Over six hundred muscles, about forty percent of you
The muscular system combines skeletal, cardiac and smooth muscle to move the body, hold it upright, pump blood and keep it warm. An adult has more than 600 skeletal muscles, the exact count depending on how you tally them, and together they make up about 40 percent of body mass.
Paired with the skeleton, the muscles form the musculoskeletal system. Each muscle is a bundle of thousands, sometimes tens of thousands, of fibres, and each fibre holds many fine fibrils. In vertebrates the nervous system runs most of it, although the heart can beat on its own, paced by the sinus node under the influence of the autonomic nerves. Heart muscle is striped like skeletal muscle, yet its fibres are joined side to side and it works involuntarily, as smooth muscle does. Smooth muscle answers to nerves, hormones and local chemicals, contracting slowly and steadily and adjusting to stretch, which helps keep blood and food moving.
A skeletal muscle is organised into hundreds of motor units, each a nerve cell controlling its own bundle of fibres, so the body grades force by choosing how many units to fire. At the junction between nerve and muscle, acetylcholine sets off an electrical change in the fibre. Calcium released from internal stores exposes binding sites on actin, and myosin heads swivel against the actin, burning ATP with every cycle. Muscles store little ATP, so they must regenerate it constantly, with creatine phosphate as a fast backup; once contraction ends, calcium is pumped back into storage. Tendons transmit the pull to bone.
At rest and during long, gentle exercise, mitochondria make ATP aerobically from oxygen, carbohydrate and fat, without tiring by-products. Harder bursts switch to faster anaerobic routes such as the phosphagen system and anaerobic glycolysis.
Muscular dystrophy, a group of more than 30 inherited disorders including the Duchenne and Becker forms, causes progressive weakness and loss of muscle and occurs at about 20 to 25 cases per 100,000 person-years worldwide. Myositis and myasthenia gravis are the main autoimmune muscle conditions.
Source: Muscular system
