Which muscle forms ridges within the atrial walls known as pectinate muscles?

• A. Pectinate muscle
• B. Papillary muscle
• C. Interventricular septum
• D. Tricuspid

Answer: (A)

Pectinate muscles are the comb-like bands of cardiac muscle that line the atrial walls, especially prominent in the right atrium and its appendage. These ridges increase the surface area of the atrial interior and help the atrial myocardium contract effectively to push blood into the ventricles. Papillary muscles reside in the ventricles and anchor the chordae tendineae of the AV valves, not in the atria. The interventricular septum is the wall between the two ventricles, and the tricuspid refers to the valve between the right atrium and right ventricle, not a muscle. So the muscle forming those atrial ridges is the pectinate muscle.