Can the Brain Ever be “Full”?

We have answered another great science question posed to us from Ask a Scientist!

Roksha, a first-grader, learned about working with a growth mindset at school.  In class, she has discussed how having a growth mindset means focusing on school work and putting forth your best effort.  Roksha’s class talked about how your brain is a dynamic structure that changes when you work hard to learn something new, and after thinking deeply about this, she sent us this insightful question:

“I know you grow dendritic spines when you learn.  But what if you grow too many dendrites and spines?  Does your brain ever get full?  Is there enough room?”

Great question, Roksha!  It may surprise you to know how many scientists have asked this same question over the years.  In fact, figuring out how the brain learns and stores memories is one of the fundamental goals in neuroscience.  Let’s start with some good news: The idea that your brain can be “full” is a myth.  We know that a brain is never too full to learn more, and cannot be filled to capacity.

Human Brain, Image Credit

Now that you know your brain will never be too full to learn, let’s journey into the process of how the brain makes room for new information.  Instead of filling up, the brain is a remarkably plastic structure, which changes constantly so that you can learn, forget, remember what’s important, and unlearn what you no longer need to know.

Neurons typically communicate with other neurons through electrical and chemical signals.  Neurons send these signals down an axon, across a small synapse, and into the dendritic branches of another cell.  One type of neuron, called a Purkinje neuron, is pictured below.  Here, you can see many dendrites branching like tree branches in varying shades of blue.  The spines are the part that looks like fuzz around the branches.  This entire neuron is only about 200 µm long!  In other words, if you lined up five of these neurons end-to-end, you could fit about five of them into one tiny millimeter.  For reference, one millimeter is about the size of the tip of a pen.

Cerebellar Purkinje neuron with dendrites in blue
Image Credit: Dana Simmons, Christian Hansel Lab, The University of Chicago. www.dana-simmons.com

Synaptic Plasticity

As you go through life, the neurons in your brain change based on your experiences.  When you learn, some of the neurons in your brain, like the one pictured above, generate dendritic spines.  Dendritic spines are little knobs that stick out of neuronal dendrites.  At these knobs, you can find synapses, which are widely considered to be locations where learning happens at the molecular level.  Synapses are tiny spaces (30-50 nm) between two neurons, across which one neuron sends a message to the other.  It has been estimated that there are ~86 billion neurons (Azevedo FA 2009, Voyteck B 2013) and ~100 trillion synapses in the human brain.  In the photo below, you can see a green dendritic branch with yellow spines sticking out of it.

Dendrite in green, dendritic spines in yellow Image Credit: Hotulainen P, Hoogenraad CC

Synaptic Plasticity Diagram: Neurons in blue, changes in synaptic strength in green/red. Neuronal stimulation indicated by red lightning bolts. Image Credit: HnWilliamson

When neurons communicate more with each other, synapses and spines are generated or strengthened (Hebb DO 1949, Bliss and Lømo 1973).  When neurons communicate less with each other, spines can be retracted (Bear MF 1995).  The dynamic process of building and eliminating synapses is continues throughout your entire life.  The brain is constantly remodeling itself by inserting new spines and removing unimportant spines.  The ability to remodel based on learning and experiences is called plasticity, or neuroplasticity.

In the past, people thought brains were like computers because you learn and store information.  However, a hard drive has a finite amount of space to store information.  Once a hard drive is full, it will be completely unable to store any more information.  In order to make new space, something must be deleted.  The hard drive is incapable of deciding what can be deleted on its own.  In contrast, we have learned that the brain is so much more capable than a hard drive because a brain never gets too full, can remodel itself.  The brain is remarkably dynamic and is constantly remodeling itself in response to your experiences.  More important pathways are solidified, while unnecessary pathways weaken and disappear, making room to store new things you learn.  When you’re young, your brain is extra plastic, because it has to learn everything from the beginning.  As you get older, your brain will still be plastic, but less so.  One advantage to having a less plastic brain as an adult is that it better preserves your most important memories.  

Refining The Young Brain

When we are born, the brain is full of many extra synapses and miscellaneous connections.  As a newborn learns and experiences the world, the brain begins to learn which synaptic connections are important.  The important synaptic connections get stronger, while the unneeded synapses disconnect and retract.  Through this process of pruning, our brains develop into highly functional adult brains that are less plastic and more refined for our daily activities.

Uniquely Human

Compare the texture of this dog brain to the human brain in the photo at the beginning of this article. Which looks more wrinkly? Image Credit

While the myth that the brain becomes full is false, it is interesting to note that the brain is an extremely crowded place.  The human brain has more folds and wrinkles than the brain of any other mammal.  These folds, which form the mountain-valley appearance, increase the surface area of our brains while maintaining a small-ish volume so that we can squeeze in as much neuronal tissue and circuitry as possible.  Throughout mammalian evolution, the large frontal lobes of brains have expanded and become more wrinkly.  A mouse brain has smooth frontal lobes, a cat’s brain has a few folds, a monkey brain will show more prominent folds, and a human brain has the most folds (See examples here).  If our brains stayed smooth instead of folding, it’s possible that our heads would be enormous and too heavy to support.  While your overall, large brain structure stays consistent, the individual neurons within your brain will change as you change and experience life.

-Dana Simmons, neurobiologist and Co-Editor of the ISC Blog

Dana is pursuing a Ph.D. in Neurobiology at The University of Chicago, where she researches autism and the cerebellum, and explores science through the lens of art.  Follow Dana on Twitter @dhsimmons1 for updates on her research, ISC articles, and neuroscience-based art.

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Sources
Azevedo FA, Carvalho LR, Grinberg LT, Farfel JM, Ferretti RE, Leite RE, Jacob Filho W, Lent R, Herculano-Houzel S. “Equal numbers of neuronal and nonneuronal cells make the human brain an isometrically scaled-up primate brain.” J Comp Neurol. 2009 Apr 10;513(5):532-41
Bear MF. 1995. “Mechanism for a sliding synaptic modification threshold”. Neuron. 15 (1): 1–4.
Bliss TVP, Lømo T. 1973. “Long lasting potentiation of synaptic transmission in the dentate area of the anaesthetized rabbit following stimulation of the perforant path.” J. Physiol. 232, 331-356.
Chudler EH. “Comparative Neuroanatomy: Neuroscience for Kids” 2008.  
Hebb, DO. The Organization of Behavior: A Neuropsychological Theory. New York: Wiley, 1949.
Voyteck B. “Are There Really as Many Neurons in the Human Brain as Stars in the Milky Way?” Nature.com May 13, 2013.