Do you like to dance? Most of us do, or we may simply enjoy watching good dancers. The grace and coordination involved in dancing result from the interaction of many of the organ systems, but the one you think of first is probably the muscular system.
There are more than 600 muscles in the human body. Most of these muscles are attached to the bones of the skeleton by tendons, although a few muscles are attached to the undersurface of the skin. The primary function of the muscular system is to move the skeleton. The muscle contractions required for movement also produce heat, which contributes to the maintenance of a constant body temperature.
The other body systems directly involved in movement are the nervous, respiratory, and circulatory systems. The nervous system transmits the electrochemical impulses that cause muscle cells to contract. The respiratory system exchanges oxygen and carbon dioxide between the air and blood. The circulatory system brings oxygen to the muscles and takes carbon dioxide away.
These interactions of body systems are covered in this chapter, which focuses on the skeletal muscles. You may recall from Chapter 4 that there are two other types of muscle tissue: smooth muscle and cardiac muscle. These types of muscle tissue will be discussed in other chapters in relation to the organs of which they are part. Before you continue, you may find it helpful to go back to Chapter 4 and review the structure and characteristics of skeletal muscle tissue.
In this chapter, we will begin with the gross (large) anatomy and physiology of muscles, then discuss the microscopic structure of muscle cells and the biochemistry of muscle contraction.
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 (see Fig. 7–3 later in this chapter). 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 aponeurosis. (See Fig. 7–9 later in this chapter for the epicranial aponeurosis, but before you look, decide what epicranial means.)
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.
Muscles are arranged around the skeleton so as to bring about a variety of movements. The two general types of arrangements are the opposing antagonists and the cooperative synergists
Antagonists are opponents, so we use the term antagonistic muscles for muscles that have opposing or opposite functions. An example will be helpful here— refer to Fig. 7–1 as you read the following. The biceps brachii is the muscle on the front of the upper arm.
The origin of the biceps is on the scapula (there are actually two tendons, hence the name biceps), and the insertion is on the radius. When the biceps contract, it flexes the forearm, that is, bends the elbow (see Table 7–2 later in this chapter). Recall that when a muscle contracts, it gets shorter and pulls. Muscles cannot push, for when they relax they exert no force.
Therefore, the biceps can bend the elbow but cannot straighten it; another muscle is needed. The triceps brachii is located on the back of the upper arm. Its origins (the prefix tri tells you that there are three of them) are on the scapula and humerus, and its insertion is on the ulna. When the triceps contract and pulls, it extends the forearm, that is, straightens the elbow.