The study of the
interactions between the
internal body functions and the body's
response to the external environment.
An uncommon dis-
order that is inherited. Also called von
Recklinghausen's disease, neurofibro-
matosis is characterized by numerous
neurofibromas (soft, fibrous swellings,
varying significantly in
grow from nerves, and by
café au lait
(pale, coffee-coloured patches) on
the skin, usually on the trunk and pelvis.
If neurofibromas occur in the central
nervous system, they may cause
and other complications. Neurofibro-
matosis can lead to bone deformities.
Rarely, neurofibromas become cancerous.
Surgical removal of neurofibromas is
necessary only if there are complications.
Anyone with this disorder, and parents
of an affected child, should seek
if planning a pregnancy.
The medical discipline con-
cerned with the study of the
and its disorders (see also
A noncancerous tumour of
tissue. In most cases, the cause is
unknown; rarely, a neuroma develops as
a result of damage to a nerve.
A neuroma may affect any nerve in the
body. Symptoms vary, but there is often
intermittent pain and sometimes weak-
ness and numbness in the areas that
are supplied by the affected nerve.
If symptoms are troublesome, the
tumour may be surgically removed. (See
The term used to describe a
nerve cell. A typical neuron consists of a
cell body, several branching projections
called dendrites, and a filamentous pro-
jection called an axon (also known as a
nerve fibre). An axon branches at its
end to form terminals through which
electrical signals are transmitted to tar-
get cells. Most axons are coated with a
speeds the transmission of the signals.
The myelin sheath is punctuated along
its length by gaps called nodes of Ran-
vier, which help this process. Because
the myelin sheath is nonconductive, ion
exchange (depolarization) only occurs
at a node, and signals leap from node
to node along the length of the axon.
The nervous system contains billions
of neurons, of which there are 3 main
types: sensory neurons, which carry sig-
nals from sense receptors into the
(CNS); motor neurons,
which carry signals from the CNS to