The Sarcomere

A sarcomere is mainly made up of two types of filaments— thin filaments containing actin, troponin, and tropomyosin and thick filaments containing myosin.

Troponin and tropomyosin-bound actin filaments flank and interlock with the myosin filaments in a sarcomere unit.

Another filamentous protein called titin runs from both ends of a sarcomere to its middle, anchoring thick filaments and giving elasticity to the whole unit.

The arrangement of these filaments gives myofibrils a characteristic banding pattern or striations.

The dark A band consists of myosin filaments at the center, forming the H zone. At the periphery of the A band, thin filaments overlap with myosin, creating the zone of overlap.

At both ends of the A band lies the lighter and less dense I band, composed of only thin filaments.

At the center of the I bands are the dense Z-discs, which mark the ends of a sarcomere.

The myomesin proteins at the center of each sarcomere connect the adjacent thick filaments and form the M line.