Chapter 5 The Muscular System 159
Muscle Functions
Despite the different properties of the
three types of muscle, certain behavioral
characteristics are common to all muscle tissue.
In the case of skeletal muscles, there are also
certain functional roles that muscles can play in
contributing to different movements of the body.
Behavioral Properties
All muscle tissues have four behavioral
characteristics in common: irritability,
extensibility, elasticity, and contractility. Two of
these—extensibility, the ability to be stretched,
and elasticity, the ability to return to normal
length after a stretch—are common not just to
muscle, but to many types of biological tissues.
For example, when a muscle group such as the
hamstrings (on the posterior side of the thigh)
is stretched over a period of time, the muscles
lengthen, and the range of motion at the hip
increases, making it easier to touch the toes. The
stretched muscles do not return to resting length
immediately, but shorten over a period of time.
1. What is the difference between voluntary
and involuntary muscles?
2. Categorize each muscle type as
voluntary or involuntary.
3. What are the three layers of tissue that
run the length of a skeletal muscle?
Check Your Understanding
Another behavioral characteristic common
to all muscle is irritability, or the ability to
respond to a stimulus. Muscles are routinely
stimulated by signals from the nerves that
supply them. Muscles can also be irritated by a
mechanical stimulus, such as an external blow to
a muscle. The response to all forms of stimuli is
muscle contraction.
As mentioned in the chapter introduction,
contractility, the ability to contract or shorten,
is the one behavioral characteristic unique to
muscle tissue. Most muscles have a tendon
attaching to a bone at one end and a tendon
attaching to another bone at the other end. When
a muscle contracts, it pulls on the bones at the
attachment sites. This pulling force is called a
tensile force, or tension. The amount of tension
developed is constant throughout the muscle,
tendons, and attachment sites.
Tension and Types of Skeletal
Muscle Contraction
Although we commonly use the term
contraction (which implies shortening) to mean
that tension has developed in a muscle, muscles
do not always shorten when they develop
tension. When a skeletal muscle develops
tension, one of three actions can happen: the
muscle can shorten, remain the same length, or
actually lengthen. Let’s look at the familiar large
muscle groups, the biceps and triceps, on the
anterior and posterior sides of your upper arm,
to see examples of these three different types of
tension.
Figure 5.3 Muscle Categories
Characteristic Skeletal Smooth Cardiac
Cell structure
varying lengths, thread-
shaped, striated
short, spindle-shaped, no
striations
branching interconnected
chains, striated
Nucleus multinucleate one nucleus one nucleus
Control voluntary involuntary involuntary
Location most attach to bones; some
facial muscles attach to skin
walls of internal organs
other than the heart
walls of the heart