Three kinds of muscle

The body has more than 600 skeletal muscles, and skeletal muscle makes up about 40 percent of body weight.

Muscle you steer, muscle that runs on its own

Lift a cup of tea and you decide every part of the movement: when to start, how high and how fast. Meanwhile your heart keeps beating and your stomach keeps churning, and nobody asks you about either. All of these jobs are done by muscle, but by different kinds of it. The body has three types of muscle tissue: skeletal muscle, cardiac muscle and smooth muscle. This unit compares them. The rest of the course follows skeletal muscle from the whole arm down to single molecules.

What all three share

All muscle tissue is excitable. Its cell membrane can change its electrical state and pass a wave called an action potential along its length. All three kinds contract in the same basic way, with a protein called myosin pulling on a protein called actin. All of them need ATP to keep contracting, and all of them relax when calcium ions are taken away again. Muscle can also be stretched, which is called extensibility, and it springs back to its resting length afterwards, which is called elasticity. The one thing muscle cannot do is push. It can only pull, which is why the arm in the model needs two muscles to bend the elbow and straighten it again.

Skeletal muscle

Skeletal muscles are attached to bones by tendons, and they are the only muscles under voluntary control. Under a microscope their cells look striped, or striated, because the actin and myosin inside are lined up in a very regular pattern. Each cell is long and cylindrical, so it is usually called a muscle fibre, and it carries many nuclei. Skeletal muscle depends completely on the nervous system: without a nerve signal, the fibre does not contract. The body has more than 600 skeletal muscles, and the exact count differs with how muscles are grouped. In adults of ordinary body weight, skeletal muscle makes up about 40 percent of body mass.

Moving the skeleton is the obvious job, but not the only one. Skeletal muscles hold you upright against gravity with small, constant adjustments, and they keep joints from slipping out of place. Rings of skeletal muscle at the openings of the digestive and urinary tracts let you decide when to swallow or when to empty the bladder. The muscles of the belly wall shield the organs behind them. And because breaking down ATP releases heat, working muscle warms the body. Shivering is skeletal muscle contracting in quick, random bursts for exactly that reason.

Cardiac muscle

Cardiac muscle is found only in the heart. It is striated like skeletal muscle, but its cells are shorter, branched and usually have a single nucleus. Their ends are joined at intercalated discs. Gap junctions in these discs let electrical current pass straight from one cell to the next, so the whole heart contracts as one coordinated unit, while desmosomes hold the cells together so they do not pull apart. Special pacemaker cells start each beat on their own. The autonomic nervous system and some hormones can speed the rate up or slow it down, but you cannot stop your heart by deciding to.

Smooth muscle

Smooth muscle lines the walls of hollow organs such as the stomach, intestines, bladder and uterus, and of passageways such as arteries, veins and airways. It also changes the size of the pupil, shapes the lens of the eye and raises the hairs on your skin in the cold. Its cells are spindle-shaped, about 30 to 200 micrometres long, each with a single nucleus. They contain actin and myosin, but not in neat rows, so there are no stripes. Smooth muscle is involuntary. Hormones, autonomic nerves and local conditions, even being stretched, can set it off. It works slowly and steadily for long periods while using relatively little energy.

In the model, press Play and watch the two muscles of the upper arm take turns. That is skeletal muscle doing the job it is built for: pulling on bone.

This is educational material about how the body is built and works. It is not medical advice and is not for diagnosing or treating anyone.