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NEW YORK — It was a staple of medical thinking dating to the 1910s that stress was the body’s alarm system, switching on only when terrible things happened, often leaving a person with an either-or choice: fight or flight.

The neuroscientist Bruce S. McEwen trailblazed a new way of thinking about stress. Beginning in the 1960s, he redefined it as the body’s way of constantly monitoring daily challenges and adapting to them.

Dr. McEwen, who died on Jan. 2 at 81, described three forms of stress: good stress — a response to an immediate challenge with a burst of energy that focuses the mind; transient stress — a response to daily frustrations that resolve quickly; and chronic stress — a response to a toxic, unrelenting barrage of challenges that eventually breaks down the body.

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It was Dr. McEwen’s research into chronic stress that proved groundbreaking. He and his research team at Rockefeller University in Manhattan, where he received his doctorate in 1964, discovered in 1968 that stress hormones had a profound effect on the brain.

In studies using animals (five rats in the initial one), Dr. McEwen and his colleagues demonstrated that toxic stress atrophied neurons near the hippocampus, the brain’s memory and learning center. Their findings also paved the way for a later discovery by other scientists: that toxic stress also expands neurons near the amygdala, an area of the brain that promotes vigilance toward threats.

Their discoveries, published in the journal Nature in 1968, ignited a new field of research that would reveal how stress hormones and other mediators change the brain, alter behavior, and impact health, in some cases accelerating disease.

By the end of his career Dr. McEwen had expanded his research to look at the impact of stress on communities, finding that chronic stress disproportionally affected marginalized people and increased their risk of illnesses.

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At his death — caused by complications of a stroke, a Rockefeller spokeswoman said — Dr. McEwen was the Alfred E. Mirsky professor and head of the Harold and Margaret Milliken Hatch Laboratory of Neuroendocrinology at the university.

He published more than 1,000 scientific articles, and his work has been cited in others more than 130,000 times. He also wrote popular books.

Bruce Sherman McEwen was born on Jan. 17, 1938, in Fort Collins, Colorado. He leaves his wife, Karen Bulloch, a Rockefeller research associate professor with whom he often collaborated; a brother, Craig McEwen, a professor emeritus of sociology at Bowdoin College; two daughters, Carolyn McEwen and Sarah McEwen Kelly; two stepchildren, Kimberly McGrath and Scott Muryasz; and eight grandchildren.