Q.In each of the following questions (Q. No. 15, 16, 17 & 18), a statement of Assertion (A) is given followed by a corresponding statement of Reason (R) just below it. Of the statement, choose the correct answer using the codes given below. Assertion (A) : In muscle contraction, length of both A-bands and I-band decreased.
Reason (R) : Both myosin of A-band and actin of I-band are contractile proteins and decrease in size during muscle contraction.

The sarcomere — the segment of a myofibril between two successive Z-lines, containing overlapping actin and myosin filaments — is the smallest structural and functional unit of contraction in a muscle fibre.

Within a skeletal muscle fibre, many thread-like myofibrils run the length of the cell. Each myofibril is divided along its length into repeating segments called sarcomeres, marked off by thin, dense boundary lines called Z-lines (or Z-discs). Within one sarcomere, thin actin filaments and thick myosin filaments overlap in an ordered arrangement. During contraction, myosin heads on the thick filaments pull the thin actin filaments inward (the sliding filament mechanism), shortening the sarcomere; because sarcomeres are arranged end to end along …

During muscle contraction the actin (thin) filaments slide over the myosin (thick) filaments; the A-band (length of the myosin filament) stays constant, only the I-band and H-zone shorten, and neither actin nor myosin filaments themselves shorten.

According to the sliding filament theory of muscle contraction:

• The A-band (the region occupied by myosin, or thick, filaments) corresponds to the length of the myosin filaments and does not change during contraction.
• The I-band (the region containing only actin, or thin, filaments) decreases in width as the actin filaments slide further into the A-band.
• The H-zone also decreases as the actin filaments approach each other in the centre of the sarcomere.

So the Assertion — that both the A-band and I-band decrease — is factually false, since the A-band length remains unchanged; only the I-band decreases.

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The sarcomere — the region of a myofibril between two Z-lines, containing thin (actin) and thick (myosin) filaments — is the basic functional unit of muscle contraction.

Each skeletal muscle fibre contains many myofibrils, and each myofibril is divided lengthwise into repeating units called sarcomeres, bounded by Z-lines.

• Actin and myosin are individual contractile proteins that make up the thin and thick filaments respectively, but neither alone is 'the functional unit'.
• Sarcoplasm is simply the cytoplasm of the muscle fibre, containing myofibrils, mitochondria and stored glycogen — a component, not the functional unit. …