Skeletal muscles play a huge role in the way our bodies function because without muscles, our bones would not properly be held together.
Most of the body’s muscle tissue is skeletal muscle. It interacts with the skeleton to move body parts. It’s long, thin cells are called fibers and they have more than one nucleus. Their structure gives them a striped look. The muscles are considered voluntary, which means that the contractions can be controlled.
A skeletal muscle contains bundles of muscle cells. Inside each cell are threadlike myofibrils, which are divided into sarcomeres, the unit of contraction. They are bundled together by connective tissue that extends past it to form tendons. A tendon is a strap of dense connective tissue that attaches a muscle to a bone. Tendons make joints more stable by keeping the adjoining bones appropriately aligned.
The human body has more than six hundred skeletal muscles. One end of the muscle is called the origin, which stays relatively motionless during contractions. The other end is called the insertion. It is attached to a bone that moves mostly while contractions occur. When a skeletal muscle contracts, it pulls on the bone it’s attached to. Because skeletal muscle exerts force only when it contracts, a second muscle, a flexor, is needed to flex or bend the joint.
Muscles attach very close to muscle joints, and are arranged in either pairs or groups. Some of the muscles work in reciprocal innervation, or opposition. That is, when one muscle stimulates, no signals are sent to the other group. Therefore, it does not contract. Other muscles work in a synergistic role. That is, it supports and the contraction adds force or helps stabilize another muscle.
The body has two basic types of skeletal muscle, “fast” and “slow.” Slow muscles, or red muscles, are slow, long-lasting contractions. The cells are packed with myoglobin, a reddish protein that binds oxygen for the cell’s use in making ATP. They are served by a large number of capillaries. The cells contract fairly slowly therefore, the slow cells are well equipped to make ATP. Fast or white muscles, are found in the hand and have fewer capillaries. There are fewer mitochondria and less myoglobin. Fast muscles can contract rapidly and powerfully for short periods. Muscle of the back and leg are called postural muscles, because they aid body support. Postural muscles must contract for long periods when a person is standing. They have a high proportion of red muscle cells.
A muscle contracts when its cell shortens. When a muscle cell shortens, many units of contraction inside that cell are shortening. Each of these basic units of contraction is a sarcomere. Bundles of cells in a skeletal muscle run parallel with the muscle. This is because the arrangement tries to focus the force of a contracting muscle onto a bone in the same direction. The nervous system controls muscle contractions. The motor neuron begins contraction of the sarcomeres....