The shape of the brain - why does it look that strange?

Last week, I decided to ask one of our (four) daughters," Which one question about the brain would you wanna ask?". "Well, why does it have that weird shape?" was her reply. This blog is my answer.

The brain has folds to maximize its surface area. To allow for more brain cells to fit inside the skull. If the brain were flat on its surface, there wouldn't be enough space for all the neurons and connections that help us think, move, and experience the world. 

With a flat surface brain, we would have to either accept being a dumber species or have much bigger skulls, which would be a problem in labour and probably the end of our species.

The folds increase the surface area, allowing more neurons to be packed into a limited space. This folding also helps different parts of the brain communicate efficiently. Areas responsible for specific tasks, like movement, language, and memory, can be tightly packed and organized within these folds. 

In short, the brain folds up to fit in more of the brain cells we need to think, learn, and interact with the world, and the folds are essential for our brains to work efficiently.

Evolution of folds
The human brain is one of our largest organs, with the cerebral surface (the cortex) making up most of its mass. The distinctive folds of the cortex are a feature of many mammals, including humans.

The folds on the surface are caused by the brain growing and expanding during development, especially in mammals like humans, who have more complex brains.

The most superficial layer, the neocortex, is a relatively new addition to the brain in evolutionary terms. It has undergone dramatic expansion in humans compared to our closest relatives, the great apes. 

The emergence of the neocortex is linked to the development of unique cognitive abilities that set humans apart from other species. Although scientists have made significant progress in understanding the structure and function of the neocortex, many questions remain, particularly about how its unique shape and organization evolved.

Not all animals have these cortical folds. Species like mice and rats have smooth cortices. Interestingly, the structure of the cortex in animals with smooth brains, like mice, still shares many similarities to folded brains. This commonality in cortical development helps explain how studying brains in animals like mice can still offer valuable insights into human brain formation.

The theory that cortical folding emerged in mammals as brain size increased is still debated. Research comparing different species like marsupials and larger rodents suggests that cortical folding may have originated much earlier in evolutionary history. 

The mechanisms that fold the brain
Cortical folding, which gives the human brain its characteristic convoluted appearance, is influenced by several factors that interact in complex ways.

The brain grows in a way that involves an expanding outer layer, which is destined to become the neocortex (containing the brain cells), and an inner zone, which will develop into white matter (containing the connections).

The outer zone grows faster than the inner zone. This rapid expansion of the outer layer causes the cortex to fold in on itself. The areas that expand more quickly will fold inwards into valleys(sulci), while slower-growing regions will form the ridges (gyri).

Genetic factors also influence this process, determining the specific pattern of folds that emerge during development. Twin studies have shown that there is a heritable component to these patterns, suggesting that genetic factors help guide the folding.

Additionally, the behavior of the brain's stem cells (progenitor cells) plays a central role in regulating the folding process. As the stem cells divide and generate more neurons, the physical expansion of the cortex contributes to the development of folds.

In species with more complex brains, such as humans, the proliferation of these cells is particularly robust, leading to a greater number of neurons and more pronounced cortical folds.

Folding in diseases
While the biomechanical forces of the cortex are crucial in driving folding, it is also essential to consider how developmental and environmental factors influence the process.

For instance, in some neurodevelopmental disorders, such as autism and Williams syndrome, atypical cortical folding patterns have been observed. Abnormal folding is also linked to more severe congenital conditions like lissencephaly (flat surface brain) and polymicrogyria (too many too small folds), which can lead to pronounced cognitive and developmental impairments.

Environmental influences can also impact cortical folding. Premature birth, growth restriction in the womb, and exposure to substances like alcohol can all lead to abnormal folding patterns and associated cognitive deficits.

All of these abnormalities underscore the significance of cortical folding in brain function and have spurred research into how disruptions in normal brain folding contribute to these disorders.


About the scientific paper:

First author:  Shyam K. Akula, USA
Published: Cell, December 2023
Link to paper: https://www.cell.com/developmental-cell/fulltext/S1534-5807(23)00580-4?_returnURL=https%3A%2F%2Flinkinghub.elsevier.com%2Fretrieve%2Fpii%2FS1534580723005804%3Fshowall%3Dtrue