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