the MUSCULar SYSteM 115is life-threatening. The bacterial toxin travels to the spinal
cord, where it blocks nervous system signals that release
skeletal muscles from contraction. The muscles go into
continuing spasms called spastic paralysis. A patient’s fists
and jaw may stay clenched (which is why the disease some-
times is called “lockjaw”) and the spine may arch in a stiff
curve. Death comes when paralysis reaches the heart and
muscles used in breathing.
Today a tetanus vaccine can confer immunity to the dis-
ease, and in developed countries such as the United States
nearly all people are immunized as children, with periodic
“booster” shots recommended for adults. Vaccines were
not available for soldiers who sustained battlefield wounds
in early wars, and many suffered an agonizing death due
to tetanus (Figure 6.18). Globally, the disease kills about
200,000 people each year, mostly women who must give
birth in unsanitary conditions.Cancer may develop in muscle tissue
Cancers that affect the body’s soft tissues are a form of
sarcoma (the prefix sarc- means “tissue”). Luckily, can-
cer that begins in muscle tissue is relatively rare—only
about 1 percent of each year’s new cancer cases. It is
most common in children and young adults, and about
two-thirds of cases involve malignancies that develop
in skeletal muscle. This form of cancer is known as
rhabdomyosarcoma.
The exact cause of rhabdomyosarcoma is not known,
although, as with all cancers, genetic changes are the direct
triggers. Having certain rare connective tissue disorders
increases the risk. Experience shows that patients must be
treated with a three-pronged therapy: surgery to remove
as much of the tumor as possible, then chemotherapy and
radiation to kill any remaining cancerous cells. When
patients undergo this demanding treatment regimen, the
chances of a cure are excellent.Duchenne muscular dystrophy (DMD) is the most com-
mon form in children (Figure 6.17). It is caused by a single
mutant gene that interferes with the ability of sarcomeres
in muscle cells to contract. Affected youngsters usually
are confined to a wheelchair by their teens, and most die
by their early twenties.
Myotonic muscular dystrophy is usually seen in
adults. It generally affects only the hands and feet and is
not life-threatening. Myo- means “muscle,” and the name
of this disorder indicates that affected muscles contract
strongly but don’t relax in the normal way.
Scientists have recently made strides in efforts to
develop effective treatments for muscular dystrophies. As
yet there is no cure, but that sad fact is simply spurring the
pace of research.
Bacterial infections can interfere
with nervous system signals to muscles
Section 6.4 mentioned the use of Clostridium botulinum toxin
for Botox injections. This microorganism normally lives in
soil. When it contaminates food in unsterilized cans or jars,
it produces the botulinum toxin, which causes the deadly
food poisoning called botulism. The toxin stops motor
neurons from releasing ACh, the neurotransmitter that
triggers muscle contractions. As a result, muscles become
paralyzed. Swift treatment with an antitoxin is the only
way to prevent death due to paralysis of the heart muscle
and the skeletal muscles involved in breathing.
A similar microbe, Clostridium tetani, lives in the gastro-
intestinal (GI) tract of animals such as cattle and horses.
(It may also inhabit the human GI tract.) C. tetani spores,
a resting stage of the microbe, may be in soil that contains
manure. If the spores enter a wound, the microbe becomes
active and produces a toxin that causes the disease tetanus.
Unlike the healthy state of steady, low-level muscle con-
traction of the same name (Section 6.6), the disease tetanus
Figure 6.18 The disease tetanus “freezes” muscles in a contracted state. A This painting depicts a soldier dying of the disease
tetanus in a military hospital in the 1800s after the bacterium Clostridium tetani infected a battlefield wound. b The tetanus vaccine
has saved countless lives in countries where it is readily available. (A: The Royal College of Surgeons of England; B: Photopat med/Alamy)
A bCopyright 2016 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).