Welcome back to Medical Neuroscience. In this tutorial, I'd like to talk to you about the neuromechanisms for mechanosensation. This topic relates to really half of the core concepts in the field of neuroscience, which gives you a sense of really what the breadth of this topic is and how it might apply. So I would suggest that, once again, as we understand how our sensory systems work, And certainly mechanosensation is a great example of how our sensory systems work. We are again confronted with the complexity of the brain in ways that make it really different from studying any other organ within the human body. We are going to be learning about circuits in the brain. Circuits that connect, different parts of the brain, and many of them are based upon the genetic instructions that, are unfolding in the course of early brain development. So, there are genetically determined circuits that are the foundations for the nervous system. And our sensory systems, certainly are an important part of that foundation. Now as we'll see in later sessions, that's not to say that they're not subject to use dependent plasticity. They certainly are. But the foundation of those circuits is largely based on genetic information. Well, because of such circuits in the brain our brain provides us with this, just fantastic capacity for curiosity. And I hope that we exercise that capacity well as we approach our studies of our sensory and motor systems over the next several tutorial sessions. And then lastly we'll increasingly apply our knowledge of sensory and motor systems to gain some insight into the diseased or the damaged nervous system. So I hope you'll see how fundamental knowledge, fundamental discovery about the brain can be used to help people who have been injured or have suffered affliction in their central nervous system. Well, we have several learning objectives today. So I want you to be able to identify and to characterize the major sensory endings that mediate sensations that are elicited by touch, by vibration, by proprioception, these are all elements of mechanosensation. And I will introduce the receptors for pain and temperature, although we will focus on those in a subsequent tutorial. I want you to be able to understand the concept of somatotopy. That is of how the body is mapped in the structure of the nervous system and so we'll confront that. In the primary somatic sensory cortex in this tutorial. And I want you to be able to discuss what happens to mechanosensory information as it enters cortical networks. We will spend a fair amount of our attention focused on the organization of the post central gyrus, but I want you to have a sense of where does that information go from there? So that will come up at the end of the tutorial today. Okay, well let's get started by considering just some broad organizational concepts as they will play out in the mechanosensory systems. Beginning with this notion of there being, multiple parallel pathways that mediate different aspects of somatic sensation. So we're going to focus today on our mechanosensory pathways, and even within our mechanosensory pathways we'll see this principle of parallel organization played out. We'll leave for another session, as I mentioned just a moment ago. Consideration of our pain and temperature systems. Okay, well speaking broadly about these two subsystems within our somatic sensory nervous system. Let's consider some of their basic functions. On the mechanosensory side, one important basic function is that the mechanosensory system helps us to identify the shape. Of objects that we encounter as well as the texture of surfaces that we might experience with our skin. Especially the very sensitive surfaces on our hands and in our face so, one very important means we have to explore the environment will be with our somatic sensory sensitive surfaces. A very important role played by our pain and temperature systems, is that these systems help to inform us about potentially threatening circumstances that our bodies might encounter in the environment. So, well, some of us, might wish that pain would go away at times, pain actually, is very adaptive. It informs us about the potential threat that might be done to the body. Or the occurrence of injury, and the potential for further exacerbation of that injury. So, these pain pathways are very important for moderating our behavior with respect to the world around us. Another important aspect of mechanosensation is that it helps us to monitor the forces that are acting upon the body. Some of those are internal, they reflect the activities of the body, itself largely, the activities of our musculoskeletal system. But we also monitor the forces acting on the outside of the body. Especially in certain activity, such as sport, where we are encountering physical objects, and it's of course critically important for success in those activities that we be able to monitor those interactions and respond accordingly. And then lastly I would suggest that all of these somatic sensory systems, our mechanosensory systems and even our pain and temperature system help to build up in our brain a image of our body, a sense of self, a self awareness. We call this proprioception. And this is tremendously important and it's something that we often take for granted. Somewhere within our brain we have an image of our body, and in one hemisphere we have an image of the opposite side of the body. And somehow between the two hemispheres, we integrate this into a united body schema. Well, as you might expect, that body schema