Dominic Stone
Muscle fibres, or myofibers, are long, cylindrical cells that make up skeletal muscle. Each muscle fibre is composed of several hundred to several thousand myofibrils, which are the contractile organelles responsible for muscle contraction. Myofibrils are long, thread-like structures made up of the proteins actin and myosin. They run parallel to the muscle fibre and attach at their ends to the plasma membrane, or sarcolemma. As myofibrils shorten, they cause muscle contraction.
| Characteristics | Values |
|---|---|
| What is a muscle fiber? | A muscle fiber is a type of cell that is long and cylindrical in shape. |
| Are muscle fibers organelles? | No, muscle fibers are not organelles. They are composed of many organelles, such as mitochondria and myonuclei, and are themselves a part of an organ. |
| What are some organelles found in muscle fibers? | Organelles found in muscle fibers include myofibrils, mitochondria, and the endoplasmic reticulum (specialized as the sarcoplasmic reticulum) |
| What are myofibrils? | Myofibrils are contractile organelles composed of actin and myosin filaments. They are responsible for muscle contraction and are the smallest functional unit of a skeletal muscle fiber. |
| What do mitochondria do in muscle fibers? | Mitochondria are essential organelles that provide ATP molecules to fuel muscle cells and enable muscle contraction. |
| What does the endoplasmic reticulum do in muscle fibers? | The endoplasmic reticulum, specialized as the sarcoplasmic reticulum, assists mitochondria in filtering molecules and maintaining homeostasis. It also stores, releases, and retrieves calcium ions. |
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What You'll Learn
Muscle fibres are made up of organelles called myofibrils
Muscle fibres, or myofibers, are indeed made up of organelles called myofibrils. A myofibril is a basic rod-like organelle of a muscle cell. Each myofibril has a diameter of 1–2 micrometres. 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, thin, and elastic myofilaments, which repeat along the length of the myofibril in sections or units of contraction called sarcomeres. The sarcomere is the smallest functional unit of a skeletal muscle fibre. It is a highly organized arrangement of contractile, regulatory, and structural proteins.
The actin and myosin filaments each have a specific and constant length on the order of a few micrometres, far less than the length of the elongated muscle cell. The filaments are organized into repeated subunits along the length of the myofibril. These subunits are called sarcomeres, which are around 3 μm in length. 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 the cell its striped or striated appearance.
When a muscle contracts, the actin is pulled along the myosin towards the centre of the sarcomere until the actin and myosin filaments are completely overlapped. The H zone becomes smaller and smaller due to the increasing overlap of actin and myosin filaments, and the muscle shortens. Thus, when the muscle is fully contracted, the H zone is no longer visible. It is important to note that the actin and myosin filaments themselves do not change length, but instead slide past each other.
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Myofibrils are composed of actin and myosin
Muscle fibres, or myofibers, are long and cylindrical skeletal muscle cells. They are commonly referred to as muscle fibres because of their shape and structure. Each skeletal muscle is an organ that consists of various integrated tissues, including muscle fibres.
Within a muscle fibre, proteins are organised into organelles called myofibrils. Myofibrils are composed of actin and myosin filaments, as well as support proteins. Actin is a thin filament, while myosin is a thick filament. The arrangement of actin and myosin gives skeletal muscle its microscopic striated appearance and creates functional units called sarcomeres.
Sarcomeres are the smallest functional unit of a skeletal muscle fibre and are a highly organised arrangement of contractile, regulatory, and structural proteins. The actin-myosin bond is essential for muscle contraction, as it releases phosphate. During contraction, binding sites for myosin on actin filaments are uncovered, and calcium ions are released. The Z line, or Z disc, acts as an anchor for the actin filaments, while the M line is where myosin filaments are anchored.
The actin and myosin filaments are arranged longitudinally, with the M line in the centre of the sarcomere. The H band contains the M line and consists solely of myosin filaments. The A band is larger and contains the entire myosin filament, as well as regions of actin and myosin overlap. The support proteins within the sarcomere include titin, desmin, myomesin, C protein, nebulin, and plectin, which help to stabilise the structure.
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Myosin and actin filaments interact within sarcomeres
Muscle fibers are long and cylindrical skeletal muscle cells commonly referred to as myofibers. Each muscle fiber contains many myofibrils, which are bundles of actin and myosin filaments. The sarcomere is the smallest functional unit of a skeletal muscle fiber and is a highly organized arrangement of contractile, regulatory, and structural proteins. It is the shortening of these individual sarcomeres that lead to the contraction of individual skeletal muscle fibers.
Myosin is the prototype of a molecular motor—a protein that converts chemical energy in the form of ATP to mechanical energy, thus generating force and movement. The most striking variety of such movement is muscle contraction. Actin filaments, usually in association with myosin, are responsible for many types of cell movements. The thick filaments of muscle consist of several hundred myosin molecules, associated in a parallel staggered array by interactions between their tails. The globular heads of myosin bind actin, forming cross-bridges between the thick and thin filaments.
