[MUSIC] Okay, let's try and understand how the cerebellum does what it does. It gets 40 times more information than it sends out. So there's 40 times more input than there is output. Is, if we count numbers of axons. So let's think about, what are the types of inputs that the cerebellum gets? It gets two basic types of inputs. One is what we call, let's call it efference copy. So. We have to make sure that everyone understands the difference between an afferent and an efferent. Only one letter. But afferents come in, efferents go out. So efference copy, what that means is, as you, as you, as your brain initiates a movement. That command gets sent to motor neurons, or to motor, inner neurons and then out to the muscles. But a copy of that command also gets sent to the cerebellum, and that's an efference copy. But the cerebellum is very interested in what you intent to do. And it, it wants to make sure that the game of telephone worked. So not only do, does the cerebellum get efference copy from the cerebral cortex. So, primary motor cortex which is going to go down to this spinal cord and tell the motor neurons how to move the muscles. It's going to send a copy of its message to the cerebellum. In addition, the motor neurons themselves, actually send a copy of the message that they received, back to the cerebellum. So, we make sure that the message that the, that the motor neuron received and the message that the, the cerebral cortex sent, both of those messages are sent to the, to the cerebellum. So this is a, an example of efference copy. I, I for some reason I always think of, airplanes and, wing flaps. So, the wing flaps have to go down. Somebody, the pilot presses the button, and the wing flaps go up or down. And then there is a sensor out there to make sure that the message was received by the wing flap mover. And so that is too different versions of ef, of efference copy. Now in addition, that's, that gets compared, the critical thing that the cerebellum does is to compare that efferernce copy with sensory reafference. And what I mean by sensory reafference, I mean the afferent stuff that comes in. Not, not initially, but as a result of an action that we have done. So, this is reafference. So, another words, If I move my arm. The reafference is, that this joint is now extended. That this muscle is extended, this muscle is contracted. That's reafference. It is the input that results from the movement that I made. So in the, in the, the airplane analogy, sensory reafference would be a little sensor that says indeed, not only did the the, do hickey that moves the wing flap receive the message. But in fact, the, the wing flap, wing flap is up. So, you're getting the information back, the sensory information back. Now, what's the point? Well, the point is that the sensory reafference and the efference copy, they should all match. We should make the movements that we intend to make. The intended movement, the actual movement should match the intended movement. When it. When they actual movement does not match the intended movement, the cerebellum makes a course correction. And that is, that is how it produces learning. And the type of learning that the cerebellum does is called associative learning. So, essentially, what it's doing, you can think about it like this. The cerebellum is trying to associate a sensory picture with a part of a movement. As I go to serve a tennis ball, it should feel like this at this point. And then, as I move my hand forward, it should feel like this. And then it should feel like this. And then it should feel like this. So I should anticipate what the right sensory feedback should be, if I'm doing the movement correctly. And in, you can think about it in terms of, if you, if you make an incorrect movement. Let's say I botch a serve, I usually know the moment where it didn't feel right. And why didn't if feel right? Not because I didn't do it right but because the sensory feedback feels wrong, it felt wrong at a given instant. And that's because it, it we've, we've deviated from the association between the correct sensory feedback and the, and the intended movement. So it, it's, this marries the idea that the cerebellum is all about motor control but it uses sensory information to produce a smooth and coordinated motor control. In the next couple of segments we're going to look at two different examples of cerebellar learning
