Question

During muscle contraction
A. Size of A-bands remains the same.
B. Size of H-zone becomes smaller.
C. Size of I-bands decreases.
D. All of the above.

Answer 1

Hint:-Muscle contraction involves complex physiological, electrical, biochemical and molecular events. Muscle contraction is best explained by the sliding filament theory. According to this theory, the sliding of the thin filaments over the thick filaments of a muscle fibre causes muscle contraction. The contraction of skeletal muscles is voluntary, which includes complex changes. Smooth muscles contract spontaneously even in the absence of chemical signals.

Complete Answer:-
The vertebral skeletal muscle fibres have a repeating striated pattern of light and dark bands due to the specific location of thick and thin filaments within the myofibrils. Myofibrils are long cylindrical fibres, occupying most of the sarcoplasm. The light bands seen in a striated muscle are called I-bands or isotropic bands and the dark bands are called A-bands or anisotropic bands. The coincidence of these bands of many myofibrils gives the muscle fibre its striated appearance. The light and dark bands are arranged alternatively so that the A-band is bounded on each side by an I-band. In the central region of the A-band, there is a lighter region known as the H-zone or Henson zone. Each I-band is bisected by a thin narrow line called the Z–line or Zwischenscheibe line. In the centre of the H-zone, a dark line called the M-line binds all the thick filaments in a sarcomere. Sarcomere is the portion of the myofibril present in between two adjacent Z-lines. The thin filaments in a myofibril are called the actin filaments and the thick filaments are called myosin filaments. Muscle contraction takes place when the active elements are drawn towards the centre of A- band. The thick filaments are attached to the A-band and the size of the A-band remains the same during muscle contractions. The thin filaments are firmly attached to the Z-line. As these actin filaments slide over the thick filaments, the size of the I-band decreases. The length of the H-zone also decreases, as the actin filaments within a myofibril move closer.
Thus, the right option is D.

Note:- Among the many theories explaining the principle of muscle contraction, the most accepted theory explaining the mechanism of muscle contraction was the `Sliding filament theory’ proposed by A. F. Huxley and J. Hanson, independently. Muscle contraction is initiated by a neural signal reaching the junction between a sarcolemma and a motor neuron. This electric potential makes some chemical changes by causing muscle contraction.