Skeletal muscle is attached to the skeleton, is voluntarily controlled and has cells that are very long “fibres” that are multinucleated and striated. Each skeletal muscle is an organ and is named. The names may seem strange but they contain information about the muscle. The name may indicate the location of the muscle in the body (femoris, brachii), the shape of the muscle (deltoid, trapezius), its action (adductor, pronator) or its relative size (maximus, longus) of the muscle. The names may indicate the origin and insertion points (sternocleidomastoid) of the muscle or the number of origins (biceps, quadriceps) or the direction (rectus, transversus) in which the muscle fibres lie. Or they may be named whimsically (sartorius). Skeletal muscle cells are distinctly different from cardiac and smooth muscle cells. They are also distinct from other body cells. The cells are called fibres, their plasma membrane is called sarcolemma, the cytoplasm is called sarcoplasm, the endoplasmic reticulum is called sarcoplasmic reticulum and the contracting structures within a myofibril of a muscle cell are called sarcomeres. Within a sarcomere are bundles of thick and thin myofilaments. The sarcolemma bundles many myofibrils together into a cell somewhat like a packet of spaghetti. Sarcomeres are joined end to end to form a long strand called a myofibril. The thick myofilaments are composed of the protein myosin. The thin myofilaments are composed of the protein actin, while the proteins, troponin and tropomyosin along with calcium ions and ATP participate in the physiology of contraction of a sarcomere. Each cell/fibre is surrounded by a membrane called the endomysium which overlies the sarcolemma. The endomysium contains nerve axons and blood capillaries. Nerve axons make contact with muscle fibres via a structure called the neuromuscular junction. A bundle of muscle fibres is called a fascicle and is surrounded by a connective tissue membrane called the perimysium. A muscle is a bundle of fascicles and the membrane surrounding a muscle is called the epimysium.