Appendix - The Body and How it Works

Musculoskeletal System

The musculoskeletal system is the framework of the body within which organs and body systems function. This framework includes bones, muscles, tendons, and ligaments. Bones act as levers for muscle action; muscles shorten to produce movement; tendons attach muscles to bones; ligaments attach bones to bones at the joints. The musculoskeletal system protects organs, supports the body, and provides for its movement.

Muscles

Muscles are made of a special kind of tissue that contracts (shortens) when stimulated by nerve impulses. Generally, body movement is caused by several muscles working in combination; as some are contracting, others are relaxing. The nerves in the muscles carry impulses to and from the brain.

Muscles are classified as either voluntary or involuntary. Voluntary muscles are consciously controlled by the person, meaning they can be contracted or relaxed as the individual wishes. The muscles that move the skeleton are voluntary.

Involuntary muscles contract and relax rhythmically without any conscious effort on the part of the person. The heart, which has its own regulating system, is a good example of an involuntary muscle.

The diaphragm, a large dome-shaped muscle that separates the chest and abdominal cavities and is used in breathing, has characteristics of both voluntary and involuntary muscles. The contraction of this muscle, and thus the rate of breathing, can be changed at will for short periods of time.