Question

Striations in the striated muscles are due to-
A. Projections of myosin
B. Absence of myofilaments
C. Presence of myofilaments
D. Specialized arrangement of myofilaments.

Answer 1

Hint: Striations are alternate light and dark bands that are seen in certain muscle fibres, like voluntary muscles, cardiac muscles, etc. The main function of these striations is to create force and contraction in the muscle fibres. Striated muscles are named so because upon staining they give an alternate band-like appearance. Their repeating functional unit is called sarcomere.

Complete answer:
These tissues are made up of elongated fibres and occur in skeletal and cardiac muscles and in most muscles of the arthropods.
a. Skeletal Muscles- These muscles are covered by epimysium. These are encased in endomysium packed as fascicles. Each sarcomere is composed of myofibrils which are made up of actin and myosin. These muscles are also known as voluntary muscles.
b. Cardiac muscles- It lies between the epicardium and the endocardium in the heart. These are uninucleated and consist of intercalated discs with gap junctions and desmosomes.
The striations are found due to the presence of myofilaments. The myofibrils or myofilaments are actin and myosin. Myofilaments are the filaments of myofibrils. These filaments regulate the contraction of muscles. These myofibrils form partially overlapping structures and give light and dark band appearance. The dark bands are known as A bands and the light bands are known as I-bands.
The correct answer here is option ( C ), i.e., presence of myofilaments.

Note: Myofilaments have actin and myosin.
-Myosin: These are thick filaments. It has a double-headed structure.
-Actin: These are the thin filaments and are the polymers of F-actin.
A bands are bisected by the M line and the I bands are bisected by the Z line. The region between two Z-lines is called the sarcomere; myofibrils are made up of these sarcomeres. The function of myofibril is to perform muscle contraction by the sliding filament theory.