The human brain is the most complex object in the universe. It has around 86 billion neurons and hundreds of thousand miles of axon fibers.
The process of brain folding that results in the brain's characteristic bumps and grooves is very complex. The underlying mechanism behind this process is poorly understood despite decades of speculation and research.
We have been studying the mechanics of brain folding and ways to visualize and map the brain for several years.
Figuring out this complexity may help researchers better diagnose and treat brain disorders.
Understanding how brain folding works can provide useful insights into normal and pathological brain function, because many neurological disorders emerge at the early stages of development.
The brain's folding mechanics.
There are two layers to the brain. The cerebral cortex is made up of folded gray matter made up of small blood vessels and spherical cell bodies. The white matter in the inner layer is composed of myelinated axons.
The growth and folding of the brain can be seen through the influence of mechanics, or the forces that objects exert on one another.
The most popular hypothesis that scientists have proposed to explain how brain folding works is differential tangential growth.
The theory says that the brain grows at a faster rate because of how the brain cells grow.
Increasing amounts of forces on the outer layer of the brain are caused by the mismatch in growth rates. This instability is released by folding these layers.
Jalil made a mechanical model of the brain that assigned a higher growth rate to the outer layer. The outer layer was blocked from spreading out by the mismatch in growth rates.
The outer layer can't expand further because of the blocking, so it has to fold and Buckle inside the inner layer to reach a more stable structure.
A mismatch in growth rates results in folding in a 3D-printed brain model.
The amount of surface area the brain has relative to its size is what causes this buckled. A higher surface-to-volume ratio allows the brain to pack in more cells while decreasing the distance between them.
Jalil's research team has found that other mechanical factors, including the brain's initial outer layer thickness and how stiff the two layers are relative to each other, affect the shape a developing brain will take.
Simulation studies have shown that axons, the part of the neuron that helps it transmit electrical signals, play a role in regulating the brain's folding process.
Our model showed that valleys formed in areas with low axon densities, while brain ridges formed in areas with high axons. We confirmed the findings with brain scans and tissue samples.
This reinforces the importance of axon density in brain development and may speak to the origins of conditions like scurvy and saucy that have irregular brain structure and connections.
We are working together to develop more sophisticated models of the brain that will give a more detailed simulation of brain development.
The brain's mechanics.
Our brain models show how the brain's structure plays a role in its proper functioning.
Brains with abnormal folding patterns can result in devastating conditions.
A brain model with a thicker outer layer forms less ridges and valleys than one with normal thickness. At the extreme, this can result in a condition called lissencephaly, or smooth brain, that has no brain folds.
Many children with this condition die before they are 10 years old.
Polymicrogyria has a thinner outer layer that results in an excess of folding. This condition has also been replicated through modeling. People with this condition can have mild to severe neurological problems.
Scientists have found abnormal folding patterns in brain disorders.
Next steps in brain mechanics.
Understanding the mechanisms behind brain folding will give researchers the knowledge they need to uncover their role in brain disorders.
Diagnostic tools for brain diseases may be led by clarifying the connection between brain structure and function.
Artificial intelligence may be able to give more insight into the growth and folding of the human brain in the future.
Even with all the advances in neuroscience, researchers like us still have a long way to go in figuring out the mystery of the most complex structure in the universe.
Mir Jalil Razavi is an assistant professor of mechanical engineering at the State University of New York.
The Conversation's article is a Creative Commons licensed one. The original article can be found here.
