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Why does our brain shrink?

We know that the human brain atrophies and loses faculties over the years, but does the same thing happen in all species? A comparative study with chimpanzee brains shows that we are the only primates that suffer this wear. Having bigger brains and living longer could have a price for our minds.

The human brain shrinks. Even in healthy individuals, those who do not suffer from any neurodegenerative disease such as the feared Alzheimer’s, this gradual reduction in size is recorded from the age of 25 and the process accelerates after 50. Despite what the myth says, It is not a massive loss of neurons, but changes in the microstructure of these cells and the dendritic connections of the cerebral cortex. This atrophy is located mainly in the frontal lobe and the hippocampus, the area where memories are fixed, so that it has direct consequences in faculties such as our ability to reason, mental speed or episodic memory.

For their analysis, the scientists scanned with magnetic resonance the brains of 99 chimpanzees (with ages between 10 and 51) and 87 humans (aged between 22 and 88 years). Unlike what happens with human brains, which show a decrease in volume in all structures over the years, the brains of chimpanzees did not show significant changes related to age.

The rates of change in human brains, however, are important. Some regions shrink up to 25% at the age of 80 years. The shrinkage of gray matter in the frontal lobe, for example, is around 14%, and 13% in the hippocampus, while in the so-called ‘white matter’, the frontal lobe can be reduced to a 24%

Why do these differences occur? Scientists had already done similar studies with rhesus monkeys, but this is the first time that it has been compared in detail with a superior primate such as chimpanzees. All the clues point to an evolutionary origin and to the increase of longevity, we live proportionally more than chimpanzees and we have a brain up to three times bigger with a considerably higher energy demand.

The human brain consumes 25% of the energy available to the rest of the body, compared to 10% consumed by a chimpanzee’s brain. This increased energy demand has some consequences, such as the deterioration of mitochondrial efficiency or increased oxidative stress, a metabolic change that, in the long term, would explain the atrophy of our brains as we get older.

“My impression is that the neurons do basically all the best they can to maintain maximum function for as long as possible,” says Chet Sherwood in Science. ‘But age accumulates against them after years of high energy consumption’.

Exogenous causes

Other studies on brain aging in healthy subjects point to the loss of connectivity due to natural factors. ‘Theoretically, up to 40% of this connectivity is lost only due to the aging process’, explains the professor of Physiology at the UCM, Francisco Mora. ‘And this is the limit at which one can maintain normal mental faculties’. But these studies have been questioned, says Mora, ‘for not taking into account something as important as environmental factors’.

That the habits of life can shrink the brain and atrophy it, beyond age, has been amply proven. Disorders such as depression, insomnia, or habits such as prolonged extreme vegetarianism, have also been shown to shrink the brain. Among these factors, drug use is one of the most important causes of atrophy.

‘The morphological alterations produced by alcohol in the brain’, explains Koldo Callado, researcher at the brain bank of the University of the Basque Country, ‘they are very similar to Alzheimer’s, with alterations in the grooves, cortical atrophy, etc. ‘ We also know that ecstasy, or designer drugs, as Callado points out, produces destruction of neurons. “And that is seen when treating animals and applying neuroimaging,” he says. ‘We see how brain volume is reduced and certain types of neurons are lost’.

Among the external factors that have modified our brain are also considered anthropological and evolutionary reasons. A hypothetical benefit of the prolongation of life after the reproductive period in humans, explains the study, is that the grandparents could have played an essential role in the care of the offspring. “These adaptive solutions,” says the study, “also created a unique framework in which humans developed a progressive neuronal aging greater than that of our primate relatives.”

With all these elements, our species evolutionarily developed a much larger and metabolically demanding brain, in addition to a long life expectancy. These factors have given decisive advantages to humans, concludes the work, but finally these adaptations also have a ‘cost’ in the form of greater neurodegeneration. References: Aging of the cerebral cortex differs between humans and chimpanzees (PNAS) See documentary ‘The evil of the brain’

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