The muscular system is a complex network of specialized tissues that enable movement throughout the human body. It consists of over 600 individual muscles that make up 40 to 50 percent of our total body weight. These muscles work together to produce movement, maintain posture, and generate heat to keep our bodies functioning properly.
The muscular system contains three distinct types of muscle tissue. Skeletal muscle has a striated appearance and is under voluntary control, allowing us to move our limbs and body. Cardiac muscle is also striated but works involuntarily to pump blood through the heart. Smooth muscle lacks striations and operates involuntarily in our internal organs like the digestive system and blood vessels.
Skeletal muscle has a hierarchical structure with multiple levels of organization. At the highest level is the whole muscle, which contains bundles of muscle fibers wrapped in connective tissue. Each bundle consists of individual muscle fibers, which are actually single cells. Within each fiber are myofibrils, the contractile units made up of actin and myosin protein filaments that slide past each other to create muscle contraction.
Muscle contraction follows a precise sequence of events called the sliding filament theory. First, a nerve signal from the brain reaches the muscle fiber. This signal triggers the release of calcium ions from internal storage. The calcium allows myosin heads to bind to actin filaments, forming cross-bridges. Finally, ATP provides the energy for a power stroke that causes the filaments to slide past each other, shortening the muscle and creating contraction.