按食物盤子的按鈕。
另外，老鼠的行為干擾還反映為它們的基本神經(jīng)回路中出現(xiàn)了一對互補(bǔ)的改變。一方面，與大腦執(zhí)行決策制定和目標(biāo)導(dǎo)向行為有關(guān)的區(qū)域萎縮，另一方面，與習(xí)慣形成有關(guān)的大腦的區(qū)域卻增加了。
換句話說，嚙齒類目前被認(rèn)為先天喜歡重復(fù)的做同一件事，他們會在同一個死胡同似的大鼠田徑場上跑圈而不愿意尋找一個通往更方便的下水道處的輸油管道。"行為在有壓力的動物中比在對照組能夠更快的變成習(xí)慣，更糟的是，當(dāng)有更好的選擇時，有壓力的動物不能重新回到目標(biāo)導(dǎo)向行為的狀態(tài)。"Sousa博士說。"我管這個叫惡性循環(huán)。"
在斯坦福大學(xué)醫(yī)學(xué)院研究壓力的神經(jīng)學(xué)家Robert Sapolsky說，"這個模型可以很好的解釋為什么我們總是在刻板乏味的生活中混沌而終，并且在這種生活中越陷越深。"
事實上，Dr. Sapolsky 說，"當(dāng)我們標(biāo)準(zhǔn)的應(yīng)付模式玩不轉(zhuǎn)時，我們在認(rèn)知方面簡直是一團(tuán)糟。相比于思考，在做一件事時我們自發(fā)的反應(yīng)會重復(fù)這件事5次，可能是時候試試新玩意了。"
盡管恒心毅力是個令人羨慕的品質(zhì)并且是成功必不可少的要素，但是一旦過度它就變成了固執(zhí)--不可控制的重復(fù)--或者簡單的堅持。"如果我正在嘗試進(jìn)入現(xiàn)代舞的世界--在開始的幾次失敗之后，邏輯反應(yīng)可能是更多的練習(xí)。Dr. Sapolsky說，他是"為什么斑馬不得潰瘍"一書的作者。"但是在遭到第12000次拒絕以后，可能我會認(rèn)識到這對我來說不是一個有前景的職業(yè)。"
很高興，行為和大腦中壓力誘導(dǎo)的的改變可能能夠恢復(fù)。為了使大鼠的壓力反應(yīng)達(dá)到最活躍點，研究者讓實驗動物在不同的壓力環(huán)境中生存如溫和的機(jī)械振動，和吵鬧的大鼠關(guān)在同一只籠子里，延長在水中浸泡的時間。將長期壓抑的動物和沒有壓力的動物相比。受到壓力的大鼠在學(xué)習(xí)某些任務(wù)時，諸如按按鈕取得食物或者從一個鞘里得到糖水，但是他們在決定何時停止壓按鈕的時候比普通的大鼠要困難。
但是在為期四周沒有欺負(fù)沒有噪聲的假期后， 從前受到壓力的大鼠看起來很像對照組，能夠創(chuàng)造，辨別和停止按按鈕。大腦前額皮層的決定區(qū)域內(nèi)萎縮的突出連接重新生長，而習(xí)慣傾向感應(yīng)紋狀體中過度生長的樹突則萎縮了。
根據(jù)洛克菲勒大學(xué)神經(jīng)內(nèi)分泌實驗室的首席科學(xué)家Bruce S. McEwen的研究，新的發(fā)現(xiàn)為研究者剛剛開始研究的理論提供了優(yōu)異的解釋。"大腦是一個安靜有可塑性的器官，"他說。樹突和突觸可以回縮和重建，可回復(fù)的模型在整個生命進(jìn)程中都可發(fā)生。
壓力可能與后工業(yè)社會快節(jié)奏的生活有關(guān)，但是身體的壓力反應(yīng)系統(tǒng)自古便已存在。它的基本體系結(jié)構(gòu)，與神經(jīng)和分泌刺激抑制的荷爾蒙的內(nèi)分泌器官相連的網(wǎng)狀結(jié)構(gòu)和其他的必須的因素，在金魚和紅斑蠑螈中與在人類中看起來沒什么兩樣。
壓力反應(yīng)系統(tǒng)對于在動力學(xué)世界中生存是必不可少的---對于逃避捕獵者，或追捕獵物，在樹叢里搖來搖去或者抗擊疾病--它自身就是一種動力系統(tǒng)。在今天，McEwen博士說，壓力系統(tǒng)的生化干擾因素升高和降低。"皮質(zhì)醇和腎上腺素升高和降低，"他說。"我們的煽動性的細(xì)胞因子升高和降低。"
壓力荷爾蒙的目標(biāo)器官類似于和著節(jié)拍起舞：血壓升高和降低，心臟跳得快跳得慢，腸胃緊縮和放松。這個系統(tǒng)叫做拮抗平衡，即在永恒的變化中保持平衡，與將pH值和血液中氧氣的濃度保持在一個狹窄不變的范圍內(nèi)的體內(nèi)平衡機(jī)制是相反的。
不幸的是，我們的壓力控制系統(tǒng)的動力機(jī)制使之變得容易被打破，尤其是當(dāng)其超負(fù)荷工作時。在大多數(shù)動物中，嚴(yán)重的威脅存生壓力反應(yīng)系統(tǒng)刺激，同情和"或打或逃"功能的激活。但是當(dāng)危險過去，平靜的副交感神經(jīng)通路將所有的反應(yīng)都清零。
在人類中，盡管大腦可以思考，將陰魂不散的恐懼和與他人的見面，高校舞會分開，壓力反應(yīng)系統(tǒng)長時間處于高活力狀態(tài)會將整個反饋的環(huán)打破。可取的范圍內(nèi)和目標(biāo)量以內(nèi)的反應(yīng)在濫用的情況下會成為災(zāi)難。當(dāng)人們跑步的時候，身體為了更快的向肌肉中輸送氧氣，會使血壓達(dá)到峰值。但如果血壓長期屬于升高狀態(tài)，有可能引起一系列的疾病。
為什么有壓力的頭腦傾向于形成某種習(xí)慣？可能是為了將盡量多的行為轉(zhuǎn)變成自動發(fā)出的，從而更好的面對危機(jī)。然而習(xí)慣可能b變成使人裹足不前的絆腳石。就像小說家Ellen Glasgow觀察到的那樣，"在停止和死亡之間的唯一不同就是他們的程度。
