Muscles are made up of thousands of muscle fibers. Each fiber is actually a number of individual cells that have joined together during development and are encased by an outer membrane. Muscle fibers that make up individual muscles are bound together by connective tissue.
Muscles are activated when an impulse, or signal, is sent from the brain along the peripheral nerves (nerves that connect the central nervous system to sensory organs and muscles) to the neuromuscular junction (the space between the nerve fiber and the muscle it activates). There, a release of the chemical acetylcholine triggers a series of events that cause the muscles to contract.
The muscle fiber membrane contains a group of proteins-the dystrophin-glycoprotein complex-which prevents damage as muscle fibers contract and relax. When this protective membrane is damaged, muscle fibers begin to leak the protein creatine kinase (needed for the chemical reactions that produce energy for muscle contractions) and take on excess calcium, which causes further harm. Affected muscle fibers eventually die from this damage, leading to progressive muscle degeneration.
Although muscular dystrophy (MD) can affect several body tissues and organs, it most prominently affects the integrity of muscle fibers. The disease causes muscle degeneration, progressive weakness, fiber death, fiber branching and splitting, phagocytosis (in which muscle fiber material is broken down and destroyed by scavenger cells), and, in some cases, chronic or permanent shortening of tendons and muscles. Also, overall muscle strength and tendon reflexes are usually lessened or lost due to replacement of muscle by connective tissue and fat.
This answer is based on source information from the National Institute of Neurological Disorders and Stroke.
