Sulcus (neuroanatomy)

Fold in the surface of the brain

In neuroanatomy, a sulcus (Latin: "furrow"; : sulci) is a depression or groove in the cerebral cortex. It surrounds a gyrus (pl. gyri), creating the characteristic folded appearance of the brain in humans and other mammals. The larger sulci are usually called fissures.

Sulci, the grooves, and gyri, the folds or ridges, make up the folded surface of the cerebral cortex. Larger or deeper sulci are termed fissures, and in many cases the two terms are interchangeable. The folded cortex creates a larger surface area for the brain in humans and other mammals. When looking at the human brain, two thirds of the surface are hidden in the grooves. The sulci and fissures are both grooves in the cortex, but they are differentiated by size. A sulcus is a shallower groove that surrounds a gyrus. A fissure is a large furrow that divides the brain into lobes and also into the two hemispheres as the longitudinal fissure.

As the surface area of the brain increases more functions are made possible. A smooth surfaced brain is only able to grow to a certain extent. A depression, sulcus, in the surface area allows for continued growth. This in turn allows for the functions of the brain to continue growing.

The sulcal pattern varies between human individuals, and the most elaborate overview on this variation is probably an atlas by Ono, Kubick and Abernathey: Atlas of the Cerebral Sulci. Some of the more prominent sulci are, however, seen across individuals – and even species – making a common nomenclature across individuals and species possible.

In humans, cerebral convolutions appear at about five months and take at least into the first year after birth to fully develop. Development varies greatly between individuals. The potential influences of genetic, epigenetic and environmental factors are not fully understood. It has been found that the width of cortical sulci increases not only with age, but also with cognitive decline in the elderly.

The variation in the number of fissures in the brain (gyrification) between species is related to the size of the animal and the size of the brain. Mammals that have smooth surfaced or nonconvoluted brains are called lissencephalics and those that have folded or convoluted brains gyrencephalics. The division between the two groups occurs when cortical surface area is about 10 cm2 and the brain has a volume of 3–4 cm3. Large rodents such as beavers and capybaras are gyrencephalic and smaller rodents such as rats and mice lissencephalic.