Sarcomere Structure and Muscle Contraction — NEET Biology
✅ Asked in NEET 2026Both the proteins are arranged as rod-like structures, parallel to each other and also to the longitudinal axis of the myofibrils. Actin filaments are thinner as compared to the myosin filaments, hence are commonly called thin and thick filaments respectively. In the centre of each 'I' band is an elastic fibre called 'Z' line which bisects it. The thin filaments are firmly attached to the 'Z' line. The thick filaments in the 'A' band are also held together in the middle of this band by a thin fibrous membrane called 'M' line. The 'A' and 'I' bands are arranged alternately throughout the length of the myofibrils. The portion of the myofibril between two successive 'Z' lines is considered as the functional unit of contraction and is called a sarcomere. In a resting state, the edges of thin filaments on either side of the thick filaments partially overlap the free ends of the thick filaments leaving the central part of the thick filaments. This central part of thick filament, not overlapped by thin filaments is called the 'H' zone.
The sarcomere is the functional contractile unit of muscle, bounded by two Z-lines and containing thick myosin and thin actin filaments arranged in a specific pattern. Students often confuse the H-zone (the central region of thick filament not overlapped by thin filaments in resting state) with the A-band or I-band, leading to incorrect answers about muscle architecture. Remember: the H-zone only exists in resting muscle and disappears during contraction as thin filaments slide over thick filaments. NEET tests this to verify understanding of muscle structure, contraction mechanism, and the spatial arrangement of contractile proteins within sarcomeres.
Choose the correct statements regarding muscle contraction: A. A motor neuron carries a CNS signal to the sarcolemma. B. The neural signal generates an action potential causing Ca++ release into sarcoplasm. C. Increase in Ca++ inactivates actin for breaking cross bridges. D. Actin binds myosin head to form a cross bridge. E. Sarcomere shortens by pulling actin filaments toward centre of A band.
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