How the brain rewires itself to recover after a stroke

HostIf you have ever seen someone try to walk or speak after a stroke, it can look like they're learning everything from the ground up. It's a massive thing to overcome because a part of the brain has actually died, but then, slowly, bits of life start to come back. I have always wondered how that works on the inside. How does the brain go about fixing a hole in itself?

GuestWell, the funny thing is that the brain doesn't really fix the hole. Once those brain cells are gone, they're gone. But the brain is incredibly good at finding a way around the damage. Think of it like a map. When a stroke happens, it's like a huge rockslide has blocked off a main road. At first, everything stops. The whole area is in shock. But pretty soon, the brain starts looking for side streets and dirt paths to get the traffic moving again. It starts to rewire itself by using the healthy parts that are still standing.

HostSo it's just about finding a new path? I always thought it was more like the brain was growing new cells to replace the old ones.

GuestThat would be nice, but we don't do much of that as adults. Most of the heavy lifting comes from what we call the stunned area. Right after a stroke, there's a big circle of cells around the dead spot that aren't actually dead, they're just... sleeping. They have lost their power supply and they're shut down. For the first few weeks, a lot of the progress we see is just those cells waking back up as the swelling goes down. They're the lucky neighbors who survived the blast, and once they get their feet under them, they can start to help out.

HostBut waking up the neighbors can only do so much. If the part of my brain that moves my right hand is gone, those neighbors were already busy doing other things. They have their own jobs.

GuestThat's the really cool part. The brain isn't as rigid as we think. If the "hand" spot is gone, the brain starts looking at the cells next to it—maybe the ones that move the arm or the face. It starts asking those cells to pick up the slack. It's like a coworker leaving and you having to learn their job on top of your own. The cells literally start growing new little branches, like new power lines, to connect to the muscles. It's messy and it takes a lot of energy, but the brain can actually change what a specific area is responsible for.

HostWait, if the brain is so good at this, why is rehab such a slog? If I can just regrow some power lines, why does it take months of doing the same tiny movements over and over? It feels like it should be faster.

GuestThe brain is actually a bit lazy. It likes to save energy. If it's hard to move your right hand, your brain will just tell you to use your left hand for everything. If you do that, the brain thinks, well, I guess we don't need those right-hand paths anymore, and it stops trying to build them. That's why rehab is so intense. You have to force the brain to use the broken path. You have to do the same motion thousands of times to prove to your brain that this path is worth the work. You're basically carving a new road by walking over it until the grass stays down.

HostI have heard of people using mirrors to do this too. That sounds like a bit of a mind game. Does that actually help the wiring?

GuestIt's a total mind game, but it works. You put a mirror on a table so it hides your weak hand, and you look at the reflection of your strong hand. When you move the strong hand, your eyes see the reflection and tell your brain, look, the weak hand is moving perfectly! Your brain sees that visual lie and it gets excited. It sends a spark to the damaged side that says, hey, we can still do this. That little bit of trickery can be the jumpstart the brain needs to start building those new connections.

HostThere has to be a limit, though. You can't just rewire everything. If a huge part of the brain is gone, you're not getting that back no matter how many mirrors you use.

GuestNo, you're right. There's a limit. If the damage is too big, there just aren't enough neighbors left to help. And those new paths are never as good as the original ones. Think of that highway again. You can take the back roads, but they're bumpy and slow. You'll get to the store, but it'll take you twice as long and you'll be exhausted when you get there. That's why people who have had a stroke get tired so fast. Their brains are working ten times harder just to do a simple task because they're using a bunch of small, twisty side roads instead of the main highway.

HostIt's a lot of work for the brain to keep redrawing itself.

GuestThe biggest mystery we're still trying to solve is why some people can draw those new maps so much faster than others, even when the damage looks the same on a scan.

HostThe brain is already trying to draw a new way home before the person even leaves the hospital bed.