Muscular system: types of muscle

Muscle makes up roughly 40 percent of your body weight, and you use it every second, even when you sit still. Muscles move your arms and legs, but they also push food through your intestines, squeeze blood through your vessels, and keep your heart beating. The body uses three different kinds of muscle tissue for these very different jobs.

Skeletal muscle is the type you think of first. You have more than 600 of them, and they attach to bones by tendons. Under a microscope, skeletal muscle cells look striped, or striated. These are long cells called fibers. Skeletal muscle is voluntary, meaning you decide when to use it. You choose to kick a ball or raise your hand. It can contract quickly and strongly, but it tires, so you need rest.

Smooth muscle has no stripes, and its cells are short and spindle-shaped. It lines the walls of the stomach, intestines, blood vessels, and airways. Smooth muscle is involuntary: you do not control it consciously. It works slowly and steadily without tiring much. Waves of smooth muscle contraction move food through your digestive system, and smooth muscle in blood vessel walls helps control how much blood flows through. The tiny muscles in your eyes that change the size of your pupils are smooth muscle too.

Cardiac muscle is found only in the heart. Like skeletal muscle it looks striped, but like smooth muscle it is involuntary. Its cells are joined tightly together so they contract in a coordinated beat. Cardiac muscle is built to work without rest for a lifetime, and its cells are packed with mitochondria to supply the constant energy it needs.

All muscle does one thing: it contracts, which means it gets shorter and pulls. Muscle cannot push. Muscles get the energy to contract from food, released in mitochondria, and they need a steady supply of oxygen from the blood.

A common mistake is thinking involuntary muscles are less important than the ones you control. Without smooth and cardiac muscle, you could not digest food or circulate blood. Another is thinking the three types are interchangeable; each is shaped for its own job. Next lesson: how muscles produce movement.