Question

The functional unit of the contractile system in striated muscle is
a. Cross bridge
b. Myofibril
c. Sarcomere
d. Z-band

Answer 1

Hint: Striated muscle is a type of muscle tissue, which having striations like appearance when looking into the microscope, basically striated muscles are of two types, which include Cardiac muscle and skeletal muscle.

Complete answer:
- Striated muscle consists of a repeated functional unit called a sarcomere.
- Striated muscles are composed of tubular muscle cells, which are formed in the process of myogenesis.
- Usually, a muscle fiber contains tubular myofibrils, where these muscle fibers are again composed of repeated sections of the units called a sarcomere
- So sarcomere is considered as a structural and functional unit.
- When we look at sarcomere under the microscope, it shows the alternative dark and light bands.

- Generally, sarcomere comprises large filaments of proteins which slide one on each other, when they contract.
- Two important proteins are present in the sarcomere, one is myosin, which forms the thick filament and another one is actin which forms the thin filament.
- Myosin filaments have a long tail and globular head, this head binds to the actin if the actin filaments are exposed to the calcium ions, and the energy source for this filament is the ATP.
- Actin molecules are generally bound to the Z lines. Where the sarcomere is present between the two Z lines.
- Surrounding Z line there is an I band where the thick and thin filaments are not contacted each other.
- Following I band there is A band where it contains the entire length of the single thick filament.
- The paler region of the A band is the H zone and it consists of the M line.

Hence, the correct answer is option (C).

Note: During muscle contraction, A band do not change their length, whereas I band and H zone are coming close to each other, this brings the Z lines to come close to each other, during rest A band remains dark, I band becomes light and Z line is darker.