A muscle is composed of about 1000 muscle fibres which are connected together by tendons. Each fibre is, in fact, only one, giant muscle cell, usually around 100?m in diameter and a few centimetres in length. They were created from a number of cells and contain numerous nuclei. The cytoplasm contains a multitude of myofibrils. These are protein filament bundles which carry out the contractions. The cells also contains a large number of mitochondria, vital for ATP production when the muscle contracts.

If you look at a muscle cell using an electron microscope you can see that each of the myofibrils is composed of dark and light bands. Through each band runs a thin line: in the dark band this is called the M line and the light band, the Z line. The units which repeat between the Z lines is known as a sarcomere.

Looking even more closely with a very high resolution electron micrograph it’s possible to observe that a myofibril is composed of parallel filaments. There are in fact two types which alternate, known as thick and thin filaments. At intervals the filaments are linked by cross-bridges.

Thick filaments are composed of the protein myosin. A myosin molecule has a thin body with two ‘heads’ at one end. Many molecules make up a thick filament, with the thin bodies tight together to create a backbone and the ‘heads’ poking out to create the cross-bridges.

Thin filaments consist of another protein: actin. Although this is a globular protein, when it polymerises it becomes a double helix chain. Thin filaments also contain two other proteins: troponin and tropomyosin. These have important roles in muscles contraction.

A sarcomere is composed of both thick and thin filaments, joined together to create a lattice; thick filaments are connected at the M line while thin filaments connect at the Z line. However, the thick and thin filaments are not bound permanently together.