Question

The functional unit of contractile system of a striated muscle is
A. Sarcomere
B. Z-band
C. Cross bridge
D. Myofibril

Answer 1

The striated muscles are skeletal muscles and cardiac muscles that have thick and thin bands or striations when visualized under a microscope. These striations are not present in smooth muscles.

Complete answer:
Option (A) is correct. The functional unit of the contractile system of a striated muscle is sarcomere that is a repeating unit imparting striations to the striated muscles due to the presence of proteins that form thick and thin filaments.

Option (B) is incorrect. Z-band is a part of sarcomere that anchors the actin protein of sarcomere and appears as a dark line in the center of the I band. The sarcomere is present in between the two neighboring Z-bands.
Option (C) is incorrect. Cross bridge as seen under high magnification is a small bridge-like structure that is seen on a thick filament extending towards the thin filament in the overlap region.
Option (D) is incorrect. Myofibrils are composed of long filaments of actin, myosin, and other associated proteins. These proteins get arranged into a region called a sarcomere. Thus, myofibril is not the functional unit of the contractile system of a striated muscle but the sarcomere which is a region in myofibrils formed by the arrangement of proteins is the functional unit of the contractile system of striated muscles.

Hence, the correct option is (A).

Note:
In striated muscles, the striations are present due to the presence of a repeating functional unit called sarcomeres. Two important proteins i.e., myosin and actin are present in the sarcomere that forms thick (A-band) and thin (I-band) filaments respectively. In the center of each I-band lies the Z-line to which thin filaments are attached.