The ridges visible on the surface of the human brain are called gyri. These raised folds are characteristic features of the cerebrum, the largest part of the brain responsible for higher functions like thought and action. The indentations or grooves that separate these gyri are known as sulci, and together, they create the distinct, wrinkled appearance of the cerebral cortex. This highly convoluted structure represents a compact and efficient way to organize the brain’s vast neural network.
The Ridges and Valleys: Defining Gyri and Sulci
The terminology for the brain’s topography is precise. Gyri are the elevated parts, and sulci are the depressions. A gyrus is defined as a ridge on the cerebral cortex, and it is almost always surrounded by one or more sulci. The presence of these folds is known as gyrification, a defining trait of the cerebrum in humans and many other large mammals.
The main difference between a sulcus and a fissure lies in depth, with fissures being deeper, more prominent grooves. Fissures often serve to divide the brain into larger structural regions, while sulci are typically shallower indentations. For instance, the groove that separates the two large hemispheres is termed the longitudinal fissure.
Why the Brain is Folded
The reason for the brain’s folded appearance is to maximize the surface area of the cerebral cortex, the outermost layer of gray matter. This cortex is where the majority of neural processing, including consciousness, language, and memory, takes place.
The folding allows a significantly larger amount of gray matter to be packed into the limited space of the cranial vault. Approximately two-thirds of the cerebral cortex is actually buried within the sulci, hidden from view. This high degree of folding, which develops during fetal growth, is a hallmark of complex cognitive ability, enabling a greater number of neurons and more advanced processing. When this folding process is disrupted during development, it can lead to a condition called lissencephaly, or “smooth brain,” which is associated with severe intellectual deficits.
The mechanical process of folding is driven by the expansion of the outer gray matter layer relative to the inner white matter. This differential growth causes the outer layer to buckle and crease, resulting in the formation of gyri and sulci. This strategic folding also works to shorten the distance that nerve fibers must travel to connect different brain regions, optimizing the speed of signal transmission.
Mapping the Brain: Major Fissures and Lobes
The most prominent gyri and sulci serve as anatomical landmarks that divide the brain into its main functional compartments, or lobes. The cerebral hemispheres are separated vertically by the longitudinal fissure, a deep groove running down the midline. Each hemisphere is divided into four lobes: the frontal, parietal, temporal, and occipital lobes.
These lobes are demarcated by major folds consistent across individuals. The central sulcus is a groove that runs from the top of the brain down the side, acting as a boundary between the frontal lobe and the parietal lobe. The lateral sulcus (also known as the Sylvian fissure) is a deep horizontal groove that separates the temporal lobe from the frontal and parietal lobes above it. The parieto-occipital sulcus separates the parietal and occipital lobes.