The thirty three vertebrae are stacked on cartilaginous discs. When these discs slip or are ruptured, the encased nerves are irritated or damaged. This often takes the form of shooting pain in the leg, involving the sciatic nerve. The appendicular skeleton supports the limbs of the body. Even muscles that appear to generate force by extension in fact do so by contraction, often involving antagonistic pairs of muscles.
Skeletal muscle propels the skeletal system and is made up of giant cells called muscle fibers .
Smooth muscle propels food through the digestive tract, and provides tension in the bladder, uterus, mammary glands, and arteries. Cardiac muscle, found only in the heart, contracts spontaneously. All three types of muscle rely on the same biochemical events for contraction. In skeletal muscle, a nerve impulse travels to the muscle, causing the release of calcium ions into the muscle fiber. The release of calcium ions sets in motion the sequence of events resulting in muscle contraction.