Muscle structure, illustration. Muscles (top left) are formed of groups of muscle bundles, or fascicles, which are themselves formed of bundles of muscle fibres. Muscles fibres (right) are formed of many myofibrils, the basic protein unit of a muscle cell, surrounded by sarcolemma connective tissue. Within the muscle fibre are mitochondria (orange), which provide energy, and transverse tubules (purple), an extension of the cell membrane that conduct action potentials through the muscle. Each myofibril is surrounded by sarcoplasmic reticulum (blue).