explain
pain
section
3
page
56
Get to know your skin and soft tissues
A
dults have nearly 2 squaremetres of skin. Skinmakes
up about 15 to 20% of body weight. Skin is only half a
millimetre thick on the eyelids but could be as thick as six
millimetres on the soles of the feet and the back. The skin is
a critical protector and as the first physical contact with the
outside world, it containsmany alarm bells.
Much ofwhatwe know about pain is based on the skin. In this
sense, itmirrors the state of thenervous system. Interestingly,
an injury to the skin very rarely leads to chronic pain, with
the exception of severe burns. That said, painful skin zones,
changes in skin health and altered sweating or hair growth
can all be indicators of damaged nerves.
In some pain states that are initiated by joint or nerve
damage, skin can be sensitive to light touch and brushing.
Sometimes even the touch of an item of clothingmay evoke
severe pain. Of course, light touchwould not normally be
able to evoke pain unless the skin is badly damaged.
However, if there is a change in the way the nervous system
works and the alarm system is altered, gentle touch or
small movementsmay cause pain. The pain that is common
after shingles (post-herpetic neuralgia) is a good example.
We will discuss nerve changes in the next section.
The strange creature pictured on the right is known as
‘homuncularman’. Contained in a little strip of brain
(sensory cortex), which is as long as your finger and just
above your ear, is the representation of the skin in the brain
(review page 22 and 23). Thismeans that if you put a pin in
your finger, the virtual finger in the brainwould ‘light up’.
All skin (all body parts too) has a little section devoted to it
in the brain. Some areas of skinwhichwe rely onmore for
day to day activities (for example our fingers and lips) have
more brain ‘real estate’ than areas of skin that we don’t rely
on somuch, for example the back. If the bodywas shaped in
the same proportions that the virtual body in the brainwas,
then humans would look a little bit like this odd figure – not
a good look! This suggests a use-dependent brain. Just to be
clear, the areas that you usemore andwhich require the
best sensation have a larger brain representation. If you
demandmore of a body part, then that part will have a
bigger representation in the brain. For example, violinists,
cellists and guitarists have a bigger left virtual hand in the
brain thannon-musicians.
90
And London taxi drivers have a
bigger hippocampus (the part of the brain used in
navigation) than healthy non-taxi drivers.
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