Like the saying, "You can't teach an old dog new tricks," the aging human brain has a tough time learning from new experiences, suggests a study on rats showing tiny brain-cell structures needed for this process get quite rigid in their twilight years.

Rats are generally reliable models for human brain studies, so the results should hold for us, the researchers say.

The researchers looked at the prefrontal cortex, the brain region that controls various cognitive processes and plays a role in higher learning. They knew that brain cells in the prefrontal cortex of young animals are really flexible, or plastic. Life experiences, particularly those that involve learning, can profoundly alter the circuitry in this brain region. [10 Things You Didn't Know About the Brain]

For example, stress causes nerve cells to shrink and lose synapses, or the connections between nerve cells where communication occurs. Once the stressful experience ends, these brain cells recover — they are plastic, flexible — or at least they do in young animals.

Stressed brain

To find out how stress affects this plasticity in aging brains, the researchers exposed young, middle-age and old rats to a stressor known to elicit nerve cell changes in the prefrontal cortex.

After stressing out the rats, the researchers looked at close-up images of structures on nerve cells called spines that form synapses and are critical for learning. These spines "are modified when you learn something," said study researcher John Morrison, a professor of neuroscience at Mount Sinai School of Medicine. "In a sense, that's where learning occurs."

In the young rats, the brain cells lost many of their spines, which grew back after a stress-free period. However, in middle-age and old rats, the spines didn't change at all. Another change seen due to stress was a shortening of branchlike projections on neurons called dendrites. And while these dendrites recovered in young rats, they didn't in the aging rodents.

"The way we interpret that is that with aging you lose a lot of the capacity to have experience-induced plasticity," Morrison told LiveScience, adding that learning is the classic example of this type of plasticity. "So we think this gives us a really good working model for why with age you have these cognitive declines and impaired learning."

They suspect the problem occurs when a rat, or person, loses these spines as they age; the ones that go are the spry spines with lots of plasticity, leaving the more rigid ones behind. These spines can't effectively respond to stress or learning, he said.

Cognitive decline

That lack of rewiring ability may be responsible for cognitive decline in aging adults, he added.

He said that this type of study is important because it may reveal changes in brain cells that occur in an early stage of Alzheimer's disease, before the neurons actually die. It's at this early stage where doctors would want to intervene and treat the cognitive decline before it's too late, he said.

In fact, no other animal except humans show naturally occurring Alzheimer's; in animal models of the disease, researchers must modify the rats or monkeys to induce Alzheimer's.

The research is detailed in the May 25 issue of the Journal of Neuroscience.

Follow LiveScience for the latest in science news and discoveries on Twitter @livescience and on Facebook.