Athena Vale
Striations in muscle refer to the appearance of skeletal muscle tissue, which is composed of repeating functional units called sarcomeres. Under a microscope, skeletal muscle cells exhibit a striped or striated pattern of light and dark regions. These stripes are caused by the regular arrangement of actin and myosin proteins within the cells, forming structures known as myofibrils. The two types of striated muscle are skeletal muscle, which is under voluntary control and responsible for movement, and cardiac muscle, which is found in the walls of the heart and responsible for pumping blood throughout the body. Smooth muscle, in contrast, lacks striations and is found in hollow structures such as the walls of intestines or blood vessels.
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Skeletal muscle
The structure of skeletal muscle is organized into distinct layers or sheaths surrounding the muscle fibers. The outermost layer, called the epimysium, provides structural integrity to the muscle during contractions. The middle layer, the perimysium, organizes the muscle fibers into fascicles and is encased in collagen and endomysium. The innermost layer, the endomysium, surrounds individual muscle fibers. Additionally, skeletal muscle includes blood vessels, nerve fibers, and connective tissue, all contributing to its functionality.
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![Form, Endings and Relations of Striated Muscular Fibers in the Muscles of Minute Animals (Mouse, Shrew, Bat and English Sparrow.) 1888 [Leather Bound]](https://m.media-amazon.com/images/I/617DLHXyzlL._AC_UL320_.jpg)
Cardiac muscle
Striated muscle tissue features repeating functional units called sarcomeres. Under a microscope, these sarcomeres are visible along muscle fibres, giving striated muscle tissue its striated appearance. Striated muscle tissue contains T-tubules, which enable the release of calcium ions from the sarcoplasmic reticulum. The two types of striated muscle are skeletal muscle and cardiac muscle.
The functional unit of cardiomyocyte contraction is the sarcomere, which consists of thick (myosin) and thin (actin) filaments, the interactions between which form the basis of the sliding filament theory. The various light and dark bands in the myofibril are identified by letters. The thin, dark 'Z' line is the origin of the slender actin filaments, which are interleaved with the thicker myosin filaments, forming the 'A' band. The 'I' band and 'H' zone change their width during muscular contraction, as they represent the areas where the actin and myosin, respectively, are not overlapped.
Unlike skeletal muscle, cardiac muscle cells are unicellular. They are connected to each other by intercalated discs, which contain gap junctions and desmosomes. Skeletal muscle is able to regenerate far better than cardiac muscle due to satellite cells, which are dormant in all healthy skeletal muscle tissue. Adult humans cannot regenerate cardiac muscle tissue after an injury, which can lead to scarring and thus heart failure.
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Sarcomeres
The sarcomere structure consists of dark and light bands visible under a microscope. The banding pattern is due to the arrangement of thick and thin myofilaments in each unit. The A-bands (or anisotropic bands) are dark bands that contain whole thick filaments (myosin). The I-bands (or isotropic bands) are light bands that contain only the thin filaments (actin) and are located between the two thick filaments. The Z-disc is an area that traverses the I-bands and marks the point of connection between the two neighbouring actin filaments.
The sarcomere length affects force and velocity – longer sarcomeres have more cross-bridges, which results in increased force output. The sarcomere structure also affects its function, with the overlap of actin and myosin giving rise to the length-tension curve, which shows how sarcomere force output decreases if the muscle is stretched so that fewer cross-bridges can form.
The contraction of the sarcomere is induced at the neuromuscular junction via the neurotransmitter acetylcholine. The presence of calcium ions in the cell is also crucial for contraction.
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Muscle regeneration
The regeneration process of skeletal muscle is far superior to that of cardiac muscle due to satellite cells, which are dormant in all healthy skeletal muscle tissue. The three phases of the regeneration process are the inflammatory response, the activation, differentiation, and fusion of satellite cells, and the maturation and remodelling of newly formed myofibrils. This process begins with the necrosis of damaged muscle fibres, which induces the inflammatory response. Macrophages induce phagocytosis of the cell debris and eventually secrete anti-inflammatory cytokines, resulting in the termination of inflammation. Regulatory T cells (Treg) are another heterogeneous cell population that accumulates in muscle tissue after injury. Treg cells play an important role in regulating the inflammatory infiltrate at the site of tissue damage.
Other precursors and stem cell populations, either residing within the muscle or recruited via circulation in response to injury, can contribute to muscle regeneration. Muscle-resident non-myogenic cells, such as fibro-adipogenic progenitors (FAPs), are determinant components of the muscle niche, contributing to the maintenance and alteration of a homeostatic environment. The modulation of stem cell activity, survival, and differentiation indicates that the tissue niche is a critical component of muscle regeneration. The intrinsic complexity of the skeletal muscle milieu and satellite cell niche also include motor neurons, blood vessels, and the extracellular matrix (ECM).
The field of research on muscle regeneration is remarkably diverse in its approaches and prospects, and the myriad processes that are integrated to restore function and enable adaptation are fascinating.
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Muscle contraction
The muscle fibres themselves are composed of long chains of proteins that can interact and reorganise to shorten and relax. The proteins include actin and myosin filaments, which are organised into repeating units called sarcomeres. These sarcomeres give striated muscles their distinctive appearance under a microscope, with alternating bright isotropic (I-) bands and dark anisotropic (A-) bands. The contraction of the muscle occurs due to the ATP-dependent rowing motion of the myosin heads, causing a shift of the actin filaments.
The sliding filament theory explains that during contraction, the protein filaments within each skeletal muscle fibre slide past each other. This sliding motion results in a shortening of the muscle fibre, producing a contraction. The contraction can be described as a twitch, summation, or tetanus, depending on the frequency of action potentials.
The contraction process can be further classified into two types: isometric and isotonic contractions. Isometric contractions occur when muscle tension changes without any alteration in muscle length, such as holding a weight. In contrast, isotonic contractions involve maintaining muscle tension while the muscle length changes, like when lifting a weight.
The two primary types of striated muscles, skeletal and cardiac, exhibit distinct contraction mechanisms. Skeletal muscle contractions are neurogenic, requiring input from motor neurons, while cardiac muscle contractions are myogenic, initiated by the heart muscle cells themselves. Additionally, skeletal muscles are under voluntary control, whereas cardiac muscles are regulated involuntarily by the autonomic nervous system.
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Frequently asked questions
Striations are the transverse dark and light bands that can be seen in striated muscle tissue under a microscope.
Striated muscle tissue is a type of muscle tissue that features repeating functional units called sarcomeres. The two types of striated muscle tissue are skeletal muscle and cardiac muscle.
Sarcomeres are the basic contractile units that make up striated muscle tissue. They are composed of a central myosin-rich dark anisotropic (A) band and two actin-dominated light isotropic (I) bands.
The main function of striated muscle tissue is to generate force and contract to support respiration, locomotion, and posture (skeletal muscle) and to pump blood throughout the body (cardiac muscle).
