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Dean F. MacKinnon, M.D.

Dean F. MacKinnon M.D.

The Plastic Brain

How stress makes us stupid

Life molds our brains into ever more finely tuned instruments. Stress dulls them. Here's how that works.

The newborn emerges from the sensory deprivation chamber of the womb with little if any information established in the brain. The newborn brain is not only an empty vessel, ready to be filled with knowledge; it lacks the capacity to hold knowledge. The newborn brain has no paucity of neuronal connections. On the contrary, there are way too many indiscriminant connections. The capacity to hold information expands through the sculpting of permanent networks from the existing tangle of connections, while the application of this information to guide adaptive behavior requires fresh neurons along the way.

Unlike the heart, kidneys, muscles, and other vital organs, brains do not grow very much in size, relative to the rest of the body. The process of maturation begins when the infant brain begins to record patterns of association between sensations, actions, and changes in body states. Over a lifetime, one acquires a large set of such patterns. Thus one develops the stubborn stability of character and worldview that characterize middle age.

Neurobiologists used to think that an adult brain could produce no new neurons. They now know that new neurons and connections can be formed well into advanced adulthood, so in a healthy brain there may always be new cellular material with which to learn new things. This adds little to the size of the brain, but it provides the capacity for plasticity--the functional restructuring of neurons. The capacity for new learning is integral to adaptive living. As long as the capacity for plasticity exists, new links can be established, and thus add new twists to old memories.

Experience thus alters the structure of the brain. A moderate degree of arousal, in the form of dopamine, serotonin, and norepinephrine secreting neurons, enhances performance and information retention. Overarousal, on the other hand, may harm the brain.

Humans often seem to swim in a sea of stress. Excessive intensity and duration of arousal comprises the biological side of stress.

A classic laboratory experiment illustrates the point that the inescapable anticipation of threat produces stress. Two rats are kept isolated in identical, restrictive cages. They can do virtually nothing but spin a wheel with their forepaws. Each rat has an electrode on its tail that administers a shock. Each rat receives a series of simultaneous, identical shocks. However, one rat sees a light before the shock, and then may delay or diminish the shock by working the wheel. The other rat sees no cues and thus has no way to anticipate the shock or to influence its delivery.

Why don't both rats become stressed? At the start of the experiment, both rats likely experienced an unpleasant state of pain and alarm upon receiving the shock. The first rat has a finite period of time in which to be aroused to anticipate the shock, and has motivation to expend energy to control it. After the shock, the rat can relax until the next appearance of the cue.

Meanwhile, the other unfortunate, uncued rat has no way to anticipate the shock and no way to act on the anticipation of a shock, thus no cue to stand down from a state of arousal. There is no end point to the anticipation of a shock; hence the rat remains in a state of stress.

If this were a human experiment, of course, no shock would be necessary. Imagine you were placed in a restrictive cell. If you were told that you would remain there for a specific amount of time and were provided a clock, you might be bored and uncomfortable, but you would feel far less stressed than if you were told nothing and given no way to mark the hours.

Whether rat or human, stress induces your brain to tell your adrenal glands to secrete cortisol. Cortisol has widespread physiologic effects, including mobilization of glucose stores from the liver and diversion of amino acids from protein-building to energy consumption. In a state of truly prolonged threat, such as a famine or siege, the ready availability of physiologic resources may prove life saving even if maintenance of this state impairs long-term survival and quality of life.

Prolonged exposure to high levels of cortisol has physiologically damaging effects, however. Because cortisol inhibits the creation of new protein, tissue comprised of cells in which there is high turnover tends to waste and weaken. The immune system falters and stomach ulcers develop. The damage is magnified in the brain. Certain populations of overexcited neurons are vulnerable to toxicity as a direct result of overstimulation, and cortisol inhibits the growth of new neurons to take their place.

If the brain is inhibited from manufacturing new neurons and new connections, it can't adapt. If the brain can't adapt, that is, modify its responses to environmental challenges, then behavior becomes poorly suited to the circumstances. That is, you end up doing the wrong thing and the wrong time. This is how stress can make us stupid.


About the Author

Dean F. MacKinnon, M.D.

Dean F. MacKinnon, M.D. studies and treats affective disorders and teaches medical students at the Johns Hopkins University School of Medicine.


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