This figure shows us Nissl-stained section through the lumbar level of the human spinal cord. And if you've not yet taken the time to see my overview of the internal anatomy of the spinal cord, this might be a good time to pause and have a look at that particular tutorial. I think that will help you out, with some aspects of where we're going to go over the next few minutes. if you have seen that, great. Let's go ahead and move on. So I won't go through the detail of the organization of the gray matter and the white matter of the spinal cord. Rather I'll draw your attention to the ventral horn, and to a set of columns of neurons that we call laminae seven through nine. And that's these columns that contain our lower motor neurons, and even at this lower magnification, I think you can see that in this region of the ventral horn, we have these large neurons that are present. And, of course, I'd like to show you those in more detail. And so, just to give you a higher magnification view of what they actually look like. here we see some lower motor neurons, and they have a very characteristic morphology to them. They're, first of all, are very large cells. They have, sort of, a multi-polar shape, this is a Nissl stain, so we're not seeing all of the dendrites that are extending off in various directions. but what we see is a cytoplasm that's just full of darkly staining Nissl substance. Which is indictive of rough endoplasmic reticulum in the production of proteins. We see quite a large nucleus and a very prominent, and single nucleolus near the center of that nucleus. So this is the characteristic appearance of a typical motor neuron. Here we have another one. The plane of focus is just a little bit off the nucleolus but I think you can see that, nuclear membrane there and then the nucleolus here in the center. Well that's what the motor neurons actually look like. how are they distributed in the ventral horn, is the topic that I'd like to spend just a few minutes discussing with you next. Now you already know that along the long axis of the spinal cord, there are two regions in which the diameter of the spinal chord is significantly enlarged. There is the cervical enlargement and the lumbosacral enlargement. And these enlargements reflect the additional circuitry and the additional numbers of neurons, that are in those segments of the spinal chord that serve the upper extremities and the lower extremities. Well that's what's going on along the length of the spinal cord as we look at the gross organization of the structure. If we look at a finer scale, what we see are that within the cervical enlargement, or within the lumbosacral enlargement. The neurons that supply a given muscle are also extended over some distance across multiple segments of the spinal cord. And we see in this figure the result of an experiment that demonstrates this point. So what was done in this experiment is that the gastrocnemius muscle in an animal model was injected with a dye substance. That is taken up at the neuromuscular junction by the axons of the alpha motor neurons that innervate that muscle. And then this dye is transported back to their cell bodies. And then the experimenters can look into the spinal chord in histological sections, and plot the location of the cells that were labeled by injecting the specific muscle. Well, the same kind of experiment was also done for the soleus muscle on the opposite side of the body. And as a result, what we see, is a column of cells on one side of the spinal cord that reflects the distribution of motor neurons that supply the gastrocnemius. And on the other side of the spinal cord, are the labeled motor neurons that innervate the soleus muscle. And the point here is that for each muscle. There is column of cells. Perhaps not unlike this writing device that I'm holding. That is distributed across multiple segments of the spinal cord. And when we look at their positions, we find that they're not overlapping necessarily. There may be some order to their distribution in the medial to lateral aspect of the ventral horn. In fact, this is a very important principle of ventral horn organization that I want to stress for you. If we look at the arrangement of these columns of motor neurons that innervate specific muscles. What we see is a beautiful and, and hopefully memorable, somatopic organization. Again, I'll use this word somatotopy that we talked about when we considered the somatic sensory system, to refer to the mapping of the body. So we see a kind of somatotopy now in the organization of our lower motor neurons, in the ventral horn of the spinal chord. What we see are neurons that are present in the medial part of the ventral horn, are supplying muscles that are found in the more proximal portions of the extremities. Or perhaps in the trunk muscles themselves, if we're talking about the thoracic spinal cord. Whereas as we get to progressively more lateral positions in the vental horn, we move from proximal to distal, such that the muscles that govern the movements of the distal extremities. Which is where so much of the skill is expressed especially via the activities of our arms and hands. These motor neurons are found out here in lateral part of the ventral horn. So it's very important to recognize this menial to lateral somatotopic organization in the ventral horn. Because that will provide an important framework for understanding the descending projections from upper motor neurons. We'll recognize that the descending projections that reside in this ventral, or interior