Washington ? Scientists have discovered how stress — in the form of emotional, mental or physical tension — physically reshapes the brain and causes long-lasting harm to humans and animals.

“Stress causes neurons (brain cells) to shrink or grow,” said Bruce McEwen, a neuroscientist at Rockefeller University in New York. “The wear and tear on the body from lots of stress changes the nervous system.”

He said that stress is “particularly worrying in the developing brain, which appears to be programmed by early stressful experience.”

Stress in early life, even in the womb, can later lead to undesirable changes in behavior and the ability to learn and remember. Other consequences may be substance abuse and psychiatric disorders, researchers said at a conference of neuroscientists in Washington this week.

“Prenatal stress can change the brain forever,” said Tallie Baram, a neurologist at the University of California, Irvine. “Stress changes how genes are expressed throughout life.”

Even short-term stress can be harmful, Baram said. She described her work with laboratory mice, which were immobilized for five hours and subjected to loud rock music. The ordeal reduced the number of delicate fibers that carry signals between neurons, an MRI brain scan of the stressed-out mice showed.

The experiment offered “insights into why some people are forgetful or have difficulty retaining information during stressful situations,” Baram said. She said that neuroscientists hope they’ll be able to “design drugs to prevent the damage due to stress.”

Long-lasting, chronic stress also physically affects the brain, according to Fred Helmstetter, a researcher at the University of Wisconsin, Milwaukee. After laboratory rats were tightly restrained for six hours daily for 21 days, without food or water, the animals’ hippocampus — a brain region involved in learning and memory — shrank by 3 percent.

Another researcher, Lauren Jones of the University of Washington in Seattle, found that rats subjected to 60 minutes of restraint and electric tail shocks lost their ability to decide which path in a maze to take to receive a reward.

“If uncontrollable stress disrupts rats’ abilities to adjust their behavior,” she said, “how influenced by stress are people’s frequent and complex daily decisions?”

Nim Tottenham, a neuroscientist at the Weill Cornell Medical School in New York, studied children adopted from orphanages abroad who suffered from anxiety and had difficulty controlling their emotions.

Brain scans showed that these children’s stressful upbringing increased activity in the amygdala, a region involved in emotion. “Adverse rearing environments can produce long-lasting changes in the ability to regulate emotion,” Tottenham said.