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Sulcus (neuroanatomy)

Fold in the surface of the brain

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Mammals that have a folded cortex are known as 'gyrencephalic', and the small-brained mammals that have a smooth cortex, such as rats and mice are termed lissencephalic.

Structure

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. A sulcus is a shallow groove that surrounds a gyrus or part of a gyrus. A fissure is a deeper furrow that divides the brain into lobes, and also into the two hemispheres as the longitudinal fissure. Fissures are the most prominent and invariable of the sulci.

The pia mater, the membrane surrounding the brain follows the surface of the brain into each sulcus, but the arachnoid mater stretches across all sizes of the sulci, except the longitudinal fissure where it follows the pia mater. Consequently the inner sides of almost all sulci are separated only by the pia mater and the subarachnoid space, in which the cerebrospinal fluid circulates. Sulci may be considered as extensions of the subarachnoid space.

The approximate depth of a sulcus ranges between one and three centimetres. Other parameters of sulcal shape are length, width, and surface area. Within a sulcus there may be smaller gyri, collectively known as transverse gyri.

A sulcus is not necessarily a single structure. Some sulci have one or more parts that may branch in different directions. Such parts may be short, long, isolated, or connected to other sulci.

Variations

The sulcal pattern varies between human individuals, but the sulci and gyri do have a generalised arrangement, making a common nomenclature possible.

Types

On the basis of formation:

  1. Large primary sulci: formed before birth, independently, such as the precentral sulcus and the central sulcus.
  2. Short primary sulci include rhinal sulcus, olfactory sulcus, occipital sulcus, and lateral sulcus.
  3. Short branched sulci sulci with several branches such as the subparietal sulcus, and orbital sulcus.
  4. Short free supplementary sulci include the lunate sulcus, and medial frontal sulcus.
  5. Sulci with side branches that connect adjacent parallel sulci.

On the basis of function:

  1. 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.
  2. 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.
  3. 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:

  1. Complete sulcus is very deep so as to cause elevation in the walls of the lateral ventricle. Examples are the collateral and calcarine sulci.
  2. Incomplete sulci are superficially situated and are not very deep, E.g. paracentral sulcus.

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