The human brain’s intricate folds and grooves are a hallmark of complex brains. These convolutions are not merely cosmetic; they represent a highly organized structure fundamental to the brain’s advanced capabilities. Exploring these folds reveals much about how our brains achieve higher cognitive functions.
The Anatomy of Brain Folds
The folds on the brain’s surface are anatomical structures. The raised ridges are known as gyri (singular: gyrus), while the grooves or furrows that separate them are called sulci (singular: sulcus). These structures are characteristic of the cerebral cortex, the outermost layer of the cerebrum, the largest part of the human brain.
The cerebral cortex is divided into two hemispheres, covered with these gyri and sulci. This folding pattern allows a large cortical surface area to fit within the confined space of the skull. The overall arrangement of these folds is consistent across human brains, enabling scientists to map and study different brain regions and their functions.
Why the Brain Needs Its Folds
The primary reason for the brain’s folded appearance is to increase the surface area of the cerebral cortex. The cerebral cortex is a thin layer of gray matter, only about 2 to 4 millimeters thick, where neurons are concentrated. The average adult human brain has a cortical surface area of approximately 2000 cm², with about two-thirds of this area hidden within the sulci.
This expanded surface area allows for a greater number of neurons and neural connections to be packed into the skull than would be possible with a smooth brain. A higher density of neurons and connections is directly linked to higher cognitive functions, including language processing, memory formation, problem-solving, and conscious thought. The folding pattern also brings different cortical areas closer together, which can reduce the amount of white matter needed to connect these regions, optimizing brain efficiency. This increased capacity for information processing provides an evolutionary advantage, supporting complex behaviors and intellectual abilities.
How Brain Folds Develop
The process of cortical folding, known as gyrification, begins during fetal development. The brain initially starts as a smooth sheet of cells, but as gestation progresses, it begins to fold. This folding is driven by the rapid proliferation and migration of cells, particularly neurons, which cause the cortical plate to expand faster than the underlying brain layers and the skull that contains it.
Folding, especially the formation of primary sulci, begins between 20 and 28 gestational weeks. The lateral sulcus, also known as the Sylvian fissure, is one of the first and most prominent sulci to form, followed by others like the central sulcus. This process continues with the emergence of secondary and tertiary folds, some of which continue to develop and mature even after birth.
When Brains Aren’t Wrinkly
In some rare conditions, the brain does not develop its characteristic folds, a disorder known as lissencephaly, meaning “smooth brain.” This condition results from abnormal neuronal migration during embryonic development, where nerve cells fail to move from their origin to their correct permanent locations in the cerebral cortex. The result is a brain surface that lacks or has reduced gyri and sulci.
Lissencephaly is associated with neurological impairments. Symptoms can include developmental delays, intellectual disability, muscle spasms, and seizures. Individuals may also experience feeding difficulties, an unusual facial appearance, and sometimes anomalies of the hands, fingers, or toes. Lissencephaly can have a genetic basis, with mutations in various genes, such as LIS1, DCX, and RELN, implicated in its development.