environment and about the body's inter-
nal state, to the
central nervous system.
Information is collected by millions of
found throughout body
tissues and in special sense organs, such
Certain sensory information,
mainly that from the special sense
organs and skin receptors, enters the
of the brain, where sensations
are consciously perceived. Other types of
sensory information, for example about
body posture, are processed elsewhere
and do not produce conscious sensation.
sensation, abnormal Dulled, unpleas-
ant, or otherwise altered
the absence of an obvious stimulus.
common abnormal sensations. The spe-
cial senses can be impaired by damage
to the relevant sensory apparatus (see
vision, disorders of; smell; deafness; tinni-
Other causes of abnormal sensation
include peripheral nerve damage caused
diabetes mellitus, herpes zoster
tion, or pressure from a tumour, and
disruption of nerve pathways in the
spinal injury, head
Pressure on or damage to nerves can
sometimes be relieved by surgery or by
treatments for the cause. In other cases,
distressing abnormal sensation can be
relieved only by cutting the relevant nerve
fibres or by giving injections to block
the transmission of signals.
sensitization The initial exposure of a
person to an allergen or other substance
recognized as foreign by the
, which leads to an immune res-
ponse. On subsequent exposures to the
same substance, there is a much stronger
and faster immune reaction. This forms
the basis of allergy and other types of
to problems with the inner ear, nerves,
or the brain's auditory area.
sensory cortex A region of the outer
in which sensory
information comes to consciousness.
Pressure, pain, and temperature sen-
sations from the skin, muscles, joints,
and organs are perceived in the parietal
lobes, as is taste. Visual sensations are
perceived in the occipital lobes at the
back of the cerebrum; sound is perceived
in the temporal lobes at the sides.
sensory deprivation The removal of
normal external stimuli, such as sight
and sound, from a person's environment.
Prolonged sensory deprivation can pro-
duce feelings of unreality, difficulty in
anxiety The feelings of
distress a young child experiences when
parted from his or her parents or home.
This is a normal aspect of infant behav-
iour and usually diminishes by age 3 or 4.
In separation anxiety disorder, the
reaction to separation is greater than
that expected for the child's level of de-
velopment. The anxiety may manifest as
physical symptoms. Separation anxiety
disorder may be a feature of
sepsis Infection of a wound or body tis-
sues with bacteria that leads to the
or to the multiplication
of the bacteria in the blood. (See also
bacteraemia; septicaemia; septic shock.
septal defect A congenital
mality in which there is a hole in the
septum between the left and right ven-
tricles of the heart or, more rarely,
between the left and right atria. Usually,
the cause is unknown. The hole allows
freshly oxygenated blood to mix with
deoxygenated blood in the heart.
A small defect has little or no effect. A
large ventricular hole may cause
weeks after birth,
causing breathlessness and feeding dif-
ficulties. A large atrial defect may never
cause heart failure, but there may be
fatigue on exertion.
may develop in both types of defect.
Diagnosis may be aided by a
Atrial holes are repaired surgically if
they cause symptoms or if complications
develop. As the child grows, small ven-
tricular holes often become smaller, or
even close, on their own. A ventricular
defect that is causing heart failure is
If the hole does not close spontaneously,
it may be repaired by
open heart surgery
septicaemia A potentially life-threaten-
ing condition in which there is rapid
multiplication of bacteria and in which