Bma Illustrated Medical Dictionary

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-
monesand 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 dystrophyand myasthenia gravis.
muscle-relaxant drugsA 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-
rosisand 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 generalanaesthesia.
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 spasmSudden 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 drugsmay

also be needed. (See also spasticity.)

muscular dystrophyA 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

MUSCLE-RELAXANT DRUGS MUSCULAR DYSTROPHY

M

MUSCLE

MUSCLE FIBRES MYOFIBRIL

Thin

myofilament

Thick myofilament

Muscle

fibre

Blood vessel

Myofibril

SECTION OF SKELETAL MUSCLE

Fascicle
Muscle fibres