Muscles are the contractile tissues responsible for all external and internal bodily movements. Of the three types of muscle tissue, only skeletal muscles are under voluntary control: and are thus sometimes called voluntary muscles. Cardiac muscles and smooth muscles are involuntary muscle tissues which respond to internal stimuli.
Skeletal striated muscle consists of bundled muscle fibres held together by connective tissue. The muscle fibres themselves are made up of long, narrow, multinucleated cells composed of actin and myosin myofibrils. These bundles are the cause of muscle striation.
Skeletal muscle fibres are divided into fast twitch muscle fibres and slow twitch muscle fibres. Fast twitch muscles give high, fast power, but have little endurance. Slow twitch muscles can maintain their contracting abilities over a long period of time, but cannot generate great power. Muscles are composed of both, although the ratio changes from muscle to muscle as well as from person to person.
One end of a skeletal muscle is attached to a fixed point, or origin. The other end of the muscle is attached to a part of its associated bone or tissue which can move. Depending on exactly how they are connected, skeletal muscles can move a bone or tissue in one, two, or all three planes of orientation.
Skeletal muscles are capable of great power due to their leverage, especially when linked to the long bones of the body. In other places, such as their control of the human eye, they can also be used for fine motor control. The skeletal muscles which control the long bones of the body are the most energy-intensive muscles in the human body.
The organisation of the muscle fibres determines the type of skeletal muscle. In parallel muscles, the bundles of muscle fibres all run parallel to each other, providing great potential power. In convergent muscles, the muscle fibres fan out from a fixed point so that they do not pull in exactly the same directly: which reduces power but allows more versatility. In pennate muscles, the muscle fibres actually run at an angle to the associated tendon. Finally, in sphincter muscles, the muscle fibres run concentrically around a body opening.
Muscle tissue can only respond to neural instructions to contract. To do the opposite, a skeletal muscle must be physically stretched by an outside force. Most skeletal muscles thus come in mutually antagonistic pairs, which are attached on opposite sides of their associated bone or tissue. When one muscle contracts, the other is stretched: and vice versa. A few skeletal muscles do not need a direct antagonist, since the opposing force is provided by gravity or internal pressure from peristalsis.
Smooth muscle includes all the involuntary muscles of the body, with the exception of the cardiac muscles. These are the muscles which govern the digestive tract, urinary tract, respiratory tract, and even the tiny arrector pili in the skin which cause goosebumps.
Unlike skeletal and cardiac muscle, smooth muscle does not contain regular bundles of muscle fibres, and thus has no striation. Instead, smooth muscle cells spread out into sheets or tubes.
The interaction of the autonomic nervous system with the smooth muscle sheet determines the type of smooth muscle. In single-unit smooth muscle tissue, stimulation of a single muscle cell spreads so rapidly to the surrounding muscle cells that the entire sheet or tube contracts nearly as one. In multi-unit smooth muscle tissue, the stimulus response does not spread, so individual cells can be stimulated individually for fine control.
Cardiac muscle combines some characteristics of smooth muscle with some characteristics of skeletal muscle. It is found only in the heart.
Like skeletal muscle, cardiac muscle is striated, but the striations are not parallel as with skeletal muscle. Instead, the bundles which make up cardiac muscle connect at branching, irregular angles. These intercalated discs allow cardiac muscle to contract in the short, intense, coordinated bursts needed to pump blood efficiently through the body. As with single-unit smooth muscle, stimulation spreads so rapidly from cell to cell within paired heart ventricles that each pair of ventricles pulses nearly as one.
Cardiac muscle is also highly resistant to fatigue: a function of the relatively high number of mitochondria in cardiac muscle cells. Mitochondria are capable of storing a limited amount of oxygen in myoglobins. Together with an adequate blood supply, healthy cardiac cells can theoretically keep beating forever.
Yet that same aerobic efficiency also comes at a price. While other types of muscle can function, albeit inefficiently, in anaerobic conditions, cardiac muscle is so highly adapted to oxygen-based metabolism that no more than 10% of cardiac muscle energy - 1% under normal conditions - can be derived from anaerobic metabolism. Over a relatively short period of time, lack of adequate oxygen will cause the heart to stop beating.