This is what happens inside children when they are forcibly separated from their parents.

Their heart rate goes up. Their body releases a flood of stress hormones such as cortisol and adrenaline. Those stress hormones can start killing off dendrites — the little branches in brain cells that transmit mes­sages. In time, the stress can start killing off neurons and — especially in young children — wreaking dramatic and long-term damage, both psychologically and to the physical structure of the brain.

“The effect is catastrophic,” said Charles Nelson, a pediatrics professor at Harvard Medical School. “There’s so much research on this that if people paid attention at all to the science, they would never do this.”

That research on child-parent separation is driving pediatricians, psychologists and other health experts to vehemently oppose the Trump administration’s new border crossing policy, which has separated more than 2,000 immigrant children from their parents in recent weeks.

The American Academy of Pediatrics, the American College of Physicians and the American Psychiatric Association have all issued statements against it — representing more than 250,000 doctors in the United States. Nearly 7,700 mental-health professionals and 142 organizations have also signed a petition urging President Trump to end the policy.

“To pretend that separated children do not grow up with the shrapnel of this traumatic experience embedded in their minds is to disregard everything we know about child development, the brain, and trauma,” the petition reads.

Nelson has studied the neurological damage from child-parent separation — work that he said has often reduced him to tears.

In 2000, the Romanian government invited Nelson and a team of researchers into its state orphanages to advise them on a humanitarian crisis that the country’s previous policies had created.

For decades, Romania’s communist dictator Nicolae Ceausescu had banned birth control and abortions, and imposed a “celibacy tax” on families with fewer than five children. Ceausescu believed that ratcheting up the country’s birthrate would boost Romania’s economy. Instead, the government ended up opening massive state-run orphanages to deal with more than 100,000 children whose parents couldn’t afford to raise them.

At those orphanages, Nelson said, “we saw kids rocking uncontrollably and hitting themselves, hitting their heads against walls. It was heartbreaking. We had to make up a rule for ourselves as researchers that we would never cry in front of the children. Whenever one of us felt ourselves tearing up, we would walk out of the room.”

As the children grew older, Nelson and his colleagues began finding unsettling differences in their brains.

DO NOT USE THIS IMAGE. THIS IMAGE IS FOR ONE TIME USE WITH AMP-STORIES ONLY. MCALLEN, TX - JUNE 29: Karina Lopez's one year old daughter hugs her mother after they were detained by Border Patrol after crossing illegally into the United States on Thursday, June 29, 2017, in McAllen, TX. The mother and daughter with her 16 year old niece were coming to the U.S. in search of a better life. (Photo by Salwan Georges/The Washington Post)

Those separated from their parents at a young age had much less white matter, which is largely made up of fibers that transmit information throughout the brain, as well as much less gray matter, which contains the brain-cell bodies that process information and solve problems.

The activity in the children’s brains was much lower than expected. “If you think of the brain as a lightbulb,” Nelson said, “it’s as though there was a dimmer that had reduced them from a 100-watt bulb to 30 watts.”

The children, who had been separated from their parents in their first two years of life, scored significantly lower on IQ tests later in life. Their fight-or-flight response system appeared permanently broken. Stressful situations that would usually prompt physiological responses in other people — increased heart rate, sweaty palms — would provoke nothing in the children.

What alarmed the researchers most was the duration of the damage. Unlike other parts of the body, most cells in the brain cannot renew or repair themselves.

The reason child-parent separation has such devastating effects is because it attacks one of the most fundamental and critical bonds in human biology.

From the time they are born, children emotionally attach to their caregiver and vice versa, said Lisa Fortuna, medical director for child and adolescent psychiatry at Boston Medical Center. Skin-to-skin contact for newborns, for example, is critical to their development, research shows. “Our bodies secrete hormones like oxytocin on contact that reinforces the bond, to help us attach and connect,” Fortuna said.

A child’s sense of what safety means depends on that relationship. And without it, the parts of the brain that deal with attachment and fear — the amygdala and hippocampus — develop differently. The reason such children often develop PTSD later in life is that those neurons start firing irregularly, Fortuna said. “The part of their brain that sorts things into safe or dangerous does not work like it’s supposed to. Things that are not threatening seem threatening,” she said.

Research on Aboriginal children in Australia who were removed from their families also showed long-lasting effects. They were nearly twice as likely to be arrested or criminally charged as adults, 60 percent more likely to have alcohol-abuse problems and more than twice as likely to struggle with gambling.

In China — where 1 in 5 children live in villages without their parents, who migrate for work — studies have shown that those “left-behind” children have markedly higher rates of anxiety and depression later in life.

Other studies have shown separation leading to increased aggression, withdrawal and cognitive difficulties.

“If you take the moral, spiritual, even political aspect out of it, from a strictly medical and scientific point of view, what we as a country are doing to these children at the border is unconscionable,” said Luis H. Zayas, a psychiatry professor at the University of Texas at Austin. “The harm our government is now causing will take a lifetime to undo.”