LPT 321: Meat Science (1+1)

SARCOMERE 

• The unit of the myofibril between two adjacent Z lines (an ‘A’ band and two half ‘I’ bands located on either side of the ‘A’band) is called a sarcomere.
• The sarcomere is the repeating structural unit of the myofibril and it is also the basic unit in which the events of the muscle’s contraction-relaxation cycle occur. Thus, the sarcomere is the functional unit of muscle.
• Sarcomere length is not constant and its dimensions, as well as those of I band, are dependent on the state of contraction at the time the muscle is examined.
• In the central region of the ‘A’ band, there is an area that has slightly less density than the remainder of the band. This lighter region is called the H zone which consists only of myosin filaments.
• Additionally, a region of relatively low density appears within the H zone on either side of the M line. This low-density region is referred to as the pseudo H zone, which comprises only the tail fraction of myosin.
• The H zone is less dense than the rest of the ‘A’ band because it is the centre region between the ends of the opposing actin filaments and contain only myosin heads. The pseudo H zone consists only of myosin tail.
• The width of the H zone varies with the state of contraction of the muscle.
• The densest area of the ‘A’ band is on the either side of the H zone, where both the actin and myosin filaments are present.
• Since I band contains only the thin actin filaments, it is the least dense band of the entire myofibril.
• The myosin filaments in the H zone region of the sarcomere are oriented in a definite hexagonal pattern – six thin filaments surround each thick filament.
• The H zone is completely obliterated when the muscle contracts fully, as the actin filaments are pulled towards itself by the myosin head, while the pseudo H zone is not obliterated by any amount of contraction.