Where Myofibrils Reside In Muscle Cells

Myofibrils, also known as muscle fibrils or sarcostyle, are rod-like organelles of a muscle cell. They are contractile structures composed of myofilaments. Each myofibril has a diameter of 1–2 micrometres and extends the entire length of the muscle fibre. 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. In this text, we will explore the structure and function of muscle myofibrils and their role in muscle contraction and growth.

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Myofibril structure

Myofibrils are rod-like organelles of a muscle cell that are 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. Each myofibril has a diameter of 1–2 micrometres and they make up about 80% of the volume of a whole muscle. The number of myofibrils per muscle fibre varies, ranging from 50 per muscle fibre in a fetus to about 2000 per fibre in an untrained adult.

Myofibrils are composed of thick, thin, and elastic myofilaments, which repeat along the length of the myofibril in sections or units of contraction called sarcomeres. Each sarcomere is delimited by two very dark-coloured bands called Z-discs or Z-lines, which 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 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 thick and thin filaments within each sarcomere are responsible for muscle contraction and relaxation. 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, a relatively brighter central region in the A band, becomes smaller and smaller due to the increasing overlap of actin and myosin filaments, and the muscle shortens. 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.

The sarcoplasmic reticulum, a membrane system, surrounds each myofibril and controls the level of calcium ions in the sarcoplasm. The terminal cisternae within the sarcoplasmic reticulum are the sites from which calcium ions are released when the muscle is stimulated, and the longitudinal tubules are where calcium ions are removed from the sarcoplasm.

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Myofibril composition

Myofibrils are composed of long proteins including actin, myosin, titin, tropomyosin, troponin, and other proteins that hold them together. The thick filaments are composed of myosin, and the thin filaments are predominantly actin, along with two other muscle proteins, tropomyosin and troponin. The thick and thin filaments are held together by cross-bridges, formed by the myosin heads.

The filaments are organised into repeated subunits along the length of the myofibril, with each subunit referred to as a sarcomere. Sarcomeres are the smallest functional unit of a skeletal muscle fibre and are highly organised arrangements of contractile, regulatory, and structural proteins. They are around 3 μm in length and are composed of light and dark regions, which give the cell its striated appearance.

The light regions are called I-bands or Isotropic Bands, and the dark regions are called A-bands 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.

Within the A-band, there is a brighter central region called the H-zone, which has no actin/myosin overlap when the muscle is in a relaxed state. The H-zone is bisected by a dark central line called the M-line. The sarcomeres are delimited by two very dark Z-discs or Z-lines, which are dense protein discs that do not allow the passage of light.

Myofibril function

Myofibrils are contractile structures that make up around 80% of the volume of a muscle fibre. They are composed of long proteins, including actin, myosin, and titin, which are organised into thick, thin, and elastic myofilaments. These myofilaments are further organised into repeated subunits called sarcomeres, which are the smallest contractile units of muscle tissue. Each sarcomere is delimited by two dark bands called Z-discs or Z-lines, which are dense protein discs that do not easily allow the passage of light. The area between the Z-discs is divided into two lighter bands called I-bands or Isotropic Bands, and a darker band in the middle called the A band or Anisotropic Band. The I-bands appear lighter because they mainly contain thin actin filaments, while the A band contains mostly myosin filaments, whose larger diameter restricts the passage of light.

The thick and thin filaments within the sarcomeres are responsible for muscle contraction. When a muscle contracts, the actin is pulled along the myosin towards the centre of the sarcomere, causing the H-zone to become smaller until the actin and myosin filaments are completely overlapped and the H-zone is no longer visible. This sliding of the thick and thin filaments past each other shortens the sarcomere, leading to the contraction of the individual muscle fibres and resulting in muscle contractions.

The number of myofibrils per fibre can vary, ranging from 50 per muscle fibre in the muscles of a fetus to approximately 2000 per fibre in the muscles of an untrained adult. The growth in the girth of the muscle fibres occurs by the splitting of the myofibrils, which can be stimulated by the development of stress on the sarcomere. This adds to the diameter or girth of the myofibers without any hyperplasia. The growth in length occurs at either end of the fibres, resulting in the addition of new sarcomeres.

