Sulci are the grooves or indentations on the surface of the brain. They sit between the raised folds, called gyri, and together these ridges and valleys give the brain its characteristic wrinkled appearance. The plural “sulci” refers to multiple grooves; a single groove is called a sulcus. While the term most commonly refers to the brain, sulci also appear in other parts of the body, including the heart and gums.
How Sulci Shape the Brain’s Surface
If you could flatten out the human brain, its outer layer (the cortex) would take up far more space than the inside of the skull allows. Sulci solve this problem by folding the cortex inward, packing a large surface area into a limited volume. Think of it like crumpling a sheet of paper into a ball: the creases let you fit a large sheet into a small space.
This folding isn’t just about saving room. It also optimizes the brain’s internal wiring. Neurons that need to communicate frequently end up physically closer together when the cortex folds, which shortens the connections between them and makes signaling faster and more efficient. When cortical folding doesn’t develop properly, the consequences are severe, often including intellectual disability and epilepsy.
Major Sulci and What They Separate
Some sulci are large enough to serve as major landmarks that divide the brain into its four main lobes.
- Central sulcus: Runs roughly from ear to ear across the top of the brain, separating the frontal lobe (involved in planning, decision-making, and movement) from the parietal lobe (involved in processing touch and spatial awareness).
- Lateral sulcus: A deep groove along each side of the brain that separates the frontal lobe from the temporal lobe. The temporal lobe handles hearing, language recognition, and memory formation.
- Calcarine sulcus: Located at the back of the brain in the occipital lobe, this groove runs through the primary visual cortex, the area responsible for processing what you see.
Dozens of smaller sulci further subdivide each lobe into specific functional regions. Their exact shape and depth vary from person to person, almost like a fingerprint, but the major landmarks are consistent enough that doctors and researchers use them to navigate the brain on imaging scans.
When Sulci Form During Development
A fetus’s brain starts out almost completely smooth. Folding begins around 20 weeks of gestation, and from there it accelerates rapidly. Development happens in three waves. Primary sulci, the large landmark grooves, appear first during the third trimester. Secondary sulci branch off from these shortly after. Tertiary sulci, the smallest and most variable folds, form late in gestation and continue developing around the time of birth and into early infancy.
The pace of folding is remarkable. The gyrification index, a measure of how folded the brain is, climbs steeply through the second trimester and accelerates even further during the third. Research published in Nature’s Communications Biology found that a significant burst of folding continues after birth, meaning the brain’s surface is still actively sculpting itself in the first weeks of life.
What Happens When Sulci Don’t Develop
A rare condition called lissencephaly, which translates to “smooth brain,” occurs when sulci and gyri fail to form normally. The brain’s surface appears flat rather than folded. Lissencephaly develops between the 12th and 24th weeks of pregnancy, when neurons that should migrate to the brain’s outer layer are disrupted.
The cause is usually a genetic mutation, though viral infections during the first trimester or a lack of oxygenated blood to the fetal brain can also trigger it. Children with lissencephaly typically experience significant developmental delays, intellectual disability, and neurological problems. The severity varies widely depending on how much of the brain is affected. Most children with the condition need support with daily tasks throughout their lives.
Sulcal Changes in Aging and Dementia
As the brain ages, its tissue gradually shrinks, and the sulci widen to fill the space. In healthy adults, this widening happens at a steady, linear rate and follows slightly different patterns in men and women. It’s a normal part of aging and, on its own, not a cause for concern.
In neurodegenerative diseases like Alzheimer’s, however, sulcal widening is faster and more pronounced, particularly in the frontal and temporal regions. Researchers have found that measuring sulcal width in these areas can help distinguish normal aging from early cognitive decline. In one study published in the Journal of Alzheimer’s Disease, the width of specific grooves in the temporal and occipital regions was highly accurate at identifying people with mild cognitive impairment who would go on to develop Alzheimer’s. As the disease progresses, sulcal width increases further while sulcal depth and surface area decrease, reflecting the ongoing loss of brain tissue. This makes sulcal measurements a potential tool for tracking disease progression on brain scans.
Sulci Outside the Brain
The term “sulcus” simply means groove, so it appears in other areas of anatomy too. On the surface of the heart, the coronary sulcus is the groove that circles the heart and separates the upper chambers (atria) from the lower chambers (ventricles). Major blood vessels, including the coronary arteries, run along these grooves. The anterior interventricular sulcus marks the line between the heart’s two ventricles on the front surface.
In dentistry, the gingival sulcus is the shallow space between a tooth and the surrounding gum tissue. In healthy adults, this groove measures roughly 2 to 3 millimeters deep. When gum disease develops, the sulcus deepens into what dentists call a “pocket,” which is one of the key measurements used to assess gum health during a dental exam.