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Muscle tissue that is marked by transverse dark and light bands is known as striated muscle tissue. It is made up of elongated, usually multinucleated fibres and includes skeletal muscle, cardiac muscle, and most muscle of arthropods. The two types of striated muscle are skeletal muscle and cardiac muscle. Skeletal muscle is part of the locomotor system and is responsible for moving and stabilising the skeleton. Cardiac muscle, on the other hand, is only found in the heart and is responsible for heart contractions and pumping blood throughout the body.
| Characteristics | Values |
|---|---|
| Appearance | Striated or banded |
| Structure | Transverse dark and light bands |
| Composition | Elongated, multinucleated fibres |
| Types | Skeletal muscle, cardiac muscle, and arthropods |
| Function | Contraction and force creation |
| Control | Skeletal muscle is voluntary, cardiac muscle is involuntary |
| Regeneration | Skeletal muscle regenerates better than cardiac muscle |
| Metabolism | Anaerobic glycolysis or aerobic oxidation of substrates |
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What You'll Learn
Skeletal muscle
The primary function of skeletal muscle is contraction, which enables breathing, movement, and posture maintenance. Skeletal muscles are attached to bones by collagen-rich tendons, allowing for a wide range of movements and functions. They are also found in organs such as the tongue, mimic musculature, and the diaphragm.
The structure of skeletal muscle is organised into layers. The outermost layer is the epimysium, which provides structural integrity during contractions. Within the epimysium are compartments, each containing a bundle of muscle fibres known as a fasciculus. Each fasciculus is surrounded by a layer of connective tissue called the perimysium. Moving inward, the endomysium surrounds each individual muscle fibre. These layers of connective tissue provide support, protection, and pathways for blood vessels and nerves.
The development of skeletal muscle begins during embryogenesis, with the differentiation of the para-axial mesoderm into somites. These somites are stimulated by myogenic regulatory factors to form the dermomyotome and sclerotome. In the fetus, embryonic myoblasts differentiate further to form primary muscle fibres, which then unite to create secondary myofibers. After birth, satellite cells act as stem cells, contributing to the growth and development of skeletal muscles.
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Cardiac muscle
The outside of the cardiomyocyte is surrounded by a plasma membrane called the sarcolemma, which acts as a barrier between extracellular and intracellular contents. The sarcolemma contains T-tubules, which are highly branched invaginations of the sarcolemma that function in excitation-contraction coupling, action potential initiation and regulation, maintaining the resting membrane potential, and signal transduction. The sarcolemma of cardiac muscle cells also contains voltage-gated calcium channels, specialized ion channels that skeletal muscle does not possess.
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Muscle regeneration
The regeneration process can be divided into five interrelated and time-dependent phases: degeneration-necrosis, inflammation, regeneration, maturation/remodelling, and functional recovery. The process begins with the necrosis of damaged muscle fibres, which induces an inflammatory response. Macrophages play a key role in this phase, as they phagocytose cell debris and secrete anti-inflammatory cytokines to terminate the inflammation. Regulatory T cells (Treg) are also important in regulating the inflammatory infiltrate at the site of injury.
The next phase involves the activation, differentiation, and fusion of satellite cells, which are dormant in healthy skeletal muscle tissue. These cells re-enter the cell cycle to multiply and differentiate into myoblasts. Fibro-adipogenic progenitors (FAPs) are another type of non-myogenic cell that contributes to the maintenance and alteration of the muscle environment during regeneration.
The final phases of muscle regeneration include the maturation and remodelling of newly formed myofibers, restoring the function of the muscle. The entire process is influenced by various factors, including cellular dynamics, physical activity, and muscle-tendon-bone biomechanics. Understanding the mechanisms of muscle regeneration is crucial for developing effective therapeutic strategies for muscular disorders and improving rehabilitation techniques.
