Most sources agree that there are between 640 and 850 skeletal muscles in the body, but there isn't a precise count because there are different views of what constitutes "one" muscle. Some believe a single muscle can have different parts, while others prefer to look at the "parts" as distinct muscles. Regardless of actual numbers or type of muscle, all muscles are responsible for movement.
Function
Muscles convert chemical energy into mechanical energy and this produces motion, including gross movement of the skeletal system, fine motor skills in the hands and functional movement of organs such as the heart and stomach. Muscles also support posture and help maintain body temperature by producing about 85 percent of the body's heat.
Characteristics
Muscle cells have unique characteristics that enable them to function properly. They can respond to a stimulus (excitability), and they contract when stimulated (contractility). Muscles can also stretch (extensibility) but then are able to return to their original shape (elasticity).
Types
Skeletal muscles are attached to bones and are responsible for moving the body. This is the only type of muscle that is under voluntary control. The walls of the heart are made of cardiac muscle, and smooth muscle is found in the walls of all the other hollow organs, such as blood vessels and the gastrointestinal tract. Cardiac and smooth muscle have a slightly different structure from skeletal muscle and are controlled involuntarily by the nervous system.
Structure of Skeletal Muscle
Each skeletal muscle is wrapped in connective tissue, and the connective tissue folds into the muscle, dividing it into bundles of fibers. As a result, each muscle is composed of many cylindrical bundles, called fascicles, that run the length of the muscle. Inside each fascicle are more bundles of long muscle fibers. Each muscle fiber consists of threadlike filaments that are responsible for contraction.
Contraction
Even though the filaments are the length of the muscle fiber, they are organized into sections that contain groups of thin and thick filaments. Each group has one section of thick filaments in between two sections of thin filaments. Muscles are stimulated when nerves release acetylcholine into the muscle fibers. The acetylcholine stimulates the release of calcium, and the calcium causes more chemical reactions that allow special structures in the thick filaments to attach to the thin filaments and pull them toward the center of the thick filament. As the thin filaments slide in between the thick ones, the muscle contracts. When the stimulus ends, the calcium goes back to storage, the structures let go and the muscle relaxes.
Movement
All of the connective tissue that runs through skeletal muscles to create the bundles merges together at the ends of the muscle and extends to form tendons. The tendons anchor muscles to bones so when the muscles contract, they cause the bones to move at their joints. Since muscles can only contract and pull, they work in pairs--one muscle is a flexor and its partner is an extensor. Together they pull in opposite directions to cause movement.
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