environment and about the body’s inter-
nal state, to the central nervous system.
Information is collected by millions of
sense receptorsfound throughout body
tissues and in special sense organs, such
as the eye.Certain sensory information,
mainly that from the special sense
organs and skin receptors, enters the sen-
sory cortexof 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, abnormalDulled, unpleas-
ant, or otherwise alteredsensationsin
the absence of an obvious stimulus.
Numbness and pins-and-needlesare
common abnormal sensations.The spe-
cial senses can be impaired by damage
to the relevant sensory apparatus (see
vision, disorders of; smell; deafness; tinni-
tus). Other causes of abnormal sensation
include peripheral nerve damage caused
by diabetes mellitus, herpes zosterinfec-
tion, or pressure from a tumour, and
disruption of nerve pathways in the brain
or spinal corddue to spinal injury, head
injury, stroke, and multiple sclerosis.
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.
sensesSee sensation.
sensitizationThe initial exposure of a
person to an allergen or other substance
recognized as foreign by the immune
system, 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
hypersensitivityreaction.
sensorineural deafnessDeafnessdue
to problems with the inner ear, nerves,
or the brain’s auditory area.
sensory cortexA region of the outer
cerebrumof the brainin 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 areperceived in the occipital lobes at the
back of the cerebrum; sound is perceived
in the temporal lobes at the sides.
sensory deprivationThe 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
thinking, and hallucinations.
separation anxietyThe 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 depression.
sepsisInfection of a wound or body tis-
sues with bacteria that leads to the
formation of pusor to the multiplication
of the bacteria in the blood. (See also
bacteraemia; septicaemia; septic shock.)
septal defect A congenital heartabnor-
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 heart
failureto develop 6–8 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. Pulmonary hyperten-
sionmay develop in both types of defect.
Diagnosis may be aided by a chest
X-ray, ECG, or echocardiography.
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
treated with diureticsand digitalis drugs.
If the hole does not close spontaneously,
it may be repaired by open heart surgery.
septicaemiaA potentially life-threaten-
ing condition in which there is rapid
multiplication of bacteria and in whichSENSATION, ABNORMAL SEPTICAEMIAS