contractile muscle fiber

Definition

Target type: cellularcomponent

Fibers, composed of actin, myosin, and associated proteins, found in cells of smooth or striated muscle. [GOC:go_curators, ISBN:0815316194]

Contractile muscle fibers, also known as myofibers, are highly specialized elongated cells responsible for muscle contraction. They are composed of multiple cellular components that work together to generate force and movement. These components can be broadly categorized into three main levels:

1. **Myofibrils:** These are the fundamental units of muscle contraction, composed of repeating units called sarcomeres. Sarcomeres are the basic functional unit of a muscle fiber and consist of two primary protein filaments:
* **Actin filaments:** Thin filaments that are composed of the protein actin. They are anchored to the Z-discs, which are structural proteins that define the boundaries of a sarcomere.
* **Myosin filaments:** Thick filaments that are composed of the protein myosin. They are responsible for generating the force needed for muscle contraction.

2. **Sarcolemma:** This is the plasma membrane of a muscle fiber. It surrounds the myofibrils and plays a crucial role in transmitting nerve impulses to initiate muscle contraction. The sarcolemma is also involved in regulating the passage of ions and nutrients into and out of the muscle fiber.

3. **Sarcoplasmic reticulum:** This is a specialized network of intracellular membranes that surrounds each myofibril. It is responsible for storing and releasing calcium ions, which are essential for muscle contraction. The sarcoplasmic reticulum contains specialized structures called terminal cisternae, which are located near the T-tubules (transverse tubules).

**T-tubules:** These are invaginations of the sarcolemma that extend deep into the muscle fiber. They play a vital role in transmitting the nerve impulse from the sarcolemma to the sarcoplasmic reticulum, triggering the release of calcium ions and initiating muscle contraction.

These components work in a coordinated manner to generate force and movement. Upon receiving a nerve impulse, the sarcolemma depolarizes, which triggers the release of calcium ions from the sarcoplasmic reticulum. Calcium ions bind to troponin, a protein associated with the actin filaments. This binding causes a conformational change in troponin, which in turn moves tropomyosin, another protein associated with actin, out of the way. This exposes the myosin-binding sites on actin, allowing myosin to bind and initiate the sliding filament mechanism, leading to muscle contraction.

In summary, the cellular components of a contractile muscle fiber include myofibrils, sarcolemma, sarcoplasmic reticulum, T-tubules, and the various proteins that compose these structures. Each component plays a specific role in ensuring the proper function of the muscle fiber, allowing for the generation of force and movement.'
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Proteins (2)

Compounds (78)