Biology · Locomotion and Movement · NEET
No, and NEET loves this mix-up. A muscle FIBRE is one whole muscle cell (covered by the sarcolemma, holds sarcoplasm and many nuclei). Inside that one fibre lie many parallel thread-like MYOFIBRILS. So: muscle fibre = the cell, myofibril = one of the many contractile threads inside it. Order from big to small: muscle > muscle bundle (fascicle) > muscle fibre > myofibril > myofilament (actin/myosin).
The sarcomere. NCERT says: 'The portion of the myofibril between two successive Z lines is considered as the functional unit of contraction and is called a sarcomere.' It is NOT the myofibril, NOT the muscle fibre, and NOT the M line. When you are asked for the functional or structural unit of contraction, the answer is always the sarcomere.
Between two successive 'Z' lines. One sarcomere = one full 'A' band in the centre + two half 'I' bands (one on each side), bounded by a Z line at each end. The Z line bisects each I band, so each sarcomere carries a full A band and two halves of I bands.
A myofibril is a long thread running the length of the muscle fibre, made of many sarcomeres joined end to end. A myofilament is the actual protein filament inside the sarcomere: thin filaments (actin) and thick filaments (myosin). So myofilaments are the building blocks; the myofibril is the whole thread built from them. NCERT sometimes uses 'myofilaments or myofibrils' loosely, but for exams treat filaments (actin/myosin) as the smallest level.
Because of the ordered arrangement of two proteins. The light band (I band) contains only thin actin filaments and looks pale; the dark band (A band) contains thick myosin filaments and looks dark. These light and dark bands repeat one after another along every myofibril, and since all myofibrils line up together, the whole fibre looks striped. That is why skeletal muscle is called striated muscle.
Which of the following statements are correct regarding skeletal muscle? A. Muscle bundles are held together by collagenous connective tissue layer called fascicle. B. Sarcoplasmic reticulum of muscle fibre is a store house of calcium ions. C. Striated appearance of skeletal muscle fibre is due to distribution pattern of actin and myosin proteins. D. M line is considered as functional unit of contraction called sarcomere. Choose the most appropriate answer from the options given below:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Many. Sarcomeres are arranged one after another (serially) along the length of each myofibril, like beads on a thread. When they all shorten together, the whole muscle contracts.
Two successive 'Z' lines. Each sarcomere has one central 'A' band and two half 'I' bands, with a Z line at each end.
Both terms are used for it. NCERT calls the sarcomere the functional unit of contraction; it is also the basic structural (repeating) unit of the myofibril.
Actin (thin filaments, in the light I band) and myosin (thick filaments, in the dark A band). Their ordered pattern creates the alternating light and dark bands.
No. During contraction the sarcomere shortens as the Z lines are pulled inward and the I bands reduce, while the A band keeps its length. This is covered in detail on the band-changes page.
