arranged so that the pull of one muscle
or group of muscles is opposed to
another, enabling a movement to be
reversed. Although most actions of the
skeletal muscles are under conscious
control, reflexmovements of certain
muscles occur in response to stimuli.
There are more than 600 muscles in
the body, classified according to the
type of movement they produce. An
extensor opens out a joint, a flexor closes
it; an adductor draws a part of the body
inwards, an abductor moves it out-
wards; a levator raises it, a depressor
lowers it; and constrictor or sphincter
muscles surround and close orifices.
musculoskeletalRelating to muscle
and/or bone. The musculoskeletal sys-
tem is the skeleton and the muscles
attached to it.
mushroom poisoningThere are many
species of poisonous mushrooms and
toadstools in the UK, but many of them
have an unpleasant taste and are there-
fore unlikely to be eaten in sufficient
amounts to cause problems.
Most fatal cases of mushroom poison-
ing in the UK are caused by AMANITA
PHALLOIDES(death cap). This mushroom
can be confused with the edible field
mushroom, although it has white gills
instead of pink-brown ones.
The death cap and some related spec-
ies, such as AMANITA VIROSA(destroying
angel), contain poisons called amanitins,
which attack cells in the liver, kidneys,
and small intestine. Symptoms such as
severe abdominal pain, vomiting, and
diarrhoea usually develop 8–14 hours
after eating the mushrooms. Later, there
may be liver enlargement and jaundice,
which may lead to death from liver fail-
ure. There is no antidote, and treatment
consists of supportive measures only.
For those people who survive, recovery
usually occurs after about 1 week.
AMANITA MUSCARIA(fly agaric) has a red
cap flecked with white. Symptoms of
poisoning appear within 20 minutes to
2 hours, and may include drowsiness,
visual disturbances, delirium, muscle
tremors, and nausea and vomiting. Treat-
ment of this type of poisoning (and of
other types with rapidly developing
symptoms) is with gastric lavage (see
lavage, gastric) and activated charcoal.
Recovery usually occurs within 24 hours.
“Magic” mushrooms contain the hallu-
cinogen psilocybin. These mushrooms
may also cause high fever in children.
The effects usually last for 4– 6 hours.
mutagenAny agent that increases the
rate of mutationin cells. The main
mutagens are ionizing radiation(see
radiation hazards), some chemicals, and
certain illnesses.
mutationA change in a cell’s DNA.
Many mutations are harmless; however,
some are harmful, giving rise to cancers,
birth defects, and hereditary diseases.
Very rarely, a mutation may be beneficial.
A mutation results from a fault in the
replication of DNA when a cell divides.
A daughter cell inherits some faulty
DNA, and the fault is copied each time
the new cell divides, creating a cell pop-
ulation containing the altered DNA.
Some mutations occur by chance. Any
agent that makes mutations more likely
is called a mutagen.
There are several types of mutation.
Point mutations affect only one gene
and may lead to the production of de-
fective enzymesor other proteins. In
other mutations, chromosomes(or parts
of them) are deleted, added, or rear-
ranged. This type may produce greater
disruptive effects than point mutations.
If a mutated cell is a somatic (body)
cell, it can, at worst, multiply to form a
group of abnormal cells. These cells
often die out, are destroyed by the
body’s immune system, or have only a
minor effect. Sometimes, however, they
may become a tumour.
A mutation in a germ cell(immature
egg or sperm) may be passed on to a
child, who then has the mutation in all
of his or her cells. This may cause an
obvious birth defect or an abnormality
in body chemistry. The mutation may
also be passed on to the child’s des-
cendants. Genetic disorders(such as
haemophiliaand achondroplasia) stem
from point mutations that occurred in
the germ cell of a parent, grandparent,
or more distant ancestor. Chromosomal
abnormalities(such as Down’s syndrome)
are generally due to mutations in the
formation of parental eggs or sperm.MUSCULOSKELETAL MUTATION
M