Muscles have several main functions, including locomotion, heat production, structural support and muscle contractions. Muscles are dense and account for nearly 40 percent of the body's weight.
There are three primary types of muscles: skeletal muscles, smooth muscles and cardiac muscle. Skeletal muscles control movements of the body. Limb movement occurs when muscle fibers attached to the bones contract. Muscle contraction is a voluntary response controlled by neural functions that release the energy needed to cause the reaction. The framework, posture and joints of the body are supported by skeletal muscles. Skeletal muscles also contribute to heat production when necessary, which is a sign of metabolic activity. Cardiac muscles are located exclusively in the heart. Blood circulation is dependent on cardiac muscle contractions, which supply the body with oxygenated blood.
Muscle tissue is cased in a material made up of connective tissue called fascia. Inside the fascia are bundles of muscle fibers known as fasciculus that are directly responsible for muscle function. Individual muscle fibers are multinucleated and striated, surrounded by a material called perimysium. These fibers are surrounded by sarcoplasm and a cell membrane called sarcolemma. Myofibrils are protein-based filaments found in the sarcoplasm that contain the necessary information for proper muscle fiber function.