Download PDF

CONCEPTUAL LIFE SCIENCE
MUSCULAR SYSTEM
TYPES OF MUSCLES
Smooth muscle is the type of muscle found in all ofthe involuntary organs except
the heart. The tissue is made ofindividual muscle cells, each with its own nucleus. The
cells of smooth muscle do not have striations. Striations are parallel lines that are
perpendicular to the long axis of the muscle cell. They are found in striated and cardiac
muscle but not in smooth muscle.
Striated muscle is also called voluntary muscle. It is found attached to the bones
ofthe skeleton by tendons. A tendon is a connective structure composed of dense
connective tissue. The individual cells of striated muscle are very long, as long as the
muscle itself. Cells this long must have many nuclei so muscle cells are multinucleate.
Each individual nucleus controls its own nearby area of the muscle. Striated muscle has
striations. These cross-bands result from the inner molecular structure ofthe muscle.
Striated muscle is under voluntary control. These are the muscles that you can move
when you want them to do something for you.
Cardiac muscle is the type of muscle found only in the heart. The cells of cardiac
muscle are striated. Cardiac muscle has individual cells connected to each other by their
ends at which are located intercalated discs. Each individual cell of cardiac muscle has
its own nucleus. The heart pumps constantly at an average rate of 72 beats per minute.
The heart muscle cells rest between each heartbeat. Systole is when the heart contracts
while diastole is when the heart relaxes.
Structure ofmuscle
z
z
.-- --+---A---+
Figure 15-1. Structure of muscle (relaxed)
15-1
I--~
15-2
z
z
H
Figure 15-2. Structure of muscle (contracted)
Muscle consists ofthick fibers called myosin and thin fibers called actin. The
actin fibers are attached to vertical structures called Z-lines. The actin and myosin fibers
overlap and are in close proximity to each other. This configuration is known as the
actomyosin complex. These bands cause the striations of skeletal and cardiac muscle.
The band indicated by A is the complete length ofeach section of myosin. The space
between consecutive myosin bands is called the I-band. The space between consecutive
actin bands (where there is myosin alone) is called the H-band.
Relaxed muscle is shown in Figure 15-1. Note that the Z-lines are quite far apart
and the H and I bands are wide. Then examine Figure 15-2. When muscle contracts the
actin and myosin bands slide over each other and the Z-lines get closer together. Also,
the H and I-bands become much shorter. As the muscle units get closer together, the
muscle gets shorter. This is how muscles contract. Muscles cannot elongate on their
own. Another muscle must be used in order to elongate or relax a contracted muscle.
Attachment of muscles
Muscles are attached to bones in such a way as to provide pairs of muscles for
each moving part ofthe body. The forearm is an example. When the biceps muscle
contracts, it closes the angle that the ann makes. This muscle is called ajlexor. In order
to extend the ann and make it straight, the triceps muscle is used. This muscle is called
an extensor. So a flexor closes the angle of ajoint while the extensor increases the angle
ofa joint. When the flexor contracts the extensor is relaxed and permits itself to be
stretched. Likewise, when the extensor contracts, the flexor relaxes and is allowed to be
stretched. Nonnally, these muscles do not contract simultaneously. However, during
electric shock, such as when a person accidentally grabs a live wire, all muscles are
contracted simultaneously. The stronger ones win. That is why a person cannot let go of
a live wire.