Structure of a skeletal muscle:

Muscle bundles or fascicles- Many such fascicles are held together by a common collagenous connective tissue layer called fascia to form a skeletal muscle in our body.

Sarcolemma – Plasma membrane lining each muscle fibre which encloses sarcoplasm.

Sarcoplasm – Cytoplasm of a muscle cell. Muscle fibre is a syncitium as the sarcoplasm contains many nuclei.

Sarcoplasmic reticulum -Endoplasmic reticulum of the muscle fibres which is the store house of calcium ions.

Myofilaments or Myofibrils - a large number of parallel arranged filaments in the sarcoplasm. 

• Each myofibril has alternate dark and light bands on it.
• Striated appearance of myofibril is due to the distribution pattern of two important proteins – Actin and Myosin. 

Light band contains actin and is called I-band or Isotropic band. 

Dark band called ‘A’ or Anisotropic band contains myosin. 

Both proteins are arranged as rod-like structures, parallel to each other and also to longitudinal axis of the myofibrils. 

Actin filaments are thinner as compared to myosin filaments, hence are commonly called thin filaments. 

Myosin filaments are thick filaments.

Sarcomere - Portion of myofibril between two successive "Z" lines. - is the functional unit of contraction.