Gyrification

Gyrification is the process of forming the characteristic folds of the cerebral cortex. The peak of such a fold is called a gyrus (plural: gyri), and its trough is called a sulcus (plural: sulci). The neurons of the cerebral cortex reside in a thin layer of 'gray matter', only 2–4 mm thick, at the surface of the brain. Much of the interior volume is occupied by 'white matter', which consists of long axonal projections to and from the cortical neurons residing near the surface. Gyrification allows a larger cortical surface area and hence greater cognitive functionality to fit inside a smaller cranium. In most mammals, gyrification begins during fetal development. Primates, cetaceans, and ungulates have extensive cortical gyri, with a few species exceptions, while rodents generally have none. Gyrification in some animals, for example the ferret, continues well into postnatal life.

As fetal development proceeds, gyri and sulci begin to take shape with the emergence of deepening indentations on the surface of the cortex. Not all gyri begin to develop at the same time. Instead, the primary cortical gyri form first (beginning as early as gestational week 10 in humans), followed by secondary and tertiary gyri later in development. One of the first and most prominent sulci is the lateral sulcus (also known as the lateral fissure or Sylvian fissure), followed by others such as the central sulcus, which separates the motor cortex (precentral gyrus) from somatosensory cortex (postcentral gyrus). Most cortical gyri and sulci begin to take shape between weeks 24 and 38 of gestation, and continue to enlarge and mature after birth.

One advantage of gyrification is thought to be increased speed of brain cell communication, since cortical folds allow for cells to be closer to one other, requiring less time and energy to transmit neuronal electrical impulses, termed action potentials. There is evidence to suggest a positive relationship between gyrification and cognitive information processing speed, as well as better verbal working memory. Additionally, because a large cranium requires a larger pelvis during childbirth, with implied difficulty in bipedalism, a smaller cranium is more easily delivered.

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