Healthy Minds for Life A Message from Lee Ryan, Professor and Head of the Psychology Department at the University of Arizona The Ever-Adapting Brain
As we age, we lose thousands of brain cells every day which cannot be replaced. At least, that was the prevailing view of science for more than 40 years. The idea came from several influential research studies published in the early 1950s that demonstrated significant cell death in the brains of aging humans and animals alike. According to these researchers, older adults lose up to 50% of neurons throughout the brain, including the hippocampus, the brain region that is critically important for learning and memory. Thankfully, the scientists were wrong. It took another 40 years for new and more accurate methods to be developed for counting cells in brain tissue. Once these methods were applied to studies of the aging brain in the early 1990s, the picture changed quite dramatically. These studies led to the somewhat surprising conclusion that neurons – those important brain cells that code and store information -- simply do not decline in large numbers as we get older. Instead, we now understand that the typical human loses no more than 10% of neurons across their entire lifespan – even into their 80s and beyond! Equally surprising, it appears that neuron cell loss occurs even less within the hippocampus. I should be clear that we’re talking about older adults without Alzheimer’s disease. Alzheimer’s occurs in about 1 out of 9 adults over the age of 65, and results in extensive loss of neurons, beginning in the hippocampus and spreading throughout the brain. Their loss is due to a disease process that deposits abnormal proteins in and around brain cells resulting first in their dysfunction, and ultimately their death.
But let’s get back to normal aging! Most older adults, more than 85%, will not develop Alzheimer’s disease in their lifetime. However, about a third of those individuals will experience memory changes that range from mild annoying slips of memory, to memory problems that are sufficiently severe that they interfere with quality of life and, potentially, independence. Now we have a mystery. On the one hand, I just told you that brain cells – neurons – don’t die off in the aging brain, especially in the hippocampus. On the other hand, we know that most older adults experience some degree of memory problem as they age. So, if it’s not loss of brain cells, then what accounts for age-related memory problems? Let me introduce Dr. Carol Barnes, a world-renowned neuroscientist who has studied the aging brain for more than 30 years. She is a Regent’s Professor at the University of Arizona and a member of the National Academy of Sciences. Her pioneering research has provided insights into what is really happening in the brain that can account for age-related memory changes. I’ll just highlight two of her important discoveries. First, like other researchers studying aged rats, while the number of neurons are not changing much in the brains of aged animals, the number of connections between neurons – allowing one neuron to pass information on to another – are decreasing in number. Not fewer neurons, but smaller neurons with fewer branches connecting to surrounding neurons. Importantly, however, using highly sophisticated electrophysiological methods, Barnes found that the ability
of those neurons to pass on information doesn’t decrease. In other words, although there are fewer connections in the aged brain, the remaining connections are more powerful. Aged neurons are adapting and becoming more efficient, offsetting the loss of connections between them. Second, Barnes also found that maintaining connections between neurons over long periods of time changes as we age. For example, every time a young rat moves through a familiar environment, the same neurons signal one another in a similar way, telling the animal that they’ve been there before. In aged rats, however, the connections between neurons are unstable. Put an older rat back in a familiar environment, and a new set of neurons may begin to signal one another. It’s as if the aged animal has never been in that place before. Barnes suggests that this instability in our neural connections over time is a major reason for memory problems in both older rats, and likely, older humans. It’s a myth that we lose massive amounts of brain cells as we age. Instead, we’ve learned that the aging brain has an incredible ability to adapt to changes in brain structure, becoming more efficient and stronger, even as the number of connections decrease. It appears that the brain changes leading to memory problems are very subtle. This kind of detailed understanding of the aging brain is critically important, because it can lead to meaningful interventions that can prevent or treat age-related memory problems.
May/June 2024, Never Too Late | Page 9
Pima Council on Aging
Made with FlippingBook - Share PDF online