are released by some types of bacteria
(such as those that cause tetanusand
diphtheria). Some chemical poisons, such
as arsenic and lead, are also neurotoxic.
neurotransmitterA chemical released
from a nerve ending that transmits
impulses from one neuron(nerve cell)
to another neuron, or to a muscle cell.
When a nerve impulse reaches a nerve
ending, neurotransmitters are released
from synaptic vesicles and cross a tiny
gap (synapse) to reach the target cell.
Here, they cause channels in the target
cell to open, letting through charged
particles that stimulate an impulse in
the cell. Alternatively, neurotransmitters
may inhibit nerve impulses.
Scores of different chemicals fulfil this
function in different parts of the nervous
system. Many neurotransmitters act as
both neurotransmitters and hormones,
being released into the bloodstream to
act on distant target cells.
One of the most important neuro-
transmitters is acetylcholine, which
causes skeletal muscles to contract
when it is released by neurons connect-
ed to the muscles. Acetylcholine is also
released by neurons that control the
sweat glands and the heartbeat, and
transmits messages between neurons
in the brain and spinal cord.
Another chemical, noradrenaline(nor-
epinephrine), aids the nervous control of
heartbeat, blood flow, and the body’s
response to stress. Dopamineplays an
important role in parts of the brain that
control movement. Serotoninis one of
the main neurotransmitters found in
parts of the brain concerned with con-
scious processes.
Another group of neurotransmitters is
called the neuropeptides. This group
includes the endorphins, which are used
by the brain to control sensitivity to pain.
neutrophilA type of phagocyte, or white
blood cell. They are an important part of
the immune system, and their role is to
engulf and destroy invading bacteria.
newbornAn infant at birth and during
the 1st few weeks of life (see also pre-
maturity; postmaturity).
NGUAn abbreviation for nongonococcal
urethritis.
niacinSee vitamin B complex.Diagnostic tests such as blood tests,
MRIscans, and nerve or muscle biopsy
may also be required. When possible,
treatment is aimed at the underlying
cause. If the cell bodies of the damaged
nerve cells have not been destroyed, full
recovery from neuropathy is possible.
neuropsychiatryThe branch of medic-
ine dealing with the relationship between
psychiatric symptoms and neurological
disorder. This may include the effects of
head injury and alcohol on the brain, or
disorders such as brain tumours, infec-
tions, inherited illnesses, and disorders
causing brain damage in childhood.
neurosisAn old term for a range of psy-
chiatric disorders excluding psychosis.
neurosurgeryThe specialty concerned
with the surgical treatment of disorders
of the brain, spinal cord, or other parts of
the nervous system.
Conditions treated by neurosurgery
include tumours of the brain, spinal cord,
or meninges (membranes surrounding
the brain and spinal cord); brain abscess;
abnormalities of the blood vessels sup-
plying the brain, such as an aneurysm
(balloon-like swelling at a weak point in
an artery); bleeding inside the skull (see
extradural haemorrhage, intracerebral
haemorrhage, and subdural haemorrh-
age); some birth defects (such as neural
tube defects and hydrocephalus); certain
types of epilepsy; and nerve damage
caused by illness or accidents. Neuro-
surgery may also be performed to relieve
pain that is otherwise untreatable.
neurosyphilisInfection of the brain or
spinal cord that occurs in untreated
syphilis many years after initial infection.
Damage to the spinal cord due to
neurosyphilis may cause tabes dorsalis,
characterized by poor coordination of
leg movements, urinary incontinence,
and pains in the abdomen and limbs.
Damage to the brain may cause demen-
tia,muscle weakness, and, in rare cases,
total paralysis of the limbs.
neurotoxinA chemical that damages
nervous tissue. The principal effects of
neurotoxic nerve damage are numbness,
weakness, or paralysis of the part of the
body supplied by the affected nerve.
Neurotoxins are present in the venom
of certain snakes (see snake bites), andNEUROPSYCHIATRY NIACINN