Q.Sarcolemma, sarcoplasm and sarcoplasmic reticulum refer to a particular type of cell in our body. Which is this cell and to what parts of that cell do these names refer to?

Imagine a very long railway track made of identical carriages joined end to end. If you want the whole train to become shorter, you cannot do it by looking at the whole train at once — you understand it by looking at a single carriage and watching what happens inside it. In a muscle, that single repeating carriage is the sarcomere, and understanding its layout is the first step to understanding how a muscle shortens.

Inside a muscle fibre run long thread-like structures called myofibrils. Along each myofibril you can see alternating dark and light bands, which is why this kind of muscle looks striped. If you follow the myofibril and mark the fine lines that cross it, you will notice thin elastic lines called Z-lines repeating at regular distances. The stretch of myofibril lying between two successive Z-lines is one sarcomere — and this piece is treated as the functional unit of contraction, the smallest length that can carry out the whole contracting job on its own.

Within a single sarcomere the parts are arranged in a fixed and orderly way:

• At each end sits a Z-line, to which the thin filaments are firmly anchored.
• Around each Z-line lies a light band, the I-band, which contains the thin protein actin.
• In the middle lies a dark band, the A-band, which contains the thick protein myosin. …