Actomyosin

Myofibril
Skeletal muscle, with myofibrils labeled at upper right.
Details
Identifiers
Latin	myofibrilla
MeSH	Myofibrils
Code	TH H2.00.05.0.00007
Anatomical terminology []

A myofibril (also known as a muscle fibril) is a basic rod-like unit of a muscle cell. Muscles are composed of tubular cells called myocytes, known as muscle fibers in striated muscle, and these cells in turn contain many chains of myofibrils. They are created during embryonic development in a process known as myogenesis.

Myofibrils are composed of long proteins including actin, myosin, and titin, and other proteins that hold them together. These proteins are organized into thick and thin filaments called myofilaments, which repeat along the length of the myofibril in sections called sarcomeres. Muscles contract by sliding the thick (myosin) and thin (actin) filaments along each other.

The filaments of myofibrils, myofilaments, consist of two types, thick and thin:

The protein complex composed of actin and myosin is sometimes referred to as "actinomyosin."

In striated muscle, such as skeletal and cardiac muscle, the actin and myosin filaments each have a specific and constant length on the order of a few micrometers, far less than the length of the elongated muscle cell (a few millimeters in the case of human skeletal muscle cells). The filaments are organized into repeated subunits along the length of the myofibril. These subunits are called sarcomeres. The muscle cell is nearly filled with myofibrils running parallel to each other on the long axis of the cell. The sarcomeric subunits of one myofibril are in nearly perfect alignment with those of the myofibrils next to it. This alignment gives rise to certain optical properties which cause the cell to appear striped or striated. In smooth muscle cells, this alignment is absent, hence there are no apparent striations and the cells are called smooth. Exposed muscle cells at certain angles, such as in meat cuts, can show structural coloration or iridescence due to this periodic alignment of the fibrils and sarcomeres.