What is the structure of a muscle cell?

A muscle cell consists of several myofibrils that are arranged in a compact bundle. Each individual myofibril is composed of a bundle of sarcomeres that are attached end to end. The sarcoplasmic reticulum (SR), which is a specialized form of endoplasmic reticulum, extends in and around the myofibril bundles. Muscle cells also contain densely packed mitochondria that are responsible for providing a constant flow of ATP. 

The entire cell and its contents – the myofibrils, sarcoplasmic reticulum, and mitochondria – are enclosed by the sarcolemma, a specialized cell membrane that allows nerve impulses to pass through to the transverse tubules and activate the SR.