[MUSIC] So now we're going to talk about reflexes, and we're going to spend some time on a very basic reflex, the stretch reflex. This is the reflex that the physician does in the office with tapping your, your, your knee there. There's a lot of complexity to this. This is actually, if you understand this, if I can explain this to you, then you have understand, probably, understood that probably the most complicated circuit that you could possibly construct. So before we get to the reflex we have to understand one thing about muscles, because a reflex means that there is a sensory input [SOUND] and a motor output. And the sensory input for the monosynaptic stretch reflex comes from the muscle itself. So, if this muscle, this muscle has all these muscle fibers, and when they contract, the muscle shortens. And these muscle fibers, are big and they produce force. But sitting in and amongst these muscle fibers, are little packages called muscle spindles, and in these muscle spindles, if we look at one of these muscle spindles, we see that it's made up of, about five to ten very thin muscle fibers. These muscle fibers, if, if we cut across like this, that muscle fiber is that thin, and this muscle fiber is that thin. Is that, so this is the relative diameter of these two muscle fibers. When this muscle fiber, which is what we traditionally think of as a muscle fiber, when that contracts, there is force generated. When this muscle fiber contracts, nothing happens. In fact, this muscle fiber is serving a sensory role. So, if we look at this so, let's say that we stretched this muscle. This muscle fiber would be stretched, and we're going to name these two muscle fibers, just for ease here, the small ones that don't produce force are called intrafusal, and the larger ones that produce force are called extrafusal. So, the monosynaptic reflex is pretty simple. What happens is, there is a sensory neuron that wraps itself around the interfusal fiber and it is, it's activated whenever the interfusal fiber is, is stretched. And so, this is activated. It's a primary afferent that has a cell body and a dorsal root ganglion. Here's the spinal cord. It comes in and it goes directly to a motor neuron. And this motor neuron comes out and innervates a muscle fiber in the same muscle. So, when this muscles stretched, this, this motor neuron is activated to contract. So you stretch it, you contract it. So it's a corrective mechanism. It's a feedback. You stretch, the reaction is to, to contract it. Now we name this, muscle this reflex, the monosynaptic reflex, because there's one synapse, and what we mean by one synapse is we mean that there's one synapse that's in the central nervous system. We don't count this synapse. So there's one synapse. This is a really fast reflex, it happens in a matter of a few milliseconds. And it is also called the stretch reflex, because the thing that starts it is a stretch of the muscle. These intrafusal fibers, these muscle spindles, are distributed throughout the the muscle. And so any time the muscle is stretched, either because somebody extends your arm that you weren't aware of, because you trip for any type of reason you pick up something and it has liquid in it, and you didn't know it has liquid, it's a little bit heavier than you thought. Whatever load, whatever weight you put on this muscle is going to evoke this monosynaptic reflex. Now, in the next segment, what we're going to look at is, how does the monosynaptic reflex work if we're actively contracting our muscles? [MUSIC]