The sarcoplasmic reticulum, a membrane system that surrounds each myofibril, plays a crucial role in controlling the level of calcium ions in the sarcoplasm. The terminal cisternae within the sarcoplasmic reticulum are the sites from which calcium ions are released when the muscle is stimulated, and the longitudinal tubules are where calcium ions are removed from the sarcoplasm. This removal of calcium ions is facilitated by a protein that catalyses the breakdown of ATP, releasing the energy required for the transport process.

Myofibril development

Myofibrils are created during embryonic development in a process known as myogenesis. 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.

Developing muscle cells contain thick (myosin) filaments that are 160-170 Å in diameter and thin (actin) filaments that are 60-70 Å in diameter. Young myofibres contain a 7:1 ratio of thin to thick filaments. Along the long axis of the muscle cells in subsarcolemmal locations, free myofilaments become aligned and aggregate into hexagonally packed arrays. These aggregates form regardless of the presence of Z band or M band material. Aggregation occurs spontaneously because the tertiary structures of actin and myosin monomers contain all the "information" with the ionic strength and ATP concentration of the cell to aggregate into the filaments.

The number of myofibrils per fibre varies and is regulated during the hypertrophy of muscle fibres associated with growth. For example, the number of myofibrils ranges from 50 per muscle fibre in the muscles of a fetus to approximately 2000 per fibre in the muscles of an untrained adult. The growth in girth of the muscle fibres appears to take place by splitting the myofibrils, which can be stimulated by developing stress on the sarcomere. This adds to the diameter or girth of myofibers without any hyperplasia. The growth in length occurs at either end of the fibres and results in the addition of new sarcomeres.

The exact spatial relationship of the tubules to the filaments in the myofibril depends on the species of animal. The other membrane system that surrounds each myofibril is the sarcoplasmic reticulum, a series of closed sac-like membranes. Each segment of the sarcoplasmic reticulum forms a cuff-like structure surrounding a myofibril.

Myofibril appearance

Myofibrils are rod-like organelles of a muscle cell, composed of long proteins including actin, myosin, and titin, and other proteins that hold them together. They are created during embryonic development in a process known as myogenesis. Each myofibril has a diameter of 1-2 micrometres and extends the entire length of the muscle fibre. The number of myofibrils per fibre varies, ranging from 50 per muscle fibre in the muscles of a fetus to approximately 2000 per fibre in the muscles of an untrained adult.

Myofibrils are composed of thick and thin myofilaments, which give the muscle its striped appearance. The thick filaments are composed of myosin, and the thin filaments are predominantly actin, along with two other muscle regulatory proteins, tropomyosin and troponin. The thick and thin filaments are organised into repeated subunits along the length of the myofibril, called sarcomeres. These sarcomeres are around 3 micrometres in length. The sarcomeres give the myofibril its banded, or striated, appearance, with alternating dark and light bands. The dark bands, or A-bands, contain mostly myosin filaments whose larger diameter restricts the passage of light. The light bands, or I-bands, contain mainly thin actin filaments, whose smaller diameter allows the passage of light between them.

The sarcomeric subunits of one myofibril are in near-perfect alignment with those of the neighbouring myofibrils, giving the cell its striped or striated appearance. This alignment can be seen in exposed muscle cells at certain angles, such as in meat cuts, where it can cause structural coloration or iridescence. The myofibrils are held together by proteins and other membrane systems, including the sarcoplasmic reticulum and transverse tubules, which are involved in the activation of muscle contraction.

The appearance of the myofibrils can change during muscle contraction. When a muscle contracts, the actin is pulled along the myosin towards the centre of the sarcomere, causing the H-zone to become smaller and eventually disappear as the actin and myosin filaments completely overlap. This sliding of the actin and myosin filaments past each other causes the muscle to shorten and contract.

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