Muscle cells contain long strands of the proteins actin and myosin. Nerve impulses sent to the cells cause a chemical response which suddenly and dramatically shortens the fibers, changing the cell shape and and creating a contraction in the muscle. The body contains three distinct types of muscles: smooth, skeletal and cardiac.
Smooth muscle is so called because the protein fibers within it are arranged in a relatively disorganized criss-cross pattern, giving it a smooth-textured appearance. It is generally shaped like a ring, tube or hollow sac; when it contracts it shrinks and squeezes inward. Most smooth muscle contractions are controlled are automatic and involuntary, and are subject to hormonal as well as nervous control.
Smooth muscle in the digestive tract both drives and regulates peristalsis (the movement of food through the digestive tract) and aids in mechanical digestion in the stomach. In the esophagus, food is forced downward from the mouth to the stomach in a steady wave of smooth muscle contractions, and in the small intestine chyme (a mixture of partially-digested food from the stomach and bile from the liver) is mixed and churned as it is moved along the tract by a series of alternating contractions of smooth muscle in the intestine wall. The sphincters which regulate the movement of food through the tract are composed of smooth muscle rings which contract to close and relax to open the sphincter.
Outside the digestive tract, smooth muscle is found in the circulatory system in the walls of arterioles and at certain points in capillary beds. Arterioles carry blood between the large, high pressure arteries and the tiny capillary beds where gas exchange between the blood and muscles actually occurs. When the body is at rest, the smooth muscles in the walls of the arterioles leading to the limbs and extremities contract (this is called vasoconstriction) and tiny smooth muscle sphincters at the entrances of many of the capillary beds fed by those arterioles close, limiting the blood supply to the muscles of the arms and legs to a minimum. At the same time, the smooth muscle in arterioles leading to the gut and organs will relax, increasing the blood supply available for digestion and other body processes. When the body is in motion, this process will reverse in order to divert blood from the gut to the arms and legs.
Smooth muscle is also found in a few other organs that need to contract, such as the bladder.
Unlike smooth muscle, skeletal muscle is striated. The contractile protein fibers within these cells are long and arranged in parallel strands, allowing for much more powerful contractions than would be possible with smooth muscle tissue. In the case of skeletal muscle, these fibers are connected at both ends to tendons, which are anchored to bones. These muscles are responsible for most voluntary movements.
Skeletal muscles are further divided into a number of fibre types: glycolytic, glycolytic-oxidative, and oxidative. All skeletal muscles contain a mixture of all three types in proportions that vary from muscle to muscle. Glycolytic fibers are also known as white muscles or fast-twitch muscles. They are so called because they are surrounded by supplies of glycogen (a chain of sugar molecules which can be readily broken apart into glucose) and because they produce energy through glycolysis. Glycolytic muscle relies mainly on anaerobic metabolism (metabolism not requiring oxygen) for energy. This form of metabolism can be started up very quickly, but when used for extended periods of time it begins to cause dangerous lactic acid buildup, which causes the ache in muscles after exercise and can lead to serious tissue damage if it becomes too severe. In the body these fibers are found mainly in the arms and legs where they are used for short, strong contractions like those required to throw a ball or run a short distance. Glycolytic fibers are also the thickest of the three, which is why bodybuilders focus on short, intense uses of strength such as weightlifting.
Oxidative-glycolytic fibers use both aerobic and anaerobic metabolism, therefore they can start strong and also possess significant endurance. They are also known as slow-twitch muscle. These fibers are found throughout the body, particularly in the legs where they are used for walking and long-distance running. Since they use more oxygen than glycolytic fibers, oxidative-glycolytic fibers have a much greater blood supply and are a darker red colour. In order to make full use of the aerobic functions of oxidative-glycolytic fibers one generally must be breathing heavily and have an elevated heart rate - this is why it is advisable to warm up before any intense physical activity.
Finally, oxidative fibers rely entirely on aerobic metabolism and have a correspondingly high blood supply; they are sometimes called red muscle. These fibers contain a different form of the muscle protein myosin, which is responsible for contractions, and therefore cannot contract quickly like the other two types. However, because they use aerobic respiration they do not cause lactic acid buildup and can contract indefinitely with little to no loss of strength so long as energy is available. These fibers are found mostly in the back and core and legs, where they are used for keeping the spine upright and keeping the legs straight while standing still.
The final type of muscle is striated like skeletal muscle but not attached to the skeleton. It is found only in the heart, where it contracts rhythmically to pump blood to the entire body. The cells are arranged in sheets similar to smooth muscle, however the particular arrangement is skewed somewhat so that the heart twists as it contracts, squeezing out blood the way you might wring water out of a towel.
Because the heart needs to beat constantly at a steady rhythm, cardiac muscle cells have special pores (intercalated discs) to allow the chemical signal to beat to spread through them, meaning that two separated cardiac muscle cells placed next to one another in a petri dish will naturally beat together in rhythm. Contractions are also regulated by signals which travel down the vagus nerve to the sino-atrial and atrioventricular nodes.