Muscles

About the Muscles

The muscular system has over 600 muscles, comprising 40 to 50 percent of body weight. Three types: skeletal voluntary, striated, attached to bones, 40 percent of body weight. Smooth involuntary, non striated, in walls of hollow organs and vessels. Cardiac involuntary, striated, only in the heart. Skeletal muscle structure: whole muscle to fascicle to muscle fiber cell to myofibril to sarcomere the functional unit, containing thick myosin and thin actin filaments. The sliding filament theory: myosin heads pull actin filaments toward the center of the sarcomere, shortening it. The process is triggered by calcium release from the sarcoplasmic reticulum after acetylcholine from motor neurons binds to the neuromuscular junction. ATP provides energy. Motor units are one motor neuron and all muscle fibers it innervates. Fast twitch type II fibers are for power and sprinting, fatigue quickly. Slow twitch type I fibers are for endurance, fatigue resistant. Muscle hypertrophy from resistance training increases fiber size, not number. Muscular dystrophy Duchenne is X linked dystrophin deficiency. Rhabdomyolysis is muscle breakdown releasing myoglobin, potentially causing kidney injury.