Skeletal muscle fibers are cylindrical and multinucleate. They are 10 – 30 cm in length – long enough to justify the term ‘fiber’. They are formed by fusion of several smaller cells into a multinucleate syncitium. Skeletal muscles are so named because they are attached to bones by tendons and move these bones and the loads borne by them. They are the only muscles under voluntary control.

Like all other cells, the striated muscle fiber also has a cell membrane (called sarcolemma), a smooth endoplasmic reticulum (called sarcoplasmic reticulum), cytoplasm (called sarcoplasm) and cytoskeletal proteins which are of 3 types:

(i) Contractile proteins: myosin and actin. These two proteins interact to generate the contractile force in a muscle,

(ii) Regulatory proteins: tropomyosin and troponin. Also called the ‘relaxation proteins’, these regulate the interaction between actin and myosin.

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(iii) Anchoring proteins: a-actinin, titin, nebulin and dystrophin. These proteins anchor the cytoskeletal proteins to each other as well as to the sarcolemma and the extracellular matrix.

The cytoskeletal proteins are disposed in a remarkably orderly configuration, which shows in gross morphology as striations, and on light microscopy as the dark (A) and light (I) bands. The Z-line is located in the middle of the I-band. The part of the muscle fiber that extends between two consecutive Z-lines is called a sarcomere, which is the contractile unit of muscle.