by Tenebris

Created on: December 20, 2011   Last Updated: December 22, 2011

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 muscles

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.

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