LITMIR.BIZ

Скачать книгу

run in order to get the kinks worked out and prepare it for more serious crises later in life. A child is dropped off at daycare, or gets a booster shot, or pinches his finger in a cupboard. He cries, his heart rate rises, cortisol circulates through his bloodstream. A few moments later his mother comforts him, and after a bit of coddling and gentle reassurance, the child is tottering happily through the house again, his discomfort forgotten and his cortisol levels back to baseline.

      The second level, called tolerable stress, occurs when a child undergoes a significant traumatic event. She breaks her leg, survives a major earthquake, or loses her grandmother to cancer. These experiences are not part of an ideal childhood, but they aren’t prescriptions for lifelong hardship, either. If they occur infrequently, and if the child has in her life a supportive and nurturing adult presence to help her through it, she’ll almost certainly recover with no negative long-term effects.

      However, when stress is severe and ongoing and the necessary parental support is not present, the stress becomes toxic. Aside from extreme circumstances like war and famine, the parental support, or lack thereof, is really the key distinction between tolerable and toxic stress. For the first six months of a child’s life, the HPA axis isn’t very reliable. It needs to learn to do its job, and it can’t do so if it’s constantly bludgeoned with stressful stimuli at levels that would challenge even a grown adult. Mastering hormonal self-regulation under conditions of toxic stress would be like trying to teach a child the rules of touch football by pitting him against an NFL linebacker. It’s not a fair fight. Children need shelter from the storm of stress, and parents are the ones to provide it. For the first six months of your child’s life, you are the HPA axis. Any stress they experience filters through you first, and the better equipped your own stress response system is for metabolizing adverse stimuli, the more effectively you’ll be able to shield your child from the emotional fallout of life’s many traumas.

      When a parent isn’t up to the task, due either to their own malfunctioning HPA axis or an overpowering flood of adverse stimuli (no parent, no matter how well-adjusted, could protect their child from the abominable brunt of the Holocaust if its nefarious agents kicked down their door), toxic stress bleeds through. And as its name suggests, toxic stress is dangerous stuff. It poisons children, weakening their immune systems and adversely affecting their emotional, physical, and intellectual development. Children raised in environments rife with toxic stress suffer disproportionately from a vast and deeply troubling array of conditions. Worst of all, a hyperactive stress response system undermines the body’s attempts to build a strong neural framework. Cortisol is neurotoxic, meaning it kills brain cells. If excess cortisol is present in infancy, the time when brain cells are rapidly growing and connecting, the brain doesn’t develop as it should. It becomes weaker, its neurons less densely packed, its operational capacity compromised.

      The Neural Garden

      Infancy and early childhood encompass a period of unparalleled brain growth. Neurons are rapidly generated by neuronal stem cells, and the connections between them multiply frantically. Each neuron releases a mad tangle of tiny filaments called dendrites, microscopic cables connecting one neuron to the next. The brain grows thick with them, a jungle canopy teeming with electrochemical impulses. And as in the jungle, Darwinian selection looms ever-present. When neural connections are used often they flourish, their wispy dendrites growing into sturdy trunks. Meanwhile, dendrites that go unused atrophy — a process called pruning — until the jungle of neural connections becomes sparser and more cultivated, a garden of the mind.

      This in itself is not cause for concern; dendritic pruning is an integral part of neurological development. It might seem somewhat inefficient — why waste precious energy growing dendrites when you’re just going to till them under? — but an initial surplus of synapses (or neural connections) gives our brains a certain amount of wiggle room during our early years. We call this feature of our brains “neural plasticity,” and it is vital to our ability to adapt to diverse and changing environments. By providing a dense network of synapses, our brains allow for every contingency. Once in place, the connections go on a kind of probation. If a dendrite is used often, the brain assumes it must be valuable and reinforces it. If a connection is used rarely or not at all, the brain assumes our circumstances don’t require that particular informational pathway and lets it wither. By the time we are adolescents, our brains have turned from lumps of unformed clay into highly individualized sculptures, shaped by the deft and meticulous hands of our early rearing environment. It is for this reason that children pick up new skills — especially languages — with an ease that leaves most adults flummoxed. Their brains are more pliant than ours and new abilities can be incorporated easily into the folds of their neocortex, while our synapses are far more set in their ways.

      Dendritic pruning is a highly effective early learning system, but it has one major drawback. Growing all those dendrites takes a lot of energy. Under normal conditions this isn’t a problem, but if some other part of the body starts demanding the bulk of the resources — an overactive stress response system, say — then the dendrites aren’t able to grow at an ideal rate. And if the stress response system continues to tap the body’s energy reserves at an unsustainable rate, many useful dendrites will wither alongside their unused peers. Those that do manage to survive will languish in impoverished neural soil, their stalks sickly and weak. To make matters worse, an environment rife with toxic stress isn’t likely to provide children with a lot of intellectual stimulation; not only is the child starved for physical resources, but the environment is simultaneously failing to stimulate his brain in a meaningful way. With both biological and sociological factors stacked against them, these children are effectively fighting a war on two fronts. Nurture bombards them with adverse stimuli while nature besieges them, cutting them off from the resources they need to feed their growing brains.

      Rodent Rules

      In 1957, a young researcher named Seymour Levine observed a strange phenomenon among the rats in his laboratory. He took newborn rat pups and, at regular daily intervals for the first several weeks of their lives, separated them from their mothers for 15-minute periods. The separated mice spent their brief diasporas exploring the confines of a small, unadorned cage. When the 15 minutes were up they were returned unharmed to their mothers. This procedure is called handling, and though it may not seem like much of an occurrence, it has a significant impact on a rat’s stress response system. Handled pups, when compared to their non-handled peers, react to stressful stimuli in a far more relaxed manner. The effect is both behavioural and physiological. When presented with a novel environment, handled rats are much calmer than non-handled rats, and their cortisol levels don’t surge nearly as high.

      The opposite effect can also be achieved. Should you for some reason — be it scientific or sadistic — want to produce rats with highly sensitive stress response systems, simply separate them from their mothers for more prolonged periods. An hour or two will do it. Repeat the process — called, appropriately enough, “maternal separation” — a dozen or so times, and the rats in question will grow into neurotic, fretful creatures rife with cortisol, their HPA axes pumping loads of the stuff into their bloodstreams long after their handled brethren have already returned to homeostasis.

      A number of established scientists repeated Levine’s results. As the studies piled up, not only was the efficacy of handling and maternal separation confirmed, but the reason why it worked became clear. Handling and maternal separation weren’t changing anything about the rats directly. Rather, these practices changed the way their mothers treated them. The real cause was a series of affectionate and nurturing actions collectively called licking, grooming, and arched-back nursing[34] behaviour, or LG-ABN. Rat dams (mothers) who engaged in high LG-ABN — those who licked, groomed, and nursed their offspring more often than usual — produced rat pups with tempered, easy-going stress response systems. Meanwhile, low LG-ABN mothers — those who spent little time licking and grooming their young, and made little effort to nurse them — produced jittery, easily agitated offspring with hyperactive stress response systems. Handling, it turns out, increases LG-ABN behaviour, while maternal separation decreases it. Levine’s findings remained accurate, but the context surrounding them suddenly changed. Handling and maternal separation were catalysts, not causes.

      LG-ABN behaviour is highly heritable. When the pups became mothers themselves, those raised by low LG-ABN

Скачать книгу