A gyrified brain is a brain with a folded, wrinkled appearance on its surface. This folding is a hallmark of the cerebral cortex in many mammals, including humans. Gyrification describes the process through which these folds develop.
The Nature of Brain Folds
The folded surface of a gyrified brain consists of ridges and grooves. The raised ridges are called gyri (singular: gyrus), while the depressions separating them are known as sulci (singular: sulcus). These structures form the brain’s complex pattern. The grey matter, which primarily contains nerve cell bodies and dendrites, forms the outer layer of the brain.
The Purpose of Brain Folds
Brain folds significantly increase the cerebral cortex’s surface area. This allows a larger number of neurons and neural connections to be packed within the skull. An expansive cortical surface facilitates complex cognitive functions, such as language processing, memory formation, and problem-solving. This folding mechanism supports higher cognitive capabilities.
How Brain Folds Form
Brain folds develop during embryonic and fetal stages. Initially, the brain’s surface is smooth. As development progresses, the proliferation and migration of neural cells, particularly radial glial cells and their descendant neurons, expand the cortical sheet. This expansion occurs faster than the underlying white matter’s growth, creating mechanical stress. This differential growth causes the cortical sheet to buckle and fold, forming the gyri and sulci.
Gyrification in Different Species
Not all brains exhibit the same degree of folding. Some species have “lissencephalic,” or smooth, brains, while others have highly gyrified brains. For instance, small rodents like mice and rats typically have smooth brains. In contrast, species such as humans, dolphins, elephants, and some primates display extensive cortical folds. The degree of gyrification generally correlates with brain size and cognitive complexity, though it is not the sole determinant of intelligence.
When Brain Folding Differs from the Norm
Abnormalities in brain folding can lead to various neurological conditions. Lissencephaly, meaning “smooth brain,” is a disorder where the brain lacks or has very few folds, resulting in an unusually smooth surface. This condition arises from issues with neuronal migration during development and can cause severe physical and intellectual disabilities, alongside seizures. Another condition, polymicrogyria, involves the brain developing too many small, irregular folds. Such deviations from typical brain folding patterns can disrupt normal brain function and lead to developmental delays and intellectual impairments.