thickfilament

Thick filament is the contractile filament in striated muscle, primarily composed of myosin II molecules. It forms the central core of the sarcomere’s A-band and works with actin-containing thin filaments to generate muscle contraction. In skeletal and cardiac muscle, thick filaments are highly organized, with myosin II molecules arranged in a bipolar fashion so that their heads project outward from both sides to form cross-bridges with actin. A central region known as the bare zone lacks myosin heads.

Structure and composition: Each myosin II molecule has two heavy chains forming a long coiled-coil tail and two globular heads (S1 domains). Many such molecules assemble into a roughly cylindrical thick filament about 10–20 nanometers in diameter. Accessory proteins, including myosin binding protein C (MyBP-C), are associated with the filament and help regulate assembly and function. Titin spans from the Z-disk to the M-line, helping to position and stabilize the thick filament and contribute to passive stiffness. M-line proteins such as myomesin cross-link thick filaments at the center of the sarcomere.

Function: During contraction, myosin heads form cross-bridges with actin, hydrolyze ATP, and undergo conformational changes that pull actin filaments past the thick filament. This cross-bridge cycling shortens the sarcomere and generates force. The thick filament’s length remains relatively constant during contraction, while the degree of overlap with thin filaments determines contractile output. In smooth muscle, thick filaments also rely on myosin II but are less regularly organized and regulated by phosphorylation of regulatory light chains.

Clinical and developmental relevance: Proper thick filament assembly is essential for normal muscle function; defects in myosin heavy chains or associated proteins can cause cardiomyopathies and myopathies. Thick filament organization is a key element of the sliding-filament mechanism of muscle contraction.