The orientation of myosin molecules in the thick filaments reverses at the M line of the sarcomere. The polarity of actin filaments (which are attached to Z discs at their plus ends) similarly reverses at the M line, so the relative orientation of myosin and actin filaments is the same on both halves of the sarcomere. The M line is the central-most line of the sarcomere, where myosin filaments are anchored together through binding sites within the myosin filament. The H band contains the M line and is the central region of the sarcomere that contains only myosin filaments. The A band is a larger portion of the sarcomere that contains the entirety of the myosin fibers and includes regions of actin and myosin overlap. The I band is rich in thinner filaments made of actin and contains only thin (actin) filaments.
The sliding filament theory states that the sliding of actin past myosin generates muscle tension. Because actin is tethered to structures located at the lateral ends of each sarcomere called Z discs or "Z bands," any shortening of the actin filament length would result in a shortening of the sarcomere and thus the muscle. Myosin slides along actin to contract the muscle fiber in a process that requires ATP. This movement slides the actin filaments from both sides of the sarcomere toward the M line, shortening the sarcomere and resulting in muscle contraction.
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Sarcomeres are the smallest functional unit of a muscle
Muscle fibres, or myofibers, are long and cylindrical skeletal muscle cells. They are made up of bundles of myofibrils, which are composed of actin and myosin filaments. These myofibrils are only approximately 1.2 μm in diameter, so hundreds to thousands of them can be found inside a single muscle fibre.
For a muscle cell to contract, the protein tropomyosin must be moved to uncover the binding sites on the actin. Calcium ions bind with troponin C molecules, which are dispersed throughout the tropomyosin protein, and alter the structure of the tropomyosin, forcing it to reveal the cross-bridge binding site on the actin. The concentration of calcium within muscle cells is controlled by the sarcoplasmic reticulum, a unique form of endoplasmic reticulum in the sarcoplasm.
Sarcomeres are defined as the segment between two neighbouring Z-lines (or Z-discs). They give skeletal and cardiac muscle their striated appearance. The length of the actin and myosin filaments (together referred to as sarcomere length) affects force and velocity – longer sarcomeres have more cross-bridges and thus more force, but have a reduced range of shortening.
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Mitochondria are interspersed throughout the muscle cell
Muscle fiber, or myofiber, is a term used to describe skeletal muscle cells. These cells are long and cylindrical, and they are made up of bundles of myofibrils. Each myofibril contains sarcomeres, which are the smallest functional unit of a skeletal muscle fiber. The arrangement of actin and myosin filaments within the sarcomeres gives skeletal muscle its striated appearance.
Skeletal muscle fibers are considered organelles, and they contain other organelles, including mitochondria. Mitochondria are essential for energy production in cells, including muscle cells. They are often referred to as the "powerhouse of the cell" or the cell's "power plants" due to their role in generating adenosine triphosphate (ATP), which is used as a source of chemical energy.
In skeletal muscle fibers, mitochondria are distributed within two main areas: the subsarcolemmal area (beneath the plasma membrane) and the intermyofibrillar area (between the myofibrils). The intermyofibrillar mitochondria can be further divided into two subpopulations based on their location within the muscle fiber. One subpopulation resides at the I-band, containing only the actin filament, while the other is located at the A-band, which contains both actin and myosin filaments.
The distribution of mitochondria within muscle fibers is regulated by proteins called MSP-300 and Klar, which promote even spacing of the organelles. This even distribution is important for maintaining the energy demands of skeletal muscle, which can increase rapidly during strenuous exercise. Studies have shown that diffusion alone is not sufficient to support normal muscle contraction, and mitochondria are able to rapidly distribute energy through a grid-like network.
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Frequently asked questions
A muscle fiber is a long, cylindrical muscle cell. They are also known as myofibers and are made up of myofibrils.
Myofibrils are the contractile organelles within muscle fibers that are responsible for muscle contraction. They are long, thread-like structures made up of the proteins actin and myosin.
A muscle fiber is a type of cell, whereas a myofibril is an organelle within that cell. Each muscle fiber is made up of many myofibrils.
Myofibrils are responsible for muscle contraction. They are made up of contractile proteins, regulatory proteins, and structural proteins. When a muscle fiber is stimulated to contract, the myosin and actin filaments within the myofibrils slide past one another, changing the length and shape of the muscle cell.
Muscle fibers contain a large number of mitochondria, which are essential for producing ATP through cellular respiration. They also contain the sarcoplasmic reticulum, which is a specialized form of endoplasmic reticulum that stores, releases, and retrieves calcium ions.
