Dominic Stone
Striated voluntary muscle, also known as skeletal muscle, is one of the three types of vertebrate muscle tissue, the others being cardiac and smooth muscle. Skeletal muscle is the most common and widely distributed muscle tissue in the body, accounting for around 40% of total body mass. It is attached to the bones of the skeleton and is responsible for voluntary movements, including limb movement, facial expressions, eye movements, and swallowing. The tissue has a striped appearance due to the arrangement of sarcomeres, which are repeating functional units composed of actin and myosin myofilaments. Skeletal muscle is unique in its ability to regenerate through satellite cells and its high degree of voluntary control, with signals from motor neurons causing muscle contractions.
| Characteristics | Values |
|---|---|
| Type | Skeletal muscle |
| Controlled by | Voluntary |
| Appearance | Striated |
| Structure | Not branched |
| Nuclei | Multinucleated |
| Muscle fibers | Cylindrical shape with blunt ends |
| Mitochondria | More mitochondria than smooth muscle |
| Nerve signals | Motor neurons |
| Muscle contraction | Calcium ions |
| Muscle regeneration | Better than cardiac muscle |
| Muscle phenotype | Regulated by independent signaling pathways |
| Muscle metabolism | Anaerobic glycolysis |
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Skeletal muscle
The appearance of skeletal muscle tissue is striated or striped due to the arrangement of sarcomeres. These sarcomeres are the repeating functional units that are visible along the muscle fibres under a microscope. The calcium ions released from the sarcoplasmic reticulum drive the movement of myosin and actin filaments, which causes the sarcomere to shorten and the muscle to contract. Skeletal muscles also contain blood vessels, nerve fibres, and connective tissue.
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Cardiac muscle
Each cardiomyocyte needs to contract in coordination with its neighbouring cells, working together to efficiently pump blood from the heart. If this coordination breaks down, the heart may not pump at all, as can occur during abnormal heart rhythms such as ventricular fibrillation. The coordinated contractions involve the cardiac muscle and electrical impulses. Electrical stimulation in the form of a cardiac action potential triggers the release of calcium from the cell's internal calcium store, the sarcoplasmic reticulum. The rise in calcium causes the cell's myofilaments to slide past each other in a process called excitation-contraction coupling.
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Smooth muscle
In addition to its role in bodily functions, smooth muscle is important in the disease process and medical treatments. For example, bronchodilators are used to relax airway smooth muscle in asthmatic patients, and nitrates are used in combination with ACE inhibitors to treat ischemic heart disease, improving patient mortality.
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Muscle contractions
Skeletal muscle, also known as striated voluntary muscle, is the most common type of muscle in the human body, comprising 30% to 40% of total body mass. These muscles are attached to the bones and allow us to perform a wide range of movements and functions. Skeletal muscles are under voluntary control, meaning we can decide how and when they work.
The main function of striated muscle tissue is to create force and contract. Skeletal muscle contractions enable breathing, movement, and posture maintenance. Each skeletal muscle contains multiple fascicles, or bundles of muscle fibres, and each fibre can contain thousands of sarcomeres, the functional units of muscle fibres. These sarcomeres are composed of actin and myosin filaments, which power contraction.
When a muscle contracts, the sarcomere shortens, causing the muscle fibre to contract. This process is driven by the movement of myosin and actin filaments, which are fuelled by adenosine triphosphate (ATP) molecules. ATP is hydrolysed into ADP and P, causing the myosin heads to change conformation and move towards the positive end of the actin, cocking the myosin head. The phosphate is then released, and the ADP-bound myosin binds to a new location on the actin filament. ADP is then released, causing the myosin to return to its original position, pulling on the actin filament and causing the sarcomere to contract.
Additionally, skeletal muscle contractions can occur in two ways: concentric and eccentric. A concentric contraction occurs when a muscle pulls a joint in the direction of the contraction, while an eccentric contraction occurs when a muscle works to decelerate a joint at the end of a movement, acting as a braking force to protect the joint from damage. An example of an eccentric contraction is when the muscle 'smooths out' a movement or resists gravity, such as during downhill walking.
Finally, skeletal muscles have a superior regeneration capacity compared to cardiac muscles due to the presence of satellite cells, which are dormant in healthy skeletal muscle tissue. When muscle fibres undergo necrosis, it induces an inflammatory response, activating macrophages that facilitate the proliferation and differentiation of satellite cells. These satellite cells multiply and differentiate into new muscle fibres, aiding in the regeneration process.
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Muscle regeneration
Striated muscle tissue is a muscle tissue type that features repeating functional units called sarcomeres. The two types of striated muscle are skeletal muscle and cardiac muscle. Skeletal muscle is part of the voluntary muscular system and is attached to the bones of the skeleton.
Skeletal muscle has a far superior regenerative capacity compared to cardiac muscle due to the presence of satellite cells. These satellite cells are dormant in all healthy skeletal muscle tissue. The regeneration process can be divided into three phases:
- Inflammatory response: This phase 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, which results in the termination of inflammation.
- Activation, differentiation, and fusion of satellite cells: Macrophages facilitate the proliferation and differentiation of satellite cells. The satellite cells re-enter the cell cycle to multiply and then leave the cycle to self-renew or differentiate as myoblasts.
- Maturation and remodelling of newly formed myofibrils: This final phase involves the maturation and remodelling of the newly formed myofibrils, resulting in the restoration of muscle function.
Various strategies for skeletal muscle tissue engineering have been developed to promote muscle repair and regeneration. Tissue engineering aims to repair or regenerate lost or damaged skeletal muscle tissue through a combination of cells, scaffolds, and growth factors. Three-dimensional (3D) biomaterial scaffolds can be used to guide and support tissue regeneration. These scaffolds mimic the properties of natural tissue and can be designed with specific mechanical, structural, physicochemical, and biological properties.
Hydrogel-based biomaterials, particularly those derived from natural elements, are promising candidates for skeletal muscle regeneration as they trigger the regeneration process effectively while eliciting a limited inflammatory response. The integration of extracellular matrix (ECM) components into biomaterials is especially beneficial for muscle regeneration. Collagen, gelatin, fibrin, matrigel, keratin, hyaluronic acid, silk, and alginate-based hydrogels have been extensively investigated for skeletal muscle tissue engineering.
While significant progress has been made in muscle regeneration techniques, ongoing challenges include the need for improved vascularization to ensure adequate nutrient and oxygen supply to metabolically active cells, as well as the complexity of the structure and the high cell density required for differentiation. Further research is needed to enhance our understanding of the specific cellular and molecular mechanisms involved in striated muscle regeneration to improve the efficacy of these therapeutic approaches.
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Frequently asked questions
Striated voluntary muscle, otherwise known as skeletal muscle, is the most common and widely distributed muscle tissue in the body, making up around 40% of the body's total mass. It is attached by tendons to the bones of a skeleton and is responsible for voluntary movements of the bones.
Striated muscle tissue features repeating functional units called sarcomeres. Under a microscope, these sarcomeres are visible along muscle fibres, giving a striped or striated appearance.
Striated voluntary muscle is voluntary, striated, not branched, and multinucleated. It is the only muscle tissue under the direct conscious control of the cerebral cortex of the brain, which is why it is considered voluntary muscle.
Examples of striated voluntary muscle include the biceps brachii and gluteus maximus, which are found in the eyes, throat, diaphragm, and anus.
Striated voluntary muscles produce adenosine triphosphate (ATP) molecules, which power the movement of the myosin heads. They also keep a storage form of glucose in the form of glycogen, which can be rapidly converted to glucose when energy is required for sustained, powerful contractions.
