Logan Steele
Myofibrils are contractile fibres that are found within muscle cells. They 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 thick and thin filaments slide past each other to cause muscle contraction and relaxation. The number of myofibrils in a muscle cell can vary depending on factors such as age and training. Understanding the structure and function of myofibrils is crucial in comprehending the mechanics of muscle movement and contraction.
| Characteristics | Values |
|---|---|
| Description | Long contractile fibres |
| Composition | Thick and thin myofilaments, long proteins including actin, myosin, and titin, and other proteins that hold them together |
| Diameter | 1-2 micrometres |
| Number in a muscle | 50 per myocyte in the muscles of a fetus, 2000 per myocyte in the muscles of an untrained adult |
| Function | Produce muscle contraction and relaxation |
| Appearance | Striated or striped |
| Constituent units | Sarcomeres |
| Constituent bands | A-bands (dark) and I-bands (light) |
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What You'll Learn
Myofibril structure
Myofibrils are rod-like organelles of a muscle cell, composed of long proteins including actin, myosin, and titin, as well as other proteins that hold them together. They are created during embryonic development in a process known as myogenesis. Myofibrils are made up of thick and thin myofilaments, which give the muscle its striped appearance. The thick filaments are composed of strands of the protein myosin, while the thin filaments are strands of the protein actin, along with two other muscle regulatory proteins, tropomyosin and troponin.
The thick and thin myofilaments repeat along the length of the myofibril in sections or units of contraction called sarcomeres. These sarcomeres are the repeating subunits of the myofibril, delimited by two very dark-coloured bands called Z-discs or Z-lines. These Z-discs are dense protein discs that do not easily allow the passage of light. The area between the Z-discs is further divided into two lighter-coloured bands at either end called the I-bands or Isotropic Bands, and a darker, grey band in the middle called the A band or Anisotropic Bands. 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 light passage.
The sarcomere structures give skeletal muscle its striated appearance and are readily visible under electron microscopy. The sarcomeric subunits of smooth muscle, however, have no alignment, resulting in a lack of striations and giving the cells a smooth appearance. The shortening of the individual sarcomeres leads to the contraction of the individual muscle fibres, resulting in muscle contractions. This occurs when the actin is pulled along the myosin towards the centre of the sarcomere, causing the H zone to decrease in size until it disappears as the muscle fully contracts.
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Myofibril composition
Myofibrils are long, contractile fibres, composed of thick and thin myofilaments. These myofilaments are made up of long proteins, including actin, myosin, and titin, and other proteins that hold them together. The thick filaments are composed of strands of the protein myosin, and the thin filaments are predominantly strands of the protein actin, along with two other muscle regulatory proteins, tropomyosin and troponin.
The thick and thin filaments form partially overlapping layers that are laid out in functional units called sarcomeres. Sarcomeres are the repeating subunits of myofibrils, and they are responsible for muscle contractions. Each sarcomere is delimited by two very dark-coloured bands called Z-discs or Z-lines. These Z-discs are dense protein discs that do not easily allow the passage of light. The area between the Z-discs is further divided into two lighter-coloured bands at either end, called the I-bands or Isotropic Bands, and a darker, grey band in the middle, called the A band or Anisotropic Bands. 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.
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. 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.
The myofibrils have a diameter of between 1 and 2 micrometres and are created during embryonic development in a process known as myogenesis. The number of myofibrils ranges from 50 per myocyte in the muscles of a fetus to approximately 2000 per myocyte in the muscles of an untrained adult.
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Myofibril function
Myofibrils are long, contractile fibres that run parallel to each other on the long axis of the myocytes (long single multinucleated cells that combine to form the muscle). They are composed of thick and thin myofilaments, which give the muscle its striped appearance. The thick filaments are composed of strands of the protein myosin, while the thin filaments are made up of strands of the protein actin, along with two other muscle regulatory proteins, tropomyosin and troponin.
Myofibrils are created during embryonic development in a process known as myogenesis. They have a diameter of approximately 1 μm, making up about 80% of the volume of a whole muscle. The number of myofibrils in a muscle can vary, ranging from 50 per myocyte in the muscles of a fetus to approximately 2000 per myocyte in the muscles of an untrained adult.
