Question

what do you mean by myofibrils?

Answer 1

A myofibril (also known as a muscle fibril) is a basic rod-like unit of a muscle cell.[1] 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.

Myofibril

Skeletal muscle, with myofibrils labeled at upper right.
Details
Identifiers
Latin
myofibrilla
TH
H2.00.05.0.00007
Anatomical terms of microanatomy
[edit on Wikidata]

A diagram of the structure of a myofibril (consisting of many myofilaments in parallel, and sarcomeres in series

Sliding filament model of muscle contraction
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.

Structure Edit

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

Thin filaments consist primarily of the protein actin, coiled with nebulin filaments. Actin, when polymerized into filaments, forms the "ladder" along which the myosin filaments "climb" to generate motion
Thick filaments consist primarily of the protein myosin, held in place by titin filaments. Myosin is responsible for force generation. It is composed of a globular head with both ATP and actin binding sites, and a long tail involved in its polymerization into myosin filaments.
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.[2] 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.[3]

Appearance Edit

The names of the various sub-regions of the sarcomere are based on their relatively lighter or darker appearance when viewed through the light microscope. Each sarcomere is delimited by two very dark colored bands called Z-discs or Z-lines (from the German zwischen meaning between). These Z-discs are dense protein discs that do not easily allow the passage of light. The T-tubule is present in this area. The area between the Z-discs is further divided into two lighter colored bands at either end called the I-bands, and a darker, grayish band in the middle called the A band.

The I bands appear lighter because these regions of the sarcomere mainly contain the thin actin filaments, whose smaller diameter allows the passage of light between them. The A band, on the other hand, contains mostly myosin filaments whose larger diameter restricts the passage of light. A stands for anisotropic and I for isotropic, referring to the optical properties of living muscle as demonstrated with polarized light microscopy.

The parts of the A band that abut the I bands are occupied by the both actin and myosin filaments (where they interdigitate as described above).
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Answer 2

A myofibril (also known as a muscle fibril) is a basic rod-like unit of a muscle cell.[1]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