The brainand spinal cordform the central
nervous system(CNS), which consists of
billions of interconnected neurons(nerve
cells). Input of information to the CNS
comes from the sense organs. Motor
instructions are sent out to skeletal
muscles, the muscles controlling speech,
internal organs and glands, and the
sweat glands in the skin. This informa-
tion is carried along nerves that fan out
from the CNS to the entire body. Each
nerve is a bundle consisting of the
axons (filamentous projections) of many
individual neurons.
In addition to the nervous system’s
anatomical divisions, there are various
functional divisions. Two of the most
important are the autonomic nervous
system, concerned with the automatic
(unconscious) regulation of internal body
functioning, and the somatic nervous
system, which controls the muscles
responsible for voluntary movement.
The overall function of the nervous
system is to gather and analyse infor-
mation about the external environment
and the body’s internal state, and to ini-
tiate appropriate responses, such as
avoiding physical danger.
The nervous system functions largely
through automatic responses to stimuli
(see reflex), although voluntary actions
can also be initiated through the activity
of higher, conscious areas of the brain.
Disorders of the nervous system may
result from damage to or dysfunction of
its component parts (see brain; spinal
cord; neuropathy; nerve injury). They may
also be due to impairment of sensory,
analytical, or memory functions (see
vision, disorders of; deafness; numbness;
anosmia;agnosia;amnesia), or of motor
functions (see aphasia; dysarthria;ataxia).
netilmicinAn antibiotic drug usually
prescribed only to treat serious infec-
tion in hospital, when other antibiotic
drugs have proved ineffective. In rare
circumstances, netilmicin can damage
the inner ear or the kidneys.
nettle rashA common name for urticaria.
neuralgiaPain caused by irritation of,
or damage to, a nerve. The pain usually
occurs in brief bouts and may be severe.
Some types of neuralgia are features
of a specific disorder. Migrainesuffererscommonly experience a form of neur-
algia consisting of attacks of intense,
radiating pain around the eye. Postherp-
etic neuralgia is a burning pain that may
recur at the site of an attack of herpes
zoster(shingles) for months or even
years after the illness.
Other types of neuralgia result from
disturbance of a particular nerve. In
glossopharyngeal neuralgia, intense pain
is felt at the back of the tongue and in
the throat and ear, all of which are areas
supplied by the glossopharyngeal nerve.
The cause of the pain is generally
unknown. The same is true of trigeminal
neuralgia, a severe paroxysm of pain
affecting one side of the face supplied
by the trigeminal nerve.
Neuralgia is sometimes relieved by
analgesic drugs(painkillers) such as
paracetamol. Glossopharyngeal, trigem-
inal, and postherpetic neuralgia may
respond to treatment with carbamaz-
epineor other anticonvulsant drugs, or
to tricyclic antidepressant drugs.
neural tube defectA developmental
failure affecting the spinal cord or brain
of the embryo. The most serious defect
is anencephaly(total lack of a brain),
which is fatal. More common is spina
bifida, in which the vertebrae do not
form a complete ring around the spinal
cord. Spina bifida can occur anywhere
on the spine, but it is most common
in the lower back.
There are different forms of spina bifida.
In spina bifida occulta, the only defect
is a failure of the fusion of the bony
arches behind the spinal cord, which
may not cause any problems. When the
bone defect is more extensive, there
may be a meningocele, a protrusion of
the meninges, or a myelomeningocele, a
malformation of the spinal cord. Myelo-
meningocele is likely to cause severe
handicap, with paralysis of the legs,
loss of sensation in the lower body,
hydrocephalus, and paralysis of the
anus and bladder, causing incontinence.
Associated problems include cerebral
palsy, epilepsy, and mental handicap.
Surgery is usually performed a few days
after birth. In mild cases, the defect can
usually be corrected, but in myelomen-
ingocele, some handicap will remain.NETILMICIN NEURAL TUBE DEFECTN