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Humans are able to move because of the actions of bones and
muscles working together. Together, the muscles and bones
allow us to bear weight so that we can stand, walk, and lift
things. If the muscle is not working properly, such as if
the muscle is too weak, then movement becomes more difficult,
or even impossible. There are many different reasons why a
muscle may not function the way it is supposed to. It may
not be receiving the signal from the appropriate nerve that
is "instructing" it to make a movement. Or, something
in the muscle tissue itself may not be intact or working appropriately.
Also, there are several different types of muscle tissue in
our bodies, each with a specialized structure so that it can
perform a specialized function. If there is a problem in the
structure of one type of muscle tissue, it doesn't mean that
there will be a problem in all the types of muscle tissue.
Generally, this is not the case at all.
Types of Muscle Tissue
| Muscle tissue is specialized
for contraction and movement and it is found in two main
forms: smooth and striated. Smooth muscle is present
in those body systems that are under involuntary control.
The digestive and the respiratory passages, among others,
are made primarily of smooth muscle. Striated muscle
is present in the heart and in the muscles that control
our movements and breathing. Striated muscle owes its
name to the interesting array of bands that become visible
at the microscopic level. The origin and importance of
these bands will be discussed further in this section.
Striated muscle can be divided into two subtypes: cardiac
muscle and skeletal muscle. Cardiac muscle, as
the word implies, makes up the walls of the heart and
is connected with many nerves that help keep the heart
functioning. Skeletal muscle (Figure 3) also is
connected with nerves and because movement of skeletal
muscles can be consciously controlled, it is known as
voluntary muscle. Most skeletal muscle is attached to
our skeleton (hence its name) and by a special form of
tissue called tendons. Our neuromuscular research focuses
mainly on conditions that are caused by primary defects
of skeletal muscle structure and function. Collectively,
these disorders are called myopathies. |
Figure 3.
Healthy Skeletal Muscle |
The Sarcomere
As stated previously, when striated muscle is examined under
a microscope, a distinct repeating pattern of bands can be observed
in the threadlike myofibrils. Within each myofibril there are
dense Z lines. A sarcomere (or muscle functional unit)
extends from Z line to Z line. The sarcomeres are repeating
units formed by different bands known as the Z, I, A, H, and
the M bands. The sarcomere is the product of the interaction
between the proteins that form the thick filaments and the thin
filaments that comprise these bands. The thick filaments are
made of myosin and are located at the center of each sarcomere.
Thin filaments are made of actin and anchor to the Z line. Muscles
contract by shortening each sarcomere, in a process known as
the sliding filament model of muscle contraction. This explains
how the thick filaments pull on and slide along the thin filaments
until the thin filaments meet in the middle, causing the sarcomere
to shorten and the muscle to contract. One of the goals of our
research is to characterize the proteins that form the sarcomere
(sarcomeric proteins), hoping to identify new genes that are
associated with disorders of muscle contraction.
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Figure 4.
This is a diagram of a sarcomere, which functions in the
contraction process in striated muscle. The Z-discs at
both ends of the sarcomere tie the thin actin filaments
together. The shortening or contraction of the sarcomere
happens when the actin and myosin proteins slide over
one another. |
Research has already unraveled many of the sarcomeric proteins
that are responsible for skeletal muscle contraction. The
names of some of these proteins are actin, myosin, troponin,
tropomyosin, and nubulin. When skeletal muscle fibers receive
a nerve impulse, these proteins (and others yet to be identified)
are believed to change their shape and position. Research
has also uncovered many different proteins that can be found
along the muscle cell membrane. Different mutations in different
genes affect specific proteins located along this membrane,
as indicated in the diagram below.
Figure 5. This is a diagram of muscle proteins located
along the muscle cell membrane. When all the proteins are
working properly, the muscle should be working properly. The
boxed-in words located next to a specific protein represent
diseases that may manifest if that protein is not functioning
properly.
Related Educational Web sites:
Cell
and Developmental Biology Online
Online
Biology Book
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