Page 72 - BBC Focus - August 2017

UNDERSTAND
PAIN

Capsaicin in
chillies binds to
heat receptors in
your mouth,
which is why
spicy food burns

2 receptors called nociceptors. When that different things can activate the
activated, these receptors send signals same nociceptor. As an example, let’s JARGON BUSTER
along the nerve fibres to the spinal look at temperature and food.
cord and up into the brain, where
pain, as a perception, emerges. How can food be painful? CAPSAICIN
The nociceptors can be activated by Different thermal nociceptors in our This chemical, found in
a variety of triggers: thermal (heat), body are activated by specific chillies, binds to a
mechanical (like a knife cut or temperatures, giving us a painful nociceptor that also
hammer blow) and chemical/irritant sensation of intense hot or cold. responds to extreme heat.
(for example, acid or chilli pepper). Amazingly, these same nociceptors This is why biting into a
The signals then travel along different are also activated by various natural chilli can cause pain.
V[RGU QH RCKP PGTXG HKDTG#ŤCNUQ chemicals, giving rise to the same
known as A-delta, fibres carry what experience. For instance, when we NOCICEPTORS
we call ‘first pain’ – the fast, quick bite into a chilli pepper, a chemical These are receptors on the
signal that tells you ‘ouch’ when you called capsaicin binds to the same pain nerve fibres that act
touch a hot pan. C fibres follow up nociceptor that’s activated by like a lock. Certain keys
with the ‘second pain’, which is the painfully hot temperatures of around (irritants, mechanical
slow, constant throbbing that tells you 42°C and above. That’s why we forces, temperature) open
it’s still hurting. Normal touch – perceive a curry as hot: the brain can’t them and set off messagesff
to the brain, signalling pain
feeling something like your clothes or distinguish what activated the and tissue damage.
holding a pen – is carried on different nociceptor, it just knows that your Over-the-counter
peripheral nerves called Aß (A-beta). mouth is burning! Downing a pint of painkillers target
The transmission of the pain signal lager won’t help, either, as capsaicin nociceptive pain.
to the spinal cord is helped by other is fat-soluble, not water-soluble – so
components of the nerve fibres called order a cucumber raita instead. PAIN NERVE FIBRES
ion channels, and it’s these that many Some scientists think that plants A,(A-delta) and C fibres
patients with CIP don’t have, which is produce capsaicin to discourage are nerves that carry
why they don’t feel pain. Therefore, mammals from eating their fruits. signals from nociceptors in
targeting and blocking nociceptors Birds don’t seem to react to the the skin, muscles and joints
and/or the transmission process is a chemical, allowing them to eat the to the spinal cord.
cunning way to block pain. Indeed, chillies and help with seed dispersal,
that’s what many pharmaceutical which is what the plant wants! PHANTOM PAIN
companies are currently trying to do. This is the perception of
Interestingly, many nociceptors are How does the brain generate pain? pain in a limb or organ
‘polymodal’ – a fancy word meaning Once the pain signals arrive from 2 that’s missing. This most
often occurs following the
amputation of an arm or
a leg. It’s devastating for
patients, and some
theories suggest that it’s a
result of altered nerve
LEFT: Nerves (yellow)
lie beneath the surface signals in the brain trying
of the skin to ‘fill the gap’.
PLACEBO ANALGESIA
Placebos are substances
with no active therapeutic
effect. Patients who take
them often report
improvements in their
condition. Placebo
analgesia is pain relief
from, say, a sugar pill, and
neuroimaging has shown
that it can work by PHOTOS: GETTY, SCIENCE PHOTO LIBRARY
hijacking an old and inbuilt
‘free pain relief’ system in
the brain.

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