The muscular system consists of three main types of muscles - skeletal, smooth, and cardiac. Skeletal muscles are attached to bones and allow voluntary movement. They work in pairs, with one contracting as the other relaxes. Smooth muscles are found within organs and work involuntarily to aid functions like digestion. The cardiac muscle makes up the heart and also contracts involuntarily to pump blood throughout the body. In total, the human body contains over 650 individual muscles.