Question

The filaments of myofibril are made up of
(a) Actin, troponin, tropomyosin
(b) Actin, troponin
(c) Myosin, troponin
(d) Actin, tropomyosin

Answer 1

 
Hint: Myofibrils are the contractile fibers present in the striated muscles. They are composed of thick and thin contractile filaments and muscle proteins bind them and help in carrying out the contractions of the voluntary muscles.

Complete Step by Step Answer:
Myofibrils, also known as muscle fibrils are basic rod- like units of a muscle cell. Myofibrils are composed of long contractile proteins that include actin and myosin, and other proteins that hold them together. These muscle proteins are organized into thin and thick filaments known as myofilaments that repeat along the complete length of the myofibrils in sections called sarcomeres.
The myofibrils also contain proteins such as troponin and tropomyosin, which help in the functions of muscle contraction and relaxation.

So, the correct answer is, “Actin, troponin, tropomyosin”.

Additional Information:
- The sliding filament theory explains the mechanism of contraction of muscle fibers based on the muscle proteins that slide over each other to exhibit movement.
- According to this theory, the thick myosin filaments slide over the thin actin filaments during the process of muscle contraction by pulling the actin filament, while both the filaments remain relatively constant in length.
- However, this can occur only when the actin filaments bind to the myosin filaments in the troponin I of the troponin complex which consists of troponin I, troponin T, and troponin C.
- In a relaxed muscle, there is a tropomyosin filament that covers the troponin T and inhibits the actin-myosin complex formation on the troponin I and thus contraction does not occur.

Note:
- Myofibrils and all the muscle filaments and proteins form the basic structure of muscle cells. However, these myofibrils occur only in striated muscles i.e. voluntary muscles and cardiac muscles.
- In involuntary smooth muscle cells, spindle- shaped muscle cells with a single nucleus are present instead of myofibrils.