Test: Muscle (NCERT) - NEET MCQ

Sarcomere is the functional unit of myofibril. It contains two types of protein filaments called actin and myosin. These filaments slide upon each other to bring about the contraction of the muscles.

Sarcomere is a part of myofibril between two successive Z-lines, which consists of two I-band halves, and an A-band between them. It is functional unit of myofibril.

A myofibril has dark and light bands. The dark bands are called A bands (Anisotropic bands). Light bands are called I bands (isotropic bands).

Each actin (thin) filament is made of two 'F' (filamentous) actin helically wound to each other. Each F actin is a polymer of monomeric 'G' (globular) actins. Myosin (thick) filament is a polymerised protein. Many monomeric proteins called meromyosins constitute one thick filament. Tropomyosin is a fibrous molecule that attaches to F actin in the groove between its filament. The globular head of meromyosin consists of heavy meromyosin.

Heavy meromyosin consists of two globular sub-frament (S-1) and one rod shaped fibre sub-fragment (S-2). Each S-1 segment contains an ATPase site and a binding site for actin.

The type of muscle fibre present in the wall of alimentary canal is smooth muscle fibre. Smooth muscle fibres are non-striated and involuntary.

The cytoplasm of striated muscle is called sarcoplasm.

Line in NCERT: . 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.

A complex troponin protein of three globular peptides (Troponin T - Binding to tropomyosin as well as to the other two troponin components; Troponin I - inhibiting the F-actin - myosin interaction, also binding to other components of troponin; Troponin C - calcium binding polypeptide) is distributed at regular intervals on the tropomyosin. In the resting stage of muscle fibre, a sub - unit of troponin masks the active sites for myosin on the actin filaments.

Smooth muscles are non-striated, unbranched and spindle shaped. Skeletal muscles are unbranched. Cardiac muscles fibres are uni-nucleated.