14.16: Disorders of the Skeletal Muscle
The clinical conditions affecting the skeletal muscle tissue are broadly categorized as musculoskeletal and neuromuscular disorders.
Musculoskeletal disorders
Musculoskeletal disorders involve injuries and conditions affecting the skeletal muscles and associated connective tissues. These disorders can arise from acute biomechanical stresses or chronic overuse and can occur across different age groups. Common injuries include sprains, fractures, and muscular strains, often resulting from accidents or intense physical activity. For example, a sudden slip or an improperly executed exercise can lead to muscle strains characterized by damage to muscle fibers. Additionally, repetitive movements can lead to conditions such as tendonitis, where the overuse of joints, like knees and wrists, causes inflammation in the tendons. Osteoarthritis, another prevalent condition, results from the long-term wear and tear of joint tissues and can significantly affect mobility and quality of life.
Neuromuscular disorders
Neuromuscular disorders primarily affect the neurons that control voluntary muscles or the interactions between muscles and neurons. These disorders often have a genetic cause and tend to affect most or all muscles in the body. One example of a neuromuscular disorder is muscular dystrophy, specifically Duchenne muscular dystrophy (DMD). DMD is a genetic disorder typically characterized by the mutation or absence of the dystrophin protein. Dystrophin plays a crucial role in maintaining the structural integrity of muscle cells. Dystrophin deficiency compromises the stability and strength of muscle fibers, leading to progressive muscle degeneration and weakness seen in individuals with DMD.
Another example of neuromuscular disorder is myasthenia gravis (MG), a chronic autoimmune neuromuscular disease causing varying degrees of skeletal muscle weakness. It is caused by a defect in transmitting nerve impulses to muscles. In MG, the immune system erroneously targets the synapses between the nerves and muscles, known as neuromuscular junctions. Specifically, the immune system produces antibodies that bind and obstruct the acetylcholine receptors on the muscle membrane of these junctions, thereby interrupting the transmission of signals from the nerve cells to the muscles. When these receptors are blocked or destroyed, muscle cells receive fewer nerve signals, resulting in muscle weakness. Typical symptoms of myasthenia gravis include drooping eyelids, difficulty swallowing, and general muscle weakness.