The thick and thin myofilaments of the myofibril are organised into repeated subunits called sarcomeres, which are the smallest contractile unit of muscle tissue. These sarcomeres are around 3 μm in length and are composed of actin and myosin filaments of specific and constant lengths. The sarcomeres contract through the sliding of the thick and thin myofilaments along each other, which is facilitated by the presence of adenosine triphosphate (ATP). This sliding action results in the shortening of the sarcomeres, leading to the contraction of the individual muscle fibres and, ultimately, muscle contractions.
In addition to their contractile function, myofibrils also play a role in maintaining muscle structure. They are supported by a complex cytoskeletal network of intermediate filaments and accessory proteins, which help to maintain the alignment of myofilaments and sarcomeres, as well as transfer forces from the peripheral myofibrils to the sarcolemma.
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Myofibril banding patterns
Myofibrils are composed of thick and thin myofilaments, which give the muscle its striped appearance. The thick filaments are composed of strands of the protein myosin, and the thin filaments are strands of the protein actin. The banding patterns observed in myofibrils are a result of the alternating dark and light bands formed by these thick and thin filaments.
The thick and thin filaments are arranged in a specific pattern, with the thick filaments in the middle and the thin filaments on the outside. This arrangement creates the characteristic banding pattern that is observed in muscle fibres. The thick and thin filaments do not change length but slide past each other during muscle contraction, causing the H zone to become smaller and eventually disappear when the muscle is fully contracted.
The dark bands, known as A bands or Anisotropic Bands, contain mostly thick myosin filaments whose larger diameter restricts the passage of light, making them appear darker. The A band also contains some thin actin filaments, which interdigitate with the thick filaments. The A band is associated with the development of myofibrils, as it contains the thick filaments that are present in developing muscle cells.
The light bands, known as I bands or Isotropic Bands, contain only thin actin filaments. These filaments have a smaller diameter, allowing the passage of light between them, resulting in a lighter appearance. The I bands are important for muscle contraction as they contain the thin actin filaments that are pulled along myosin towards the centre of the sarcomere during muscle contraction.
The Z-discs or Z-lines are the very dark-coloured bands that delimit each sarcomere. These dense protein discs do not easily allow the passage of light. The Z-line contains the protein a-actinin, which binds to the actin filaments. The sarcomere, which is the distance between two Z-lines, can be considered the primary structural and functional unit directly responsible for muscle contraction.
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Myofibril development
Myofibrils are created during embryonic development in a process known as myogenesis. They are basic rod-like organelles of a muscle cell, with a diameter of 1-2 micrometres. 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.
Sarcomeres are the subunits of myofibrils, with each subunit being around 3 micrometres in length. The thick and thin myofilaments of the sarcomeres give the muscle its striped appearance. The thick filaments are composed of the protein myosin, while the thin filaments are composed of the protein actin, along with two other muscle regulatory proteins, tropomyosin and troponin.
Under the influence of adenosine triphosphate (ATP), the actin and myosin form a contractile compound, actomyosin, which is required for muscle contraction. The shortening of the individual sarcomeres leads to the contraction of the individual muscle fibres, resulting in muscle contractions.
The growth in the girth of muscle fibres appears to occur through the splitting of myofibrils, which can be stimulated by the development of stress on the sarcomere. This process adds to the diameter or girth of the muscle fibres without hyperplasia. The growth in length, on the other hand, occurs at either end of the fibres, resulting in the addition of new sarcomeres.
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Frequently asked questions
Myofibrils are long contractile fibres, composed of long proteins including actin, myosin, and titin, and other proteins that hold them together. They are the basic rod-like organelles of a muscle cell.
Each myofibril is made up of many sarcomeres, which are short, repeating structural contractile units. Sarcomeres are arranged end to end and reveal two distinct bands under a microscope: the dark band known as the A-band and the light band known as the I-band.
The main function of myofibrils is to produce muscle contraction and relaxation. The contractile compound actomyosin, formed under the influence of adenosine triphosphate (ATP), is required for muscle contraction.
Myofibrils are composed of thick and thin myofilaments, which give the muscle its striped appearance. The thick filaments are composed of strands of the protein myosin, and the thin filaments are composed of the protein actin, along with two other muscle regulatory proteins, tropomyosin and troponin.
