The New Brain

How your brain—and our understanding of it—are constantly changing.

Brain Wiring

After age 20 it's all downhill

If you have ever been flummoxed struggling to connect the half-dozen wires from your TV to your DVR correctly, you can appreciate the problem of wiring up the human brain.  The human brain is stuffed with approximately 100 billion neurons.  Each one of these neurons can have 10,000 to 100,000 synaptic connections on it formed from other neurons.  Every one of these countless connections must be attached precisely between the correct neurons in the brain to form functional circuits.  The sheer number of wires, called axons, that are required to connect 100 billion neurons into functional circuits is imponderable. 

Fortunately, there are passionate people who become so intrigued by an unanswered question, they cannot rest until they have found the answer.  These would be scientists, of course.  Lisbeth Marner and her fellow neuranatomists working in Denmark, set out to measure the total length of all the axons in the brains of 36 normal Danes (18 males and 18 females).    What they found is not only astonishing, but also informative about how our brain changes with age and how wiring in the male and female brain may differ. 

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They determined that the total length of axons in the human brain of a 20-year old male is 176,000 km!  For readers less familiar with the metric system, that's long enough to wrap around the equator of the earth four and a half times.  This measurement includes only the brain "wires" that are coated with electrical insulation, called myelin.  The researchers didn't attempt to measure the bare axons as well.  The female brain, naturally, has less total length of wiring than the male brain of the same age-149,00 km.  I say "naturally", because women's bodies (and brains) are smaller than men's bodies (and brains), so less wire is needed to connect them up. 

But as we age, we begin to lose axons.  By the age of 80, the total length of wiring in the male brain has shrunk to 97,200 km, and the total axon length in the aged female brain has decreased to 82,00 km.  Throughout life, between ages 20-80, the total length of axons decreases steadily with age, at a rate of 1% loss/year for both males and females.  Our brains age in this respect until by age 80, we have lost about 45% of the total length of myelinated axons our brain once had when we were 20.  This may explain why mental activity slows as we age.  Aged individuals can eventually get the correct answer to challenging mental tasks, but it takes them longer.  We also lose about 10% of our neurons in the cerebral cortex with age, explaining in part the loss of wiring. 

The loss of axons is fairly uniform throughout different regions of the brain.  The researchers suggest that this would preferentially disrupt more integrative mental functions, but especially the frontal lobe functions, because of the numerous back and forth connections between it and other lobes.  Weakening frontal lobe connectivity would result in reduced capacity to hold and manipulate information in immediate memory, less ability to sustain attention, and weakened decision making capability. 

If you are fretting the loss of axons with age, consider this.  You have already lived through a much more drastic loss in some regions of your brain than the 45% wiring you will lose between your 20s and 80s.  You are born with an estimated 200 million axons connecting the left and right hemispheres of your brain.  Researchers, LaMantia and Rakic at Yale University, who studied the developing rhesus monkey, found that 70% of these axons are lost by adulthood.  All throughout adolescence these axons are lost at an astonishing rate of 5 axons/sec.  During the early period of life right after birth, these axons are lost at a blitzing rate of 50/sec! 

This axonal slaughter is a good thing.  The connections between neurons cannot be labeled with identifying chemicals to instruct proper assembly, as in "connect part A to part B" in putting together a new widget--there are far too many connections.  Instead, the brain wires itself roughly in fetal life.  Then after we are born the weak, unused, or wrong connections are pruned away and the useful ones get strengthened. 

In other words, our brain gets wired up according to the experiences we have in early life-through adolescence.  That's why everyone's brain is different.   The human brain develops after we are born according to the environmental experience we have in early life.  You were born with the capacity to understand and speak any language on earth, but you speak the language of your parents and struggle to understand foreign languages.  The neuronal connections that could have helped you understand odd sounds that are only heard in foreign languages are gone. 

After age 20, it's all downhill--at least in terms of total axon length in the brain.   But, your brain has been sharpened through experience to be very good at doing whatever you did with it when you were young.   

R. Douglas Fields, Ph.D., is the Chief of the Nervous System Development and Plasticity Section at the National Institute of Child Health and Human Development and the author of The Other Brain. more...

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