Explanation

Outward projection of head region of meromyosin. Each myosin (thick filament) is a polymerized protein. Many monomeric proteins called meromyosin constitutes one thick filament. Each meromyosin has two important parts, a globular head with a short arm and a tail, the former being called heavy meromyosin and the later is called light meromyosin. The HMM component, i.e., the head and short arm projects outwards at regular distance and angle from each other from the surface of polymerized myosin filament and is called cross arm. The globular head is an active ATPase enzyme and has binding sites for ATP and active sites for actin