white matter of the spinal chord, are mainly supplying the medial aspects of the ventral horn. Whereas the major fiber bundle that sits out there in the lateral column of the spinal cord is the lateral cortical spinal tract. And this system is mainly concerned with supplying the lateral aspects of the ventrical horn. So getting back to our general orginization framework then, I hope you can recognize that what's going on in this ventral and anterior white matter. Is mostly concerned with setting the stage for skill behavior in adjusting posture, setting gain, and otherwise facilitating what we do with our distal extremities. And what's happening out here in the lateral part of the spinal cord is mainly an expression of skilled behavior. Now I won't spend much time right now reviewing for you again the organiziton of the motor elements of our cranial nerve nuclei. Rather, I'll just refer you back to the tutorial regarding the internal organization of the brain stem, and the organization of the cranial nerves and their nuclei. And there, what we talked about was the outflow from somatic motor and branchial motor nuclei. That are concerned with moving muscles derived from somites or from the pharyngeal arches, respectively. And these motor nuclei are found in each of the three major divisions of our brain stem. In the mid brain, we have the oculomotor nucleus and the trochlear nucleus, And then in the pons we have the abducens nucleus. These three semantic motor nuclei govern the movements of the eyes in the orbit. And then down in the upper part in the medulla, we have the hypoglossal nucleus, that is concerned with the movements of the tongue. And with respect to governing those muscles derived from the pharyngeal arches in the pons, we have the trigeminal motor nucleus that innervates the muscles of chewing. We have the facial motor nucleus, that supplies the muscles of the face that are involved in facial expressions of emotion. And then, further down in the brainstem, in the medulla, and in the upper cervical segments of the spinal cord, we find first the nucleus ambiguous, which is governing the muscles of the pharynx and the larynx. And, in the upper cervical cord, the spinal accessory nucleus, which is interveining the pharyngeal arch derived muscles that turn the head and shrug the shoulders. Well, I do want to at least show you what these motor neurons look like, just to illustrate the point that, in the cranial nerve motor nuclei. We have alpha motor neurons, very much like the motor neurons, that we find in the ventral horn of the spinal cord. This is a detailed look at one of our branchiomeric motor nuclei, the trigeminal motor nucleus. And here is roughly the boundaries of this nucleus, and so this is where we would find our columns of cells that are providing innervation to the muscles of mastication. Those muscles that we use when we chew and consume food. And what we see are beautiful demonstrations of alpha motor neurons that have all the same histological features as what we saw in the ventral horn of the spinal cord. And lastly what I'd like to do in this tutorial is to talk about the motor unit. And describe for you just some of the physiological features of musculo-skeletal behavior, that we want to keep in mind as we move into a deeper discussion of how the nervous system controls movement. So, the alpha-motor neuron at the spinal chord is what we've been talking about and here's an illustration of an alpha-motor neuron, sitting in the ventral horn of the spinal chord. And, it sends it's axons out through the ventral root into a spinal nerve and then as that spinal nerve supplies a muscle, this axon will divide into several branches. And these branches will then go on to supply innervation to a set of muscle fibers within a muscle. And these muscles fibers tend to be broadly distributed within that muscle. So the motor unit is defined as the single alpha motor neuron plus the muscle fibers that it intervates. That's the motor unit. Now a single motor unit can be classified not just atomically, but also physiologically. There is coordination between the neuron and the muscle fibers for the functions that that motor unit must perform. So from a physiological perspective there are biochemical and even neuro specializations. That allow these three types of motor units to contribute to physiological activities of entire muscles in distinct ways. First we have a type of motor unit that we classify as being slow. This is a motor unit that doesn't generate a whole lot of force, and it does so fairly slowly. so what we're looking at is the response to a single action potential, that is applied to the axon that innervates a particular type, a particular set of muscle fibers. Now if that action potential arrives at a different collection of muscle fibers, we might see a more significant amount of force generated, and it might happen more quickly. this we call the fast fatigue resistant motor unit. And then, if we apply that action potential to yet a third motor axon. We might find a much larger amount of force generated very rapidly, by the contraction of a muscle fiber that happens to be innervated by that axon. If, so, that kind of physiological profile, we would know that we're dealing with a fast fatigable motor unit. Now, if we applied a series of pulses, not just an individual shock but a train of pulses we might find something that looks quite similar, except now we would see evidence of sustained response. And so with the activation of the slow motor units, notice how