Are Those Your Lips or Feet? How Your Brain Rewires Itself After Amputation
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Are Those Your Lips or Feet? How Your Brain Rewires Itself After Amputation


Even though it looks like a random, bumpy
mess, your brain is incredibly organized. Your brain has its own version of google maps
for navigating the world around it — but just how flexible are these maps to changes? Every sound you can hear or touch you can
feel is because neurons in a specific part of your brain are firing. Our brains are organized “topographically”so
that we can keep track of where things are happening, … that means we can tell the
difference between a touch on the arm or a touch on the face. By observing people’s brains in different
imaging machines, neuroscientists have largely mapped out touch, and other, sensations in
the brain. They found touch signals from different parts
of the body get sent to different areas of the brain — and from that we’ve made somatotopic
maps. The cool thing about these maps is that they
maintain spatial relationships, meaning the receptors in my skin that sense touches to
this part of my hand, and this one, send connections to similarly neighbouring parts of the brain. You have these maps for each of your senses. In the case of touch, they’ve drawn out
what’s called a homunculus in the brain — basically there’s a tiny man living
inside your head. Okay there’s not… but the homunculus is
an easy way to see which parts of the primary sensory cortex, right here, sense different
parts of the body. What you’ll notice is size is not totally
proportional.. Bigger parts of the body don’t necessarily
get more sensory space in the brain… instead, regions with more sensitivity get more real
estate in the sensory cortex. And this could be why our lips and fingertips
are a lot more sensitive than our arms? They’re represented as larger in the homunculus,
meaning there are more neurons paying close attention to sensations coming from those
regions. Diagrams like these make it seem like the
map is inflexible … but that might not always be true. Let’s say you lose a hand — those sensory
neurons don’t just go dark… Researchers hypothesize that when a limb is
lost, new connections may sprout into underused brain areas, or perhaps weaker connections
that were already there can now take over. When looking at nonhuman primates with amputated
limbs, researchers have seen touches to the lips light up regions of the brain corresponding
to where the limb was… which makes sense if they’re using their mouths to do tasks
they would’ve done with their hands before. There are similar observations in adult humans
too, though some argue it might depend on what the patient uses to compensate for the
lost limbs. But a case study added some surprising new
elements to this story. A young boy with both his hands amputated
received a double hand transplant. Shortly after his transplant, he didn’t
have any touch sensation in his new fingertips. But, when they put him in an MEG machine,
they saw that touches to his lips lit up the parts of his brain that used to be for his
hands. Then tests seven and a half months after the
transplant, touches to the lip activated only the lip area of the sensory cortex, and touches
to the fingertips produced unusually strong responses in the finger area of the cortex. Does this mean we can regrow and rewire massive
parts of our brains within less than a year? While it’s tempting to interpret things
this way, there’s a big debate in the field right now… and there’s no solid answer
yet. Patients with amputated limbs often report
feeling their lost or “phantom” limbs, sometimes painfully. This probably means the sensory neurons for
the lost limbs aren’t completely gone. So perhaps in the case of this boy who received
the transplant, he didn’t fully regrow new connections, but other connections were briefly
allowed to take over. While it’ll be helpful and interesting(!)
to know what’s really going on here, the good news is that regardless, follow-ups a
year and a half after the transplant, showed the boy can write, dress, and feed himself
more independently than before. There are more neurons in your brain than
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about head transplants? Crystal tackles that heady topic — here. One last thing: Have you done the two-point
test before? Test out your sensory homunculus by having
a friend poke you with one or two sharp pencils on the palm of your hand and on your upper
arm, and see how accurately you can guess if they used one or two points on either surface,
depending how close and far apart they are. Thanks for watching!

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