Muscle
fibre
MUSCLE-RELAXANT DRUGS
MUSCULAR DYSTROPHY
MUSCLE
Fascicle
_
'
Muscle fibres
Thin
Thick myofilament
myofilament
Myofibril
u
SECTION OF SKELETAL MUSCLE
Blood vessel
MUSCLE FIBRES
MYOFIBRIL
spindle-shaped cells, and contracts with
the same sliding action of actin and
myosin as skeletal muscle. This type of
muscle is stimulated by the
autonomic
nervous system
;
it also responds to
hor-
mones
and to levels of chemicals in
fluid around the muscle.
Cardiac muscle (also called myocar-
dium) is found only in the
heart
. It is
able to contract rhythmically about
100,000
times a day, and has a similar
structure to that of skeletal muscle.
Contraction is stimulated by the auto-
nomic nervous system, by hormones,
and by the stretching of muscle fibres.
The most common muscle disorders
are injury and lack of blood supply to a
muscle. Rare disorders include
mus-
cular dystrophy
and
myasthenia gravis
.
muscle-relaxant
drugs
A group of
drugs used to relieve muscle spasm and
spasticity
. Muscle-relaxant drugs are used
mainly in the treatment of nervous-
system disorders such as
multiple scle-
rosis
and painful muscular conditions
such as
torticollis
.
They are occasionally
used to relieve muscle rigidity caused
by injury. Some types are used to cause
temporary
paralysis
during
surgery
under general
anaesthesia.
Except for
dantrolene
, muscle-relaxant
drugs partly block nerve signals that
stimulate muscle contraction. Dantrol-
ene interferes with the chemical activity
in muscle cells needed for contraction.
The drugs may cause muscle weakness
and drowsiness. In rare cases, dantro-
lene causes liver damage.
muscle spasm
Sudden and involuntary
contraction of a muscle. Muscle spasm
is a normal reaction to pain and inflam-
mation around a joint. Common causes
are muscle
strain
,
disc prolapse
, and
stress. Usually, the cause of the spasm
is treated.
Muscle-relaxant drugs
may
also be needed. (See also
spasticity
.)
muscular dystrophy
A group of rare
inherited muscle disorders which cause
slow, progressive wasting away of mus-
cle fibres. This degeneration may lead
to disability and death.
The most common and severe form of
muscular dystrophy is Duchenne mus-
cular dystrophy. This is caused by a
recessive gene carried on the X chromo-
some (see
sex-linked inheritance)
. Boys
only have one X chromosome, so if they
inherit a copy of the defective gene
from their mother they develop the dis-
order. Girls (with two X chromosomes)
are not affected but become carriers of
the defective gene. Affected boys walk
with a waddle, find climbing difficult,
and may have curvature of the spine. The
disorder progresses rapidly: the ability
to walk is lost by the age of
12
, and few
boys survive beyond the teenage years.
Becker's
muscular dystrophy starts
later in childhood and progresses more
slowly. Myotonic dystrophy affects the
381
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