Biology · Locomotion and Movement · NEET
One muscle fibre is a single, very long muscle cell. Do not confuse it with a muscle bundle (fascicle), which contains many muscle fibres held together by connective tissue. The order from small to large is: myofibril, then muscle fibre (the cell), then muscle bundle (fascicle), then the whole muscle.
A muscle fibre is called a syncytium because its sarcoplasm contains many nuclei inside one single cell. It forms when many cells fuse together during development, so one long cell ends up with several nuclei. This is a common one-mark NEET fact, so remember: muscle fibre = multinucleate = syncytium.
The sarcolemma is the outer plasma membrane that lines and covers the muscle fibre. The sarcoplasmic reticulum is the internal endoplasmic reticulum inside the sarcoplasm. The sarcolemma receives the neural signal (action potential); the sarcoplasmic reticulum stores calcium ions and releases them when that signal arrives.
The sarcoplasmic reticulum is the store house of calcium ions (Ca2+). NEET asks this because calcium release from the sarcoplasmic reticulum is the trigger for muscle contraction: Ca2+ binds troponin and uncovers the active sites on actin. When contraction ends, Ca2+ is pumped back into the sarcoplasmic reticulum.
Yes. The sarcoplasm (muscle cytoplasm) contains a large number of parallel myofilaments called myofibrils. These myofibrils show the alternate dark (A) and light (I) bands and are made of the proteins actin and myosin. So the sarcoplasm holds the myofibrils, the many nuclei, and the sarcoplasmic reticulum.
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.
The sarcolemma is the plasma membrane that lines each muscle fibre and encloses the sarcoplasm. It carries the action potential that starts contraction.
The sarcoplasm is the cytoplasm of a muscle fibre. It contains many nuclei (making the fibre a syncytium), the sarcoplasmic reticulum, and the parallel myofibrils made of actin and myosin.
The sarcoplasmic reticulum is the store house of calcium ions (Ca2+). It releases Ca2+ to trigger contraction and pumps Ca2+ back to cause relaxation.
Because a single muscle fibre contains many nuclei within one shared sarcoplasm, formed by the fusion of many cells during development.
It is the endoplasmic reticulum of the muscle fibre, given a special name (sarcoplasmic reticulum) because of its role as the calcium ion store house in muscle.
