Question

The smallest functional unit of the striated muscle is called as
A) Sarcomere
B) Myosin
C) Fibril
D) Sarcoplasm

Answer 1

Hint: The striations are seen with light and dark bands and are repeated throughout the muscle fiber. This repeated unit is called sarcomere.

Complete answer:
There are two types of muscles: smooth muscle cells and striated muscle cells. Striated muscles differ from smooth muscle cells with sarcomeres. The striated muscles include cardiac muscle cells and skeletal muscles. Sarcomeres are units of striations found in striated muscles only. The sarcomeres enable them to contract as they are repeating units between two Z lines. The function of these muscles is to create force and contract. They contain T tubules which allow calcium ions from sarcoplasmic reticulum.
Surrounding the Z disc, there are I bands which are not superimposed by thick filaments. After I band, there is A band which contains both thin and thick filaments. Between the A bands, there is a paler region called H zone. A thin line within the H zone is called the M line.
Muscle contraction is initiated by a signal sent to the central nervous system.Neural signal reaches the neuromuscular junction causes the release of calcium ions into the sarcoplasm.The release of calcium ion cause the removal of masking site for myosin which pulls the attached actin filament towards A band.

So, the correct answer is A.

Note:
When muscles contract, the A bands are unaffected in size but whereas I bands and H zone shortens. Thus, Z lines become closer to each other.