[MUSIC] As you know, skeletal muscles cannot move on their own, they need motor neurons and now we've just looked at the fact that muscle fibers, skeletal muscle fibers differ. Some are dependent on oxygen, some are dependent on glycogen. Some can stay contracted for a long time and some for a very short time. Well, those different types of muscle fibers, the slow, the fast that are dependent on oxygen, I'm sorry, on glycogen alone, are innervated by different types of motor neurons and there's an intermediate type of muscle fiber that is dependent on oxygen, until the oxygen runs out and then it's dependent on glycogen. And whereas, the slow muscle fibers can last for hours and hours and hours and the fast glycogen dependent ones can last for, say, three to five minutes, the fast glycogen, oxygen and glycogen muscle fibers, can last for on the order of 30 minutes or so. Okay, so each type of muscle fiber is innervated by a different type of motor neuron and the motor neurons get the names of slow, innervating is slow, fast fatigue resistant, innervating these muscle fibers that can use either oxygen or glycogen, and fast fatigable, that can innervate these, that innervate only these fast muscle fibers that are dependent on glycogen. So every muscle fiber only gets input from one motor neuron and every motor neuron innervates some number of muscle fibers and, in general, the number of muscle fibers innervated by a motor neuron is low for the slow fibers, slow motor neurons, higher for the fast fatigue resistant motor neurons and highest for the fast fatigables. So when you're standing all day, you're using these. When you go and you lift weights, you're using these. In a disease such as polio, what you're losing is you're losing a motor neuron and all of the muscle fibers that it innervates. So in the case of, the name of this motor neuron, plus all the muscle fibers that it innervates is called the motor unit and as a motor neuron dies, which is what happens in polio, the motor neuron dies and now all of these muscle fibers are not innervated. They, they're offline and so there's fewer motor neurons to do the same amount of work and this is actually, one of the reasons why people think that there's a post polio syndrome. When people that have survived polio get to be 50 or 60, their motor neurons start to give out and the thought is that perhaps, now they've got say, 30% of the motor neurons that they've started with are doing the job of 100% and at age 60 or 70, they're tired and now they're going on the fritz and so these people are starting to have problems with the same muscles that they originally lost motor neurons in. What we're going to look at in the next segment is how we are going to make a smooth movement recruiting these different types of motor neurons. [MUSIC]
