YOUR SENSE OF THE WORLD AND MOVEMENT WITHIN ITpain and itch. Once these receptors are activated, the signal is passed
to the spinal cord. From here, the message can either zip straight to a
motor neuron that originates in the spinal cord, thus inducing a rapid
reflex response (such as when we withdraw a hand from a flame), or the
message can be routed up to the sensory centres in the brain for further
analysis, and ultimately for the conscious sensation of touch.
Inside our brains there is a kind of mini-version of ourselves – a
homunculus – where each part of the body is represented in neural
tissue. This homunculus, which forms a horizontal strip across part of
the parietal cortex, isn’t to scale, but rather, the more sensitive a part of
the body, then the more tissue is given over to representing it. A clay
model of the sensory homunculus would therefore be a hideous-looking
figure with huge hands, lips and genitals. What’s more, the sensory
homunculus in the brain isn’t actually arranged as our physical bodies
are arranged. For example, the brain tissue representing your feet is
nestled next to the cells that represent your genitals!
There’s a simple way to measure the sensitivity of a given part of your
skin, called the two-point discrimination test. Look away while a friend
places two drawing pins onto the palm of your hand. As they bring the
two points closer together there will come a distance beyond which it
feels to you as if they are using just one pin. Now do the same thing
but with the points applied to your back, and you should find that the
merging sensation happens much earlier, when the pins are still further
apart. That’s because fewer brain cells are given over to representing the
skin on your back compared with the skin on your palms.
Movement
All this sensory information evolved to serve one key purpose – to
enable us to move appropriately so that we could hunt down food, flee
from danger and chase potential mates. But having the right sensory
information is just the beginning. Moving smoothly and accurately is
an engineering task of epic proportions. First, substantial transmission
delays must be overcome. What you see now happened a moment ago,
and there will be a further delay before a command to move reaches
your limbs. This is overcome via endless prediction and anticipation,
using cues to guess where things will be by the time you get round to
responding. Second, how much force is applied to a muscle depends
on the current length of that muscle, which is controlled by the rele-
vant limb’s position. So your central nervous system – your brain and