(a) Describe the structure of a sarcomere and indicate the relationship of the sarcomere to the myofilament. (b) Explain the sliding filament theory of contraction inlcuding a relaxed and a contracted sarcomere
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STRUCTURE OF A SARCOMERE
The sarcoplasm of a muscle cell contains numerous "little threads" called myofibrils. They extend lengthwise down the muscle fiber. The myofibrils are the contractile elements of a skeletal muscle cell. They are, in turn, made up of even smaller structures called myofilaments. There are two types of myofilaments: thin filaments and thick filaments. The contraction of a muscle occurs when the thin and thick filaments slide past each other and overlap to a great degree.
However, the myofilaments inside a myofibril do not extend the entire length of the muscle fiber. Rather, they are arranged in compartmental units called "sarcomeres." These are the basic functional unit of the contracting muscle fiber. The ends of the sarcomere are marked by a thick line called the Z disc. This is what separates one sarcomere from another. Extending from the Z disc are the thin filaments. Where there are only thin filaments, the sarcomere looks "light." As a result, this band is called the "I" band (for the "I" in the word "light"). The Z disc passes right through the middle of the I band, which straddles two adjacent sarcomeres. In the middle of a sarcomere is the "A" band, so named because it is a dark band (comes from the "A" in "dark"). This is where the myosin filaments lie. These are the thick filaments. Of course, in addition, the A band contains area of overlap between the actin and myosin filaments.
A narrow "H zone" exists in the center of each A band. This is where there are only thick filaments and no thin filaments. The middle of the H zone is the M line, the anchor point for the myosin filaments which extend outward from this middle point in opposite directions.
SLIDING FILAMENT THEORY
The sliding filament theory is rather simple in concept. The sarcomere contracts as the thin and thick filaments slide past each other. In so doing, the A band stays the size size; however, the I band narrows substantially -- as does the H zone. This occurs because there is more overlap between the thick and thin filaments within the sarcomere. As sarcomeres contract in unison, the entire muscle fiber contracts.
How does it take place?
It is important to understand the basic structural shape of actin and myosin. Actin filaments are long and narrow. They have binding sites for myosin. Myosin, on the other hand, looks something like a little golf club. The "head" of the golf club binds with the actin filaments. When it does and when it gets activated by ATP, the head pivots (somewhat like a rowing movement). This pivoting action causes the thin and thick filaments to slide past one another to a certain degree. Eventually, the myosin head releases from the actin filament, reattaches, and pivots again. This series of movements works like a ratchet, slowly ratcheting in the thin filaments to a higher degree of overlap with the myosin filaments.
But there are other proteins involved: tropomyosin and troponin. And in a relaxed muscle fiber, tropomyosin covers the myosin binding sites on the actin, thus blocking myosin from binding. On top of the tropomyosin is troponin. This is the calcium binding protein. When calcium ions are released from the sarcoplasmic reticulum, the calcium binds to troponin. In turn, this causes tropomyosin to move off the myosin binding sites on the actin molecules. Now, myosin can bind. Once the binding occurs, the contractile mechanism gets initiated.