Neuromuscular junction. Computer artwork showing the juntion between a neuron (nerve cell, light blue) and a muscle cell, known as a neuromuscular junction. The structure of the muscle cell, also known as a muscle fibre, is also shown. 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 (dark blue) within the myofibrils provide the energy needed for contraction. Messages are passed from the axon of the neuron, via neurotransmitters, to the muscle telling it when to contract.