explain
pain
section
4
page
72
Altered central nervous system alarms
– the spinal cord
L
et’s hop into the spinal cord beforewemove to the brain.
Youmay need to keep your hat onhere! Read slowly and
hang in there! Remember that sensors in the tissues cause
dangermessages to be sent to the spinal cord, which in turn
cause the release of chemicals into the synapse there (pages
36 and 37). Those chemicals activate chemical sensors on
the next neurone, (the spinal messenger neurone on theway
to the brain)which open and allow positively charged
particles to rush into that neurone, bringing it closer to
firing. Remember too that chemicals released from
descending neurones from the brain activate different
sensors on the neurone. This can increase or decrease the
excitement of the spinal messenger neurone and take it
closer or further away from firing. We are now talking about
the dorsal horn of the spinal cord. See the figure below to
remind yourself where that is located.
The essential neuroscience
115
The nervous system is highly adaptable andwill respond to
most demands that it is given. So, when impulses from
inflamed, scarred, weak or acidic tissues keep arriving at
the synapse in the dorsal horn, or when neurones from the
brain release excitatory chemicals, the spinal danger
messenger neurone in the spinal cord adapts tomeet the
demand – that is, to get better at sending dangermessages
up to the brain. This adaptation begins within seconds of
the demand increasing.
In the short term, the spinal dangermessenger neurone
increases its sensitivity to the incoming excitatory
chemicals.
116,117
Thismeans that things that used to hurt
now hurt more. This is called
‘hyperalgesia’
. It alsomeans
that things that didn’t hurt before now hurt. This is called
‘allodynia’
. Hyperalgesia and allodynia are just two effects
of increased sensitivity.
The sensors then change the way they work so that they
stay open longer each time they are opened, which lets
more positively charged particles into the dangermessenger
neurone. Finally, the dangermessenger neurone increases
itsmanufacture of sensors for excitatory chemicals,
including sensors that ‘sleep’ until they are needed (this is
as though a dangermemory is placed in the cells). All of
these things change the sensitivity of the dangermessenger
neurone. Your alarm system is really looking out for you.
DRG
dorsal horn
to brain
nerve
from the
tissues
slice of
cord