each individual contraction takes some time. And we build up our steady state level of forced production, and can substain that over the duration of the brief train of pulses that is applied in this experiment. The fast fatigue resistant motor unit does considerably better with force production as can be seen, and notice that it takes, much less time to generate that force. And I want you to notice what happens when we get to our fast fatigable motor unit. Of course, this is going to give us the, greatest amount of force generated, with activation of the axon that supplies this set of muscle fibers, and the force that's generated happens very rapidly. But notice that even over the duration of a second or so of stimulation, there is some decrement. And the amount of force that can be sustained over time. And so, it's for this reason, that we talk about the fatigue profile of these motor units. So that of course, can be seen if we extend our stimulation, not just over a second or so, but over many minutes. And what we find is that our slow motor units, are those that can maintain their maximum amount of force production for a very long period of time. Not indefinitely of course, but for a long period of time. So, again, the amount of force that's generated is small, and the speed at which that force is generated at a pulse-by-pulse basis is relatively slow. Our fast fatigue resistant motor units do considerably better. they do fatigue but there is a healthy amount of sustain to the amount of force generated over a long period of time, at least over several minutes. Now if we look an hour later, now we see evidence of the fatigue that has set in and the ability of these motor units to keep pace with the commands that are coming down the axon. Lastly, our fast fatigable motor units are those that generate the most force. And they do so very quickly. But they're really only good for a few minutes of activity. They fatigue quite rapidly. And after just a couple of minutes of activity they're just not able typically to sustain the commands that are arriving down their axons. Now let's put all this together in terms of behavior. these are data from an animal experiment looking at the transition from, from standing all the way up through a more explosive type of motor activity, jumping. And what we see is that, to maintain posture, we rely on our slow motor units, so that makes good sense, because speed is not an issue. typically the amount of force that's needed to maintain posture is relatively low. And we certainly want to activate motor units that are resistant to fatigue. And so, as we transition now from maintaining a steady posture while standing, to actual locomotion. Now we engage next our fast fatigue resistant motor units. This will allow us to generate more force, to do so at a moderate pace, that's necessary for a transition from standing to walking. And even progressing through an increase in pace of locomotion. But once we get into a, a full out run, or in this animal model a gallop. Then certainly as we approach the most explosive our, of our motor outputs, we have to engage our fast fatigueable motor units. But we do so only strategically. So this sequential progression from slow to fast, fatigue resistant to fast fatigable motor units is called the size principle. Because it's the principle through which we recruit motor units of different sizes. Now a point I probably should have made in the previous slide, one reason why our slow motor units generate a small amount of force is that. They tend to be the smallest of the motor units, associated with just a few number of muscle fibers. Now these muscle fibers do have biochemical specializations that also limit the amount of force that each fiber can generate. But the size of the motor unit tends to be small. Whereas the fast fatigable motor units, these are the largest motor units that we have. Recall again that when we talk about the size of the motor unit, we're talking about number of muscle fibers, that are innervated by a single alpha motor, alpha motor neuron. And in the case of our fatigable motor units there could be several fold more muscle fibers innervated than what we find in a typical slow motor unit. And those muscle fibers that are supplied by these fast fatigable motor units, are biochemically specialized to generate a maximal amount of force very, very quickly. So, this size principle provides an important advantage for coordinating the output of our motor units. It allows for a matching, of the physiological properties of different types of motor units, with the demands of the specific behavioral task. Now, that's not to say that this plan for coordinating the outflow from our lower motor neurons to the specific types of motor units is not subject to change, it is. Even these muscle fiber characteristics that allow a muscle to either resist fatigue or to be susceptible fatigue. Even those biochemical properties are subject to plasticity, as are the physiological properties of the motor neurons that they innervate. Now this is a matter for exercise physiology. And we won't go into much depth at all, but let me suffice to say that, it is possible to slow the physiological properties of our motor unit. This is one of the principal impacts of endurance training or aerobic exercise on the physiology of our motor units. Well, let me pause there and we'll take a break and, we'll come back with our second part. Where we will consider the organization of the circuits of the spinal chord, as they express reflux of activity.
