Muscle fibre. Computer artwork showing the structure of a muscle cell, also known as a muscle fibre. The cell is surrounded by a plasma membrane called the sarcolemma (cream). Within the cell are myofibrils, bundles of actin and myosin fibres (pink) surrounded by a sarcoplasmic reticulum (yellow). The sarcoplasmic reticulum holds a reserve of calcium ions, which are needed for contraction. Mitochondria (blue) within the myofibrils provide the energy needed for contraction.