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Muscle contraction
Skeletal muscle contractions are neurogenic, requiring synaptic input from motor neurons. These contractions enable movement, posture maintenance, and breathing. During skeletal muscle contraction, the protein filaments within each muscle fiber slide past each other, resulting in a shortening or lengthening of the muscle. This process is known as the sliding filament theory. Additionally, skeletal muscles are attached to bones by collagen-rich tendons, providing structural support and stability to the body.
Cardiac muscle contractions are responsible for pumping blood throughout the body. These contractions are involuntary and are regulated by the body's autonomic nervous system. Cardiac muscle cells, also known as cardiomyocytes, are specialised striated muscle cells found only in the heart. They have a striated appearance due to the presence of densely packed myofibrils.
Smooth muscle contractions occur in the walls of blood vessels, the gastrointestinal tract, bronchioles, uterus, and bladder. Unlike skeletal and cardiac muscles, smooth muscles lack the striated structure and contract through a different mechanism. They are also under involuntary control, regulated by reflexes and the autonomic nervous system.
The process of muscle contraction, known as excitation-contraction coupling, involves a series of complex steps. It begins with an action potential causing depolarisation in the myocyte membrane. This depolarisation spreads via transverse (T) tubules, leading to a conformational change in the dihydropyridine receptors. As a result, nearby ryanodine receptors on the sarcoplasmic reticulum (SR) open, releasing calcium ions. The released calcium binds to troponin C, triggering a series of events that ultimately result in the attachment of myosin heads to actin filaments, forming cross-bridges. The cycling of these cross-bridges, fuelled by ATP, leads to muscle contraction.
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Muscle appearance
Skeletal muscle, also known as voluntary muscle, is responsible for movement and posture maintenance. It has a long and cylindrical appearance and is attached to the bones or skin to facilitate locomotion. Under a microscope, skeletal muscle tissue exhibits a striped or striated appearance due to the presence of transverse dark and light bands. This pattern arises from the repeating bands of the proteins actin and myosin, which are essential for muscle contraction. Skeletal muscle fibres are relatively thick, ranging from 20 to 100 µm in diameter and up to 20 cm in length. Each skeletal muscle fibre is surrounded by a plasma membrane called the sarcolemma, and the functional unit within the fibre is called the sarcomere.
Cardiac muscle, on the other hand, is found only in the heart and is responsible for heart contractions and pumping blood throughout the body. Cardiac muscle cells are shorter and branched, with a striated appearance. They have one nucleus per cell and are distinguished by the presence of intercalated discs. Cardiac muscle fibres are connected both mechanically and electrically by these intercalated discs, allowing them to contract in unison. Cardiac muscle cells are smaller, typically ranging from 10 to 20 µm in thickness and 50 to 100 µm in length.
The appearance of muscle fibres can also vary depending on their metabolic strategies. Muscle fibres that produce brief bursts of intense activity rely on anaerobic glycolytic processes and may appear lighter in colour due to lower myoglobin levels. In contrast, muscle fibres that need to provide moderate forces for extended periods exhibit higher myoglobin levels, resulting in a darker or reddish appearance.
Additionally, the regeneration process of skeletal muscle contributes to its unique appearance. The regeneration process involves three phases: the inflammatory response, the activation and differentiation of satellite cells, and the maturation of newly formed myofibrils. The ability of skeletal muscle to regenerate effectively through this process contributes to its overall appearance and structural integrity.
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Frequently asked questions
Striated muscle fibers are muscle fibers that are marked by transverse dark and light bands. They are made up of elongated, usually multinucleated fibers.
The two types of striated muscle are skeletal muscle and cardiac muscle.
The main function of striated muscle tissue is to create force and contract.
Skeletal muscle is attached to the bones and skin and controls locomotion and any movement that can be consciously controlled. Cardiac muscle, on the other hand, is found only in the heart and is not under conscious control.
Striated muscle fibers employ different metabolic strategies to produce energy. Anaerobic glycolytic processes are used for brief bursts of intense activity, while an efficient system of aerobic oxidation of substrates is used for moderate forces over prolonged periods.
