Skeletal muscles are specialised cells made up of bundles of long, tubular, multinucleate cells called muscle fibres. The muscle fibres are covered by a plasma membrane called the sarcolemma. The cytoplasm is known as the sarcoplasm and contains lots of mitochondria to produce ATP for muscle contraction. They have specialised endoplasmic reticulum called the sarcoplasmic reticulum and the sarcolemma is folded inwards to form transverse or T tubules.