This article was edited to fit the needs of the Science Focus publication.
本文以「科言」所需而被更改。
Itching
is an irritating sensation that
most people experience on a daily basis. Typically,
itches go as swiftly as they come, but a stubborn
itch can last for far too long that does not seem
to abate despite continuous scratching, or for the
more determined - willfully ignoring. What exactly is
the mechanism that causes our skin to itch?
Clinically known as pruritus, itching can be
triggered by various irritants. Itching occurs when
an external stimulus, such as a feather or dust,
comes into contact with the skin. As the stimulus
brushes against skin, it fires receptors situated
at the surface of the skin which then generate
nerve impulses [1], an electrical signal that passes
through a neuron. The journey of a nerve impulse
begins from one neuron to the next, stopping at the
synapse which causes the ends of the axons – a
neuron’s nerve fiber – to release a chemical known
as the neurotransmitter. This then diffuses across the
synapse before attaching to the membrane of the
destination neuron, which then generates another
impulse in a relay-like fashion. Repeating this action
from one neuron to the next passes the itch signal
onto the spinal cord and eventually, the brain
registers it as an itch.
For a long time, scientists believed that the
sensation of itching was the result of pain related
neurotransmitters and receptors, and that itching
was simply considered as a milder form of pain
[2]. This was questioned in a paper published in
2013, which reported that the neurotransmitter —
natriuretic polypeptide B (Nppb), and the sensory
receptor — natriuretic peptide receptor A (Npra),
are the primary vehicles that carry the itching
sensation [3]. Neurons containing Npra detect
Nppb from sensory neurons and then continue to
transmit the itching sensation to the brain. What is
important to note, however, is that Nppb and Npra
are specific to the itch sensation pathway, and thus
is a sensation distinct from pain.
How then, does our body relieve itself from this
sensation? The body’s natural response to an itch
is the urge to scratch. Scratching produces a mild
form of pain that overrides the transmission of the
sensation of itching into the spinal cord [4], resulting
in a temporary pleasurable feeling that relieves
the itch. What is not desired, however, is when the
brain releases a pain-controlling neurotransmitter
called serotonin in response to the pain generated
by scratching. While serotonin acts as a pain
suppressor, it can also affect itch-sensing neurons by
increasing the intensity of the itch sensation [5]. This
is why scratching sometimes simply makes things
worse and leads to a vicious itch-scratch cycle.
Having discovered the mechanisms involved in
itching, scientists are now looking for a solution to
relieve this nagging sensation. The challenge lies in
the fact that Nppb is also responsible for regulating
blood pressure, while serotonin is involved in other
bodily processes. Therefore, simply suppressing
either of these chemicals would not be a practical
option. On the bright side, there are other factors
that govern the sensation of itching which have yet
to be discovered, and scientists are investigating
whether another type of neuron exist, which may
be part of the reason why most itches go away
after scratching [6]. Although this is still underway,
it is no doubt an interesting problem to think about
the next time you feel an itch!
Itching to Know:
Why Do We Itch?
我們為什麼會痕癢?
W
inning article of the
Science Focus
Article Submission Competition.
「科言」
徵文比賽得獎文章。
By Lee Lok Sze
李樂思
S5, Diocesan Girls’ School
拔萃女書院
