Pectinate muscles are small, parallel ridges of muscle that line the inner walls of the heart’s upper chambers, the atria. Their name comes from the Latin word “pecten,” meaning comb, because the ridges resemble the teeth of a comb. They are most prominent in the right atrium, where they fan out from a larger muscle ridge called the crista terminalis and extend into the ear-shaped pouch known as the right atrial appendage.
Where Pectinate Muscles Are Located
The right atrium contains the most extensive array of pectinate muscles. They originate from the crista terminalis, a thick vertical ridge of muscle that runs along the inner wall of the right atrium, and extend anteriorly and laterally toward the tricuspid valve. The right atrial appendage, which has a broad, triangular opening, is heavily lined with these ridges, giving it a rough, trabeculated texture you can see clearly on imaging or dissection.
The left atrium is a different story. Most of its wall is smooth, and its appendage is narrower and more tubular than the right side. Pectinate muscles still exist here, but they are finer and fewer. Inside the left atrial appendage, they organize in a spiral-like pattern rather than the parallel comb-tooth arrangement seen on the right. This distinction between the two atria is one of the ways anatomists and surgeons reliably tell a morphologically right atrium from a left one.
How They Differ From Ventricular Ridges
The ventricles have their own set of internal muscle ridges called trabeculae carneae. These form during the embryologic period and line the inner surfaces of both ventricles, with the right ventricle being noticeably more trabeculated than the left. Despite the visual similarity, pectinate muscles and trabeculae carneae are distinct structures in different chambers. Pectinate muscles are confined to the atria and their appendages, while trabeculae carneae belong exclusively to the ventricles. The two also differ in thickness and arrangement: pectinate muscles are relatively small and linear, whereas ventricular trabeculae include thicker columns and bands, some of which anchor the heart valves.
What Pectinate Muscles Do
These ridges are not just decoration. They serve a mechanical purpose that relates to how the atria handle constantly changing blood volumes. The atria fill and empty with every heartbeat, and the volume of blood returning to the heart fluctuates with breathing, body position, and activity level. Pectinate muscles act as infoldings that increase the internal surface area of the atrial chamber. When the atrium stretches to accommodate more blood, these folds can unfurl and expand the chamber’s capacity, similar to how the folds of the intestinal lining maximize absorptive surface area. This allows the atria to be compliant reservoirs without requiring excessively thin or large walls.
Pectinate muscles also move in concert with the atrial wall during contraction. They are not independent structures floating inside the chamber. They contract as part of the wall itself, contributing to the coordinated squeeze that pushes blood into the ventricles. Research on electrical conduction through the atria shows that the ridges formed by pectinate muscles actually accelerate the spread of electrical impulses compared to the flat atrial tissue between them, helping the signal travel efficiently from the crista terminalis toward the atrioventricular region.
How They Develop Before Birth
During fetal heart development, the primitive atrial chambers initially lack internal ridges. The pectinate muscles appear later, after the septum that divides the left and right atria begins to form. They develop specifically in the future atrial appendages, branching from the terminal crest (the embryonic precursor of the crista terminalis) and extending toward the groove that separates the atria from the ventricles. One prominent ridge of pectinate muscles becomes continuous with the terminal crest and forms what is known as Bachmann’s bundle, a structure that plays a key role in conducting electrical signals between the two atria.
Variation Between Individuals
Not everyone’s pectinate muscles look the same. Anatomical studies have documented at least six distinct patterns. In some hearts, the ridges run perpendicular to the crista terminalis with even spacing. In others, they run parallel to it. Some individuals have prominent, thick muscular columns, while others have a more flattened arrangement where the ridges blend into the surrounding wall. The crista terminalis itself varies in width and thickness, and these variations together mean the right atrial wall is not uniform in thickness from person to person.
Why They Matter in Heart Disease
Pectinate muscles become clinically relevant in atrial fibrillation, a common heart rhythm disorder where the atria quiver instead of contracting effectively. When the atria stop squeezing properly, blood pools in the recesses created by the pectinate muscles. The rough, ridged surface combined with stagnant blood flow provides an ideal environment for clots to form, particularly in the left atrial appendage. This is why atrial fibrillation significantly raises stroke risk: clots that form in these recesses can break free and travel to the brain.
Pectinate muscles can also cause confusion on cardiac imaging. When they are particularly prominent in the left atrial appendage, they can protrude into the chamber and look like blood clots on echocardiography or CT scans. Radiologists distinguish the two by checking whether the structure moves independently (a clot may wobble on its own) or moves only with the atrial wall (pectinate muscles always do). The characteristic pattern of multiple parallel ridges, rather than a single irregular mass, also helps confirm that what appears on the image is normal anatomy rather than a dangerous clot.