And what we can do is we can take these two neurons, and force them to associate. We can take the neuron on the left and tickle it with an electrical stimulus, zap! zap! zap!, we make it fire. And if we make it fire hard enough, we can get through the axon, we can activate the next neuron, the neuron on the right. Neurons that fire together wire together. So we go prrp! prrp! prrp! and after a time, what we find is if we make those two neurons associate that the connection between them will get stronger, and we’re understanding the mechanisms by which that works.
Now the way in which the two neurons connect to each other is right over here at a place called the synapse. And over the last decades, neuroscience has really understood the synapse in ways that were just not possible before. So the next slide gives you an illustration of what the synapse looks like. Those little blue dots on the top are the transmitters released by the axon, and then they activate all of these receptors, and all of that machinery in the next neuron, and ultimately that causes the neuron to fire.
But you know there is much more to it. It’s these receptors that are actually very important. You see this receptor? It’s called an AMP receptor. It’s kind of boring. If you put more in, more comes out. In other words, if you give it a weak stimulus, it gives a weak response, if you give it a stronger stimulus, it gives you a stronger response, if you give it a really strong stimulus, it gives you a really strong response called linear.
Look at this kind, the NR receptor, and NMDA receptor it’s also called. It’s very interesting. Very undemocratic receptor. It hates weak inputs: you give it a weak input not only does it not respond, but it actually goes negative. You give it a slightly stronger input; still not very interesting. You give a strong input; it goes crazy. And when it goes crazy, what it does is it activates all this machinery down here, and the effect of all that machinery is to put in more of these ordinary boring receptors.
So what that means is if you can tickle the fancy of this NMDA receptor, you’ll put in more of these ordinary AMPA receptors into the synapse, and then the synapse will become stronger. And that actually seems to be the core mechanism of memory, of strengthening connections between two neurons, of how strong inputs and contiguity can result in a stronger synapse. And that’s actually how we think you remember today’s lecture.
“So OK, Max. That’s all been great biochemistry. I’m all excited. Fine. Good. Well, what have you done for me lately? How’s my memory going to improve from all this?” OK. So I can tell you that scientists are working very hard. All of this understanding is leading to new strategies and therapies. If you actually look here, it turns out that if you block this, this is very important in getting this whole process to happen. We’re working on drugs that will tickle this pathway to give you a better memory. But we’re not there yet.
OK, it turns out that there’s a crucial structure in your brain that seems to be actually very important for your memory. It’s called the hippocampus. So they’re all these points of light on the outside of your cortex. They all funnel down to the hippocampus which again represents the memory trace in a compressed and higher form. We can now record the activity of hundreds of points in the hippocampus, hundreds of cells, as animals, for instance, run through a maze.
What we can do now is we can understand the functions of the hippocampus so well that we can actually, without knowing where the animal is, we can say: “OK, these are the cells that are active now — the animals of the first choice point — Now is at the second choice point. Now is at the third choice point.” And we can hear all this simply by recording the activity of all these neurons inside the hippocampus.
I want to tell you about an experiment that was done at MIT about ten years ago by Matt Wilson. He was studying the hippocampus as the rat was learning the maze, he was going through the first choice, blah, blah, blah. The experiment ends. He closes up the apparatus, the animal sitting in the vestibule of the maze now, not in the maze, and he starts to write up his lab notes. He’s still listening to all these neurons. What he finds is while he’s writing up the notes, he hears the neurons, you can hear them on loudspeaker. The animals running through the maze. How could that be?
Well, it turns out he goes over, he looks at the animal, the animal is asleep, but the hippocampus is still running through the maze while the animal is asleep. And there is now overwhelming evidence that what actually happens at night, every night, after you learned stuff during the day is that during sleep you replay and rebroadcast the memories of the day back out from your hippocampus to the rest of your cerebral cortex, rehearsing those memories again, strengthening the association among all those points of light.
So what’s my advice if you want to improve plasticity? Get a good night’s sleep. It’s very important.
Here’s another thing you can do if you want to improve your memory capabilities and your brain plasticity, and that is do physical exercise. Do physical exercise. It used to be thought that we already had all the brain cells we’re ever going to have; that’s not true. We’re actually making thousands of new brain cells every day, and you can double or triple the number of brain cells that you make next week by doing physical exercise.