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Which one of the following is marked correct?
A) a: Z-line located at the centre of I-band.
B) b: Thin filament, occurs in A-band only.
C) c: Thick filament, confined to I-band.
D) d: H-zone, located at the centre of M-line.
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Last updated date: 20th Jun 2024
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Hint: Myofibrils are long filaments that run parallel to each other to form muscle (myo) fibers. The myofibrils, and resulting myofibers, may be several centimetres in length. The muscle fibres are single multinucleated cells that combine to form the muscle.

Complete step by step answer: The movement of muscle fibres help in the locomotion of humans. Muscle bundles make up each skeletal muscle. Each muscle bundle is in turn composed of muscle fibres. Sarcolemma enclosing the sarcoplasm surrounds the muscle fibre. The sarcoplasmic reticulum which is the reticulum of muscle fibres is rich in calcium. Myofibrils are formed by the arrangement of a large number of parallel filaments. Dark and light bands are the main characteristic features of a myofibril. The dark bands are referred to as A-bands, whereas the light bands are called I-bands. A stands for Anisotropic and I stands for Isotropic. A zone that is comparatively less dark is present at the centre of the A-band and is called the H-zone (Hensen zone). The H-zone has the M-line in the middle of it. Z-band which is a dark membrane is present at the centre of each I-band. Z-disc, Krause’s membrane, Dobie’s line are a few other names of Z-line. The sarcomere is the part of the myofibril which is present between two successive Z-lines.
Hence, the correct answer is option A.

Note: Some important points to remember:
i. The A-band and half of each adjacent I-band forms the sarcomere.
ii. The structural and functional unit of microfibril is sarcomere which has thick and thin myofilaments.
iii. The myosin proteins form the thick myofilaments. Actin, tropomyosin, and troponin form the thin myofilaments.
iv. The main function of the Z-line is to act as an anchor to actin filaments.