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大脑是恶性压力循环的罪魁祸首

时间:2010-07-29 13:07    来源:    作者: 点击:
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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.

    要是你能在年夜卫林奇经济状态下呆上几个月,这的房屋自发燃烧,办公室的人谜一样寻常的消散踪,布满着耳膜的是人们面对致人作古地的事项打算时发出的让人梗塞的呻吟声,这时辰,你身材的压力回响体系已经启动了自我修复机制,但终极照样安于现状。恭喜你,你很敏感。切实其实云云。

    要是说耐久的压力导致血压上升,动脉硬化,免疫体系按捺,高糖尿病染病率,烦闷,阿尔茨海默症和使人变成恶竞争者还不是太糟的话,此刻研究者们发明耐久处在压力状况下能够使年夜脑重修来进步它的恶的遭受力。

    今岁首年月夏的科学杂志上报到,波多黎各Minho年夜门生命康健研究所的Nuno Sousa 和他的火伴描写了这样一项尝试:在耐久高压下糊口的年夜鼠失踪去了他们奸狡的天性,它们甘愿拒绝吃对象,也不会考试测验回到熟习的蹊径和回想曾经经由过程作古记硬背把握的回响,譬喻逼迫性的按食品盘子的按钮。

    此外,老鼠的举动滋扰还回响反映为它们的根基神经回路中呈现了一对互补的变化。一方面,与年夜脑实行决意妄想拟定和方针导向举动有关的地区萎缩,另一方面,与风俗形成有关的年夜脑的地区却增进了。

    换句话说,啮齿类今朝被以为天赋喜欢一再的做同一件事,他们会在同一个作古胡同似的年夜鼠田径场上跑圈而不乐意探求一个通往更便利的下水道处的输油管道。"举动在有压力的动物中比在比较组能够更快的变成风俗,更糟的是,当有更好的选择时,有压力的动物不能从头回到方针导向举动的状况。"Sousa博士说。"我管这个叫恶性轮回。"

    在斯坦福年夜学医学院研究压力的神经学家Robert Sapolsky说,"这个模型可以很好的评释为什么我们老是在刻板乏味的糊口中混沌而终,并且在这种糊口中越陷越深。"

    毕竟上,Dr. Sapolsky 说,"当我们标准的应付模式玩不转时,我们在认知方面简直是一团糟。对比于思虑,在做一件事时我们自发的回响会一再这件事5次,年夜概是时辰试试新玩意了。"

    只管恒心毅力是个令人恋慕的品格并且是乐成必不成少的要素,可是一旦太过它就变成了坚定--不成节制的一再--可能年夜略的坚持。"要是我正在考试测验进入当代舞的天下--在起头的屡次失踪败之后,逻辑回响年夜概是更多的操练。Dr. Sapolsky说,他是"为什么斑马不得溃疡"一书的作者。"可是在遭到第12000次拒绝往后,年夜概我会了解到这对我来说不是一个有远景的职业。"

    很开心,举动和年夜脑中压力诱导的的变化年夜概能够规复。为了使年夜鼠的压力回响到达最活泼点,研究者让尝试动物在差此外压力环境中保留如温顺的机器振动,和嘈吵的年夜鼠关在同一只笼子里,延迟在水中浸泡的时刻。将耐久抑制的动物和没有压力的动物对比。受到压力的年夜鼠在进修某些义务时,诸如按按钮取得食品可能从一个鞘里获得糖水,可是他们在抉择何时遏制压按钮的时辰比平常的年夜鼠要坚苦。

    可是在为期附近没有侮辱没有噪声的假期后, 畴前受到压力的年夜鼠看起来很像比较组,能够缔造,鉴识和遏制按按钮。年夜脑前额皮层的抉择地区内萎缩的凸起毗邻从头发展,而风俗偏向感到纹状体中太过发展的树突则萎缩了。

    按照洛克菲勒年夜学神经内分泌尝试室的首席科学家Bruce S. McEwen的研究,新的发明为研究者刚刚起头研究的理论供应了优秀的评释。"年夜脑是一个惬意沉着荒僻默默有可塑性的器官,"他说。树突和突触可以回缩和重修,可回覆的模型在整个生命进程中都可产生。

    压力年夜概与后家产社会快节奏的糊口有关,可是身材的压力回响体系自古便已存在。它的根基系统结构,与神经和分泌刺激按捺的荷尔蒙的内分泌器官相连的网状结构和其他的必需的身分,在金鱼和红斑蝾螈中与在人类中看起来没什么两样。

    压力回响体系对付在动力学天下中保留是必不成少的---对付隐匿捕猎者,或追捕猎物,在树丛里摇来摇去可能抗击疾病--它自身便是一种动力体系。在本日,McEwen博士说,压力体系的生化滋扰身分升高和低落。"皮质醇和肾上腺素升高和低落,"他说。"我们的怂恿性的细胞因子升高和低落。"

    压力荷尔蒙的方针器官近似于和着节拍起舞:血压升高和低落,心脏跳得快跳得慢,肠胃缩短和放松。这个体系叫做拮抗平衡,即在永恒的厘革中贯串毗邻平衡,与将pH值和血液中氧气的浓度贯串毗邻在一个狭小不乱的领域内的体内平衡机制是相反的。

    不幸的是,我们的压力节制体系的动力机制使之变得轻易被冲破,尤其是当其超负荷事项时。在年夜年夜都动物中,主要的威胁存生压力回响体系刺激,恻隐和"或打或逃"成果的激活。可是当伤害已往,惬意的副交感神经通路将悉数的回响都清零。

    在人类中,只管年夜脑可以思虑,将阴魂不散的惊骇和与他人的见面,高校舞会分隔,压力回响体系永劫间处于高活力状况会将整个反馈的环冲破。可取的领域内和方针量以内的回响在滥用的情形下会成为灾祸。当人们跑步的时辰,身材为了更快的向肌肉中运送氧气,会使血压到达峰值。但要是血压耐久属于升高状况,有年夜概引起一系列的疾病。

    为什么有压力的脑子偏向于形成某种风俗?年夜概是为了将只管即便多的举动变化成自动发出的,从而更好的面对危急。然而风俗年夜概b变成使人缠足不前的绊脚石。就像小说家Ellen Glasgow不雅察看到的那样,"在遏制和衰亡之间的独一差别便是他们的程度。




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