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 pri-
mary function of the muscular systemis 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 res-
piratory 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 car-
diac muscle. These types of muscle tissue will be dis-
cussed 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 bio-
chemistry of muscle contraction.

MUSCLE STRUCTURE

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 an aponeurosis. (See Fig. 7–9 later in

this chapter for the epicranial aponeurosis, but before

you look, decide what epicranialmeans.) 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.

MUSCLE ARRANGEMENTS

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.

Antagonistic Muscles

Antagonists are opponents, so we use the term antag-

onistic musclesfor 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 contracts,

it flexesthe 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 ori-

gins (the prefix tritells you that there are three of

them) are on the scapula and humerus, and its inser-

tion is on the ulna. When the triceps contracts and

pulls, it extendsthe forearm, that is, straightens the

elbow.

138 The Muscular System