Section 2: The Muscular System
The muscular system consists of more than 600 muscles specialized to help the body move and keep you alive. A muscle is an organ that can contract and relax, providing the force necessary to move the body. Muscle tissue is made up of fibers or fused muscle cells which are able to shorten. The shortening of these fibers is called a contraction. Each fiber is made up of smaller units called myofibrils, which are made up of thick and thin proteins called myosin and actin. It’s the interaction of these proteins that makes your muscles contract. Almost all the movement in your body occurs because of muscle contraction.
There are three types of muscle tissue. Skeletal muscle attaches to the bones of the skeleton by tendons, which are tough, strong connective tissue. Skeletal muscles are voluntary muscles, meaning they are muscles that you can control. Because muscle cells can only contract, not extend, skeletal muscles must work in pairs. While one muscle contracts, the other muscle in the pair relaxes to its original length. When viewed under a microscope, skeletal muscles have a striated appearance, alternating between light and dark bands. A smooth muscle is found in internal organs. They help move food through the stomach and help expand the lungs when you breathe. They are involuntary muscles, meaning you cannot control them. They react more slowly and tire more slowly than skeletal muscles. They look smooth under a microscope because they have no striations. The cardiac muscle is found only in the heart. It’s an involuntary muscle that never tires and contracts repeatedly. These contractions are your heartbeat. Skeletal muscles are also striated but have one or two nuclei.