Muscle diseases can be neuromuscular disorders or myopathies, affecting communication between muscles and nerves, leading to weakness, paralysis, and degeneration. Myopathies can be inherited and result from defects in genes or proteins. Muscle cramps and spasms are symptoms of underlying disorders or injuries.
Common types of muscle diseases are neuromuscular disorders such as multiple sclerosis and myasthenia gravis, and conditions that develop within the skeletal muscle itself, called myopathies. Neuromuscular diseases affect the communication between muscles and the nerves that control their movement, and are often progressive and fatal. Some myopathies are inherited disorders, such as muscular dystrophy, that cause gradual weakness and loss of muscle tone, sometimes leading to paralysis. Muscle spasms and cramps are not specific types of disease, but can be caused by metabolic problems or injuries.
The different types of muscle diseases are classified according to their anatomical origin, both in the muscles, the nerves that control them, and in the surrounding connective tissue. Many muscle diseases are neuromuscular in origin. These can develop from problems at the joints where nerves join muscles, normally signaling their contraction whenever the body moves. The disorders can also be caused when the parts of the brain and spinal cord that control muscle movement are damaged or otherwise compromised, altering muscle tone until it becomes spastic or stiff. Many forms of neuromuscular disease are inherited, although some rare, often terminal conditions, such as amyotrophic lateral sclerosis, appear to have a sudden onset.
Myasthenia gravis is a disease that occurs when the nerve that supplies a particular muscle is attacked by an overactive immune system, blocking its signals at the neuromuscular junction and causing you to lose voluntary control there. Multiple sclerosis is a disease of the nervous system that destroys the insulation surrounding the nerves, preventing the brain from sending signals to the body. When muscles stop receiving communication from nerves, they cannot function and atrophy, leading to loss of control and eventual degeneration.
Some common types of muscle diseases – myopathies – result from non-neurological causes, such as an inherited weakness of the proteins that make up muscle fibers. For normal movement to occur, these fibers must remain strong and be able to contract when the adjacent nerve sends them a signal to do so. Muscular dystrophies involve defects in a gene, resulting in decreased production of a protein needed for normal contraction. They lead to progressive loss of muscle control, causing symptoms that include weakness and impaired movement, and classic facial problems such as droopy eyelids and drooling.
Muscle cramps and spasms are not considered muscle diseases per se, but are considered symptoms of underlying disorders or the result of an injury. Cramps, for example, are painful, involuntary contractions of a muscle and often affect the thigh quadriceps, abdominal muscles and lower calves. They are sometimes caused by dehydration or a buildup of lactic acid during intense bursts of athletic activity. Bacterial infections such as tetanus cause severe stiffness and blockage of muscles.
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