Victor Forge
Striated muscle, also known as skeletal muscle, is the most common type of muscle in the human body, accounting for 30% to 50% of total body mass. It is composed of highly organised tissues that convert chemical energy to physical work. Striated muscle is marked by transverse dark and light bands, giving it a striped appearance under a microscope. This unique structure is composed of repeating functional units called sarcomeres, which are responsible for the muscle's ability to contract and generate force. These muscles are under voluntary control, allowing for a wide range of movements and functions, including breathing, digestion, and maintaining posture.
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What You'll Learn
Skeletal muscle anatomy
Skeletal muscles, also known as striated muscles, are the most common type of muscle in the human body. They make up between 30% and 40% of total body mass. Skeletal muscles are attached to the bones and allow us to perform a wide range of movements and functions. They are voluntary muscles, meaning we control how and when they move and work.
Skeletal muscles are made up of skeletal muscle tissue, connective tissue, nerve tissue, and blood or vascular tissue. They consist of flexible muscle fibres that range from less than half an inch to just over 3 inches in diameter. Each muscle can contain thousands of fibres. These fibres are cylindrical in shape and are red and white in colour.
The fibres contract, allowing the muscles to move bones. Before a skeletal muscle fibre can contract, it must receive an impulse from a nerve cell. The nerve cells are composed of myelinated as well as non-myelinated nerve fibres. The nerve fibres send signals to make the muscles function. The primary artery supplying blood to a skeletal muscle generally courses parallel to the longitudinal axis of the muscle fibre. The artery gives off tributaries known as feed arteries that run perpendicularly to the primary artery and proceed towards the external connective tissue sheath of the muscle fibre called perimysium.
Skeletal muscles are highly regenerative, especially compared to cardiac muscle. This is due to satellite cells, which are dormant in all healthy skeletal muscle tissue. There are three phases to the regeneration process: the inflammatory response, the activation, differentiation, and fusion of satellite cells, and the maturation and remodelling of newly formed myofibrils.
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Cardiac muscle anatomy
Cardiac muscle, also called heart muscle or myocardium, is one of the three major categories of muscles in the human body, the others being skeletal muscle and smooth muscle. It is an involuntary, striated muscle that forms the thick middle layer of the heart, between the outer layer of the heart wall (the pericardium) and the inner layer (the endocardium). The endocardium is not cardiac muscle and is made up of simple squamous epithelial cells that form the inner lining of the heart chambers and valves. The pericardium, on the other hand, is a fibrous sac surrounding the heart, consisting of the epicardium, pericardial space, parietal pericardium, and fibrous pericardium.
Cardiac muscle cells, also called cardiomyocytes, are the contractile myocytes of the cardiac muscle. They are surrounded by an extracellular matrix produced by supporting fibroblast cells. Cardiomyocytes are tubular structures composed of chains of myofibrils, which are rod-like units within the cell. The myofibrils consist of repeating sections of sarcomeres, which are the fundamental contractile units of the muscle cells. Sarcomeres are composed of long proteins that organize into thick and thin filaments, called myofilaments. Thin myofilaments contain the protein actin, and thick myofilaments contain the protein myosin. The myofilaments slide past each other as the muscle contracts and relaxes, producing the formation of “cross-bridges”, which causes contraction of the heart and generation of force.
The outside of the cardiomyocyte is surrounded by a plasma membrane called the sarcolemma that acts as a barrier between extracellular and intracellular contents. Invaginations of the sarcolemma into the cytoplasm of the cardiomyocyte are called T-tubules, and they contain numerous proteins like L-type calcium channels, sodium-calcium exchangers, calcium ATPases, and beta-adrenergic receptors that allow for the exchange of ions with extracellular fluid surrounding the cell. The sarcolemma of cardiac muscle cells contains voltage-gated calcium channels, specialized ion channels that skeletal muscle does not possess.
Cardiac muscle cells are joined together at their ends by intercalated discs to form long fibers. Within the intercalated disc, there are three different types of cell junctions: fascia adherens, desmosomes, and gap junctions. The transverse side of the intercalated discs runs perpendicular to the muscle fibers at the Z lines and provides structural components via fascia adherens and desmosome connections. The lateral side of the discs contains gap junctions that permit intercellular communication by allowing ions from one cardiomyocyte to move to a neighbouring cell without having to be excreted into the extracellular space first. The low resistance of the gap junctions allows depolarization to spread quickly throughout the syncytium, facilitating the rapid transmission of action potentials to produce a synchronized contraction of the cardiomyocytes in unison.
