Muscle is composed of thousands of muscle fibers, each composed of a single muscle cell. A muscle cell contains a series of ultramicroscopic filaments called myofibrils. Each myofibril is a muscle cell that contains units called sarcomeres. Sarcomeres contain thick microfilaments composed of the protein myosin. Sarcomeres also contain thin microfilaments composed of the protein actin. The actin and myosin filaments are arranged parallel to one another, with the myosin filaments' molecular “heads” protruding toward the actin filaments. In skeletal muscle, the overlapping actin and myosin filaments give the muscle fiber a banded, or striated, appearance. Hence the muscle is striated muscle.
The human body has three major types of muscle. The striated muscle is given its name because of its fibers' overlapping actin and myosin filaments, giving it a banded appearance . This muscle is found in the limbs and is also called skeletal muscle. It operates under voluntary control and so is additionally known as voluntary muscle.
The second muscle type is smooth muscle, which has few actin and myosin filaments; therefore, it has few striations. Smooth muscle is found in the linings of the blood vessels, along the gastrointestinal tract, in the respiratory tract, and in the urinary bladder. Because it operates without voluntary control, it is sometimes called involuntary muscle.
The third muscle type is cardiac muscle, which is found in the heart. It has striations because it has multiple actin and myosin filaments, but it is involuntary muscle. The actin and myosin filaments in cardiac muscle exist as intertwined branches that form a conducting network for nerve impulses.
Skeletal muscle is further divided into several subtypes:
- slow oxidative, slow twitch, or "red" muscle is dense with capillaries and is rich in mitochondria and myoglobin, giving the muscle tissue its characteristic red color. It can carry more oxygen and sustain aerobic activity.
- fast twitch muscle, has three major kinds that are, in order of increasing contractile speed:
- type IIq, which,like slow muscle, is aerobic, rich in mitochondria and capillaries and appears red.
- Type IIx (also known as type IId), which is less dense in mitochondria and myoglobin. This is the fastest muscle type in humans. It can contract more quickly and with a greater amount of force than oxidative muscle, but can sustain only short, anaerobic bursts of activity before muscle contraction becomes painful (often incorrectly attributed to a build-up of lactic acid). N.B. in some books and articles this muscle in humans was, confusingly, called type IIB.
- Type IIb, which is anaerobic, glycolytic, "white" muscle that is even less dense in mitochondria and myoglobin. In small animals like rodents this is the major fast muscle type, explaining the pale color of their flesh.