All muscle cells are specialized for contraction. When these cells contract, they shorten and pull a bone to produce movement. Each skeletal muscle is made of thousands of individual muscle cells, which also may be called muscle fibers . Depending on the work a muscle is required to do, variable numbers of muscle fibers contract. When picking up a pencil, for example, only a small portion of the muscle fibers in each finger muscle will contract. If the muscle has more work to do, such as picking up a book, more muscle fibers will contract to accomplish the task.

Muscles are anchored firmly to bones by tendons. Most tendons are rope-like, but some are flat; a flat tendon is called an aponeurosis. Tendons are made of fibrous connective tissue, which, you may remember, is very strong and merges with the fascia that covers the muscle and with the periosteum, the fibrous connective tissue membrane that covers bones. A muscle usually has at least two tendons, each attached to a different bone.

The more immobile or stationary attachment of the muscle is its origin; the more movable attachment is called the insertion. The muscle itself crosses the joint of the two bones to which it is attached, and when the muscle contracts it pulls on its insertion and moves the bone in a specific direction.

Buy the Book that holds this excerpt: Essentials of Anatomy And Physiology

Related Articles