Sulcus (neuroanatomy)

Sulci, the grooves, and gyri, the folds or ridges, make up the folded surface of the cerebral cortex. Larger or deeper sulci are also often termed fissures. The folded cortex creates a larger surface area for the brain in humans and other larger mammals, without the need of increasing the size of the skull. In the human brain, two-thirds of the folded cortex is buried within the sulci, if the division of the hemispheres by the longitudinal fissure is taken into account. The sulci and fissures are shallow and deep grooves respectively in the cortex, that organise the brain into its regions. Fissures are the most prominent and invariable of the sulci. Sulci may be considered as extensions of the subarachnoid space. • Large primary sulci: formed before birth, independently, such as the precentral sulcus and the central sulcus. • Short primary sulci include rhinal sulcus, olfactory sulcus, occipital sulcus, and lateral sulcus. • Short branched sulci sulci with several branches such as the subparietal sulcus, and orbital sulcus. • Short free supplementary sulci include the lunate sulcus, and medial frontal sulcus. • Sulci with side branches that connect adjacent parallel sulci. On the basis of function: • A limiting sulcus separates at its floor into two areas which are different functionally and structurally e.g. central sulcus between the motor and sensory areas. • Axial sulcus develops in the long axis of a rapidly growing homogeneous area e.g. postcalcarine sulcus in the long axis of the striate area. • Operculated sulcus separates by its lips into two areas and contains a third area in the walls of the sulcus e.g. lunate sulcus is an operculated sulcus, separating the striate and parastriate areas. On the basis of depth: • Complete sulcus is very deep so as to cause elevation in the walls of the lateral ventricle. Examples are the collateral and calcarine sulci. • Incomplete sulci are superficially situated and are not very deep, E.g. paracentral sulcus. s, and Sonic hedgehog protein signalling promote cortical folding ==Development==

The process of cortical folding is complex and incompletely understood. It is explained by a number of hypotheses including mechanical buckling, and axonal tension factors. The hypotheses are not mutually exclusive and can include their combined effects, with that of another mechanism of tangential expansion. Tangential expansion is associated with radial glial cells, and a process of intercalation of cortical neurons in between cells of the outer cortical plate layer producing the outward buckling of a gyrus. In humans, cerebral convolutions appear at about five months and take at least into the first year after birth to fully develop. There is a hierarchy of morphological development with the fissures and main sulci developing ahead of others. The first sulci to develop are the primary sulci, followed by secondary sulci. The more constantly found sulci are those related to functional specialization. Development varies greatly between individuals. The potential influences of genetic, epigenetic and environmental factors are not fully understood. ==Sulci of note==

- green;Central - red; Lateral sulcus - dark blue; Superior temporal - light blue; Intraparietal - yellow ; Frontal lobe Superior frontal sulcus, Inferior frontal sulcus, Precentral sulcus, Olfactory sulcus, Orbital sulcus, Paracentral sulcus ; Parietal lobe Intraparietal sulcus, Marginal sulcus, Postcentral sulcus ; Occipital lobe Lunate sulcus, Transverse occipital sulcus, Calcarine sulcus ; Temporal lobe Superior temporal sulcus, Inferior temporal sulcus ; Interlobar fissures Longitudinal fissure, Central sulcus, Lateral sulcus, Collateral sulcus, Callosal sulcus, Parieto-occipital sulcus, Occipitotemporal sulcus, Subparietal sulcus, Cingulate sulcus ; Limbic lobe Hippocampal sulcus, Rhinal sulcus, Fimbriodentate sulcus, Central sulcus of the insula, Circular sulcus of insula ==Clinical significance==

The advanced cognitive abilities that have developed from the expansion of cortical folding, are shown to be adversely affected when the folds are malformed. Malformations of the cortical folds have been linked to the intellectual disabilities associated with epilepsy, schizophrenia, and autism. Anomalies in gyrification can affect the width or depth of sulci that are associated with many neurological or neuropsychiatric disorders. The widening of sulci is seen to indicate early atrophy in neurodegenerative disorders, and may be used as a biomarker in their progression. but also with cognitive decline in the elderly. Ulegyria, is a condition of scarring in the deep regions of sulci leading to disruption of the associated gyri. The sulci are valuable landmarks in microneurosurgery, and may also be used as corridors for surgeries. ==Other animals==

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, and some New World monkeys are lissencephalic. Macaque A macaque has a more simple sulcal pattern. In a monograph Bonin and Bailey list the following as the primary sulci: • Calcarine fissure (ca) • Central sulcus (ce) • Sulcus cinguli (ci) • Hippocampal fissure (h) • Sulcus intraparitalis (ip) • Lateral fissure (or Sylvian fissure) (la) • Sulcus olfactorius (olf) • Medial parieto-occipital fissure (pom) • Fissura rhinalis (rh) • Sulcus temporalis superior (ts) – This sulcus runs parallel to the lateral fissure and extends to the temporal pole and often superficially merges with it. ==Additional images==

File:Sulci of the brain.jpg|Different views of the sulci of the brain File:Gray726.png|Gray's Fig. 726 – Lateral surface of left cerebral hemisphere, viewed from the left side File:Gray727.svg|Gray's Fig. 727 – Medial surface of left cerebral hemisphere, viewed from the right side ==See also==