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Smooth muscle tissue
Smooth muscle cells are spindle-shaped and have a single nucleus, with individual cells ranging in size from 30 to 200 μm. The smooth muscle cell is 3-10 µm thick and 20-200 µm long. The cytoplasm is homogeneously eosinophilic and consists mainly of myofilaments. The nucleus is located in the center and takes on a cigar-like shape during contraction. The cell membrane forms small pouch-like invaginations into the cytoplasm, called caveolae, which are functionally equivalent to the T-tubules found in skeletal muscle.
Smooth muscle fibers group in branching bundles, forming sheets of tissue. This allows the cells to contract much stronger than those of striated musculature. The smooth muscle cells are anchored to the surrounding connective tissue by a basal lamina. Myofibroblasts represent a special type of smooth muscle cell that also has qualities of fibrocytes. They produce connective tissue proteins such as collagen and elastin.
Smooth muscle contraction is initiated when calcium ions bind to intracellular calmodulin, which then activates an enzyme called myosin kinase. This enzyme phosphorylates myosin heads so they can form cross-bridges with actin and then pull on the thin filaments. Smooth muscle can be stimulated by pacesetter cells, the autonomic nervous system, hormones, stretching, or spontaneously. Single-unit smooth muscle tissue contains gap junctions to synchronize membrane depolarization and contractions so that the muscle contracts as a single unit.
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Striated muscle functions
Striated muscles are highly organised tissues that convert chemical energy to physical work. They are responsible for the voluntary movements of bones and make up between 30% and 50% of an average human's total body mass. Striated muscles are attached to some component of the skeleton and are made up of elongated, multinucleated fibres. They are called striated muscles because they have a striped appearance, with transverse dark and light bands.
The two types of striated muscle are skeletal muscle and cardiac muscle. Skeletal muscle is the most common type of muscle in the body and is responsible for functions such as breathing, eating and moving bones. Skeletal muscles are voluntary muscles, meaning that we control how and when they move and work. They consist of flexible muscle fibres that contract, allowing the muscles to move bones so we can perform different movements. Each muscle can contain thousands of fibres.
Cardiac muscle, on the other hand, is the muscle found on the walls of the heart. Cardiac muscle cells are specialised striated muscle cells found only in the heart. Their main task is heart contraction, which pumps blood throughout the body. Cardiac muscles also produce the atrial natriuretic peptide (ANP) in the atria, which stimulates diuresis and lowers blood pressure.
Both skeletal and cardiac muscles have a striated appearance due to their densely packed myofibrils. However, they differ in their histology and physiology. Skeletal muscle cells have many nuclei, while cardiac muscle cells have a single nucleus. Skeletal muscle is also able to regenerate far better than cardiac muscle due to satellite cells, which are dormant in all healthy skeletal muscle tissue.
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Striated muscle regeneration
Skeletal muscle, or striated muscle, is the most common type of muscle in the body, making up between 30% and 40% of total body mass. These muscles are attached to the skeleton and consist of flexible muscle fibres that contract, enabling movement. Striated muscle tissue contains repeating functional units called sarcomeres, which are visible under a microscope along muscle fibres, giving a striped appearance.
Striated muscle tissue can regenerate, and this process is orchestrated by multiple steps. The regeneration process begins with the necrosis of damaged muscle fibres, which induces an inflammatory response. Macrophages induce phagocytosis of the cell debris and eventually secrete anti-inflammatory cytokines, resulting in the termination of inflammation. The macrophages can also facilitate the proliferation and differentiation of satellite cells. These are mononucleated quiescent cells that, when the muscle is damaged, are stimulated to divide and fuse with existing muscle fibres to regenerate and repair the damage.
Other cells outside the basal lamina, such as pericytes, also have myogenic potency and can produce new cells. Regulatory T cells (Treg) are another cell population that accumulates in muscle tissue after injury and are important players in muscle regeneration, regulating the inflammatory infiltrate at the site of damage.
Additionally, neural stem cells have been shown to differentiate into skeletal muscle when cocultured with skeletal myoblasts or transplanted into regenerating skeletal muscle. Myoblasts have been used as cell therapy for individuals with stress urinary incontinence (SUI), which is characterised by the loss of small amounts of urine upon coughing, laughing, sneezing, or other movements that increase intra-abdominal pressure.
Understanding the mechanisms of inflammation in muscle regeneration is critical for developing effective regenerative and therapeutic strategies for muscular disorders.
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