is built on a steady stream of information flowing through our schematic sensory systems. And that stream is then met with information coming from other modalities of sensation, such as vision. Audition. I would suggest even our chemical senses help to inform us about our body and the world around us. So it's the confluence of this information integrated at the level of the cerebral cortex with loops through deeper structures in the brain that give rise to this integrated sense of self. So we will begin a conversation about what that means today when we consider our mechanosensory pathways. Now let me just consider a couple of points about the basic organization of our, somatic sensory systems, beginning with this notion of labeled lines. I've talked about this in a different tutorial, so I won't repeat all of what I said there, but let me just remind you that the pathways for somatic sensation, as with our other sensory systems, are arranged by having a series of parallel channels. That reflect specializations that began right at the level of the receptor endings of the primary axons out there in the body, wherever they may be. In the skin surface, or perhaps in deeper structures such as muscles and joints. We call this a labeled line system. Because the channel is labeled with respect to the kind of information that it carries. And that will label will really begins with the structure and the physiology of the receptor that's at the end of the primary axon. So, so we'll see that as we go through this. So keep that thought in mind. About labeled lines as one organizational principle in sensory systems. Another basic feature of the organization of the somatic sensory systems is that we have multiple pathways that are spatially segregated within the central nervous system. So if there are labeled lines, you might wonder, well where are those lines as they run through the CNS. And that's the question we're going to address as we go through this discussion of our somatic sensory systems, including the anatomy of those systems. And what we will discover is that they are two pairs of pathways that serve somatic sensation. Really they can be further subdivided, but we'll keep it pretty broad and simple and say there are two pairs of pathways. There's a pair of pathways for the body, including the back of the head and on down below the head, and then there are a pair of pathways for the face. As you might imagine, the face is a very important part of our body with respect to somatic sensation. And for reasons that aren't entirely clear there is a dedication of considerable brain space to the processing of signals that arise from the face in a manner that's similar. To the dedication of brain space that is involved with processing signals that are derived from our hands. So we'll provide a partial explanation for why that might be so as we go along today. Okay, so we won't go into the detailed anatomy of these pathways in this tutorial, we will reserve that to dedicated tutorials that will follow. Okay now as we begin to consider the organization of these systems, let's focus in on the first order neuron and our attention will be turned primarily to the mechanosensory systems but as we'll see, for completeness. I also want you to see some of the differences between these labeled lines from mechanosensation and for pain and temperature. Alright so let's consider the general organization of this first order neuron so, the first order neuron in our pathway is for somatic sensation or dorsal root ganglion cells and so what's illustrated here. Is the connection between the sensory signals that arise in the fingertip and the central nervous system. So these dorsal root ganglion cells. They have cell bodies that, reside in this dorsal root ganglion. So this is a home for cell bodies associated with the dorsal root that attaches to the spinal cord. And these cells have a very distinctive histological morphology. These cells are pseudomonopolar. And that means that there's a cell body. It grows out an axon that then bifurcates and grows in two directions. One direction grows out to the body, and the other grows out to the spinal cord. So that's what we mean by a pseudomonopolar neuron. That's what we find in the dorsal root ganglia. So the peripheral process of this cell grows out to reach the surface for which it is going to serve. And in the case of a mechanosensory fiber which is what's illustrated here in red, there may be some kind of specialized receptor ending that we'll see in just a moment that will begin to label that line as being a mechanosensory axon. Well, we also show a different cell here, one illustrated in blue. That is going to be sensitive to pain and temperature signals. And we'll talk more about this pathway in a later session. What we'll find is that there's a different kind of receptor ending there and a different set of proteins expressed on that ending that label that line as being sensitive to pain and temperature. Well that's the peripheral process. What about the central process? One hint at where we're going to go with our studies of the anatomy here is that the central process of these two different kinds of dorsal root ganglia do very different things as they enter the dorsal root entry zone of the spinal cord. The mechanosensory axon enters that dorsal root and notice that it makes a sharp upward bend. In truth, there are other branches that do other things but we're going to ignore those for the time being. Rather, I want to emphasize to you that there is a growth pattern. For the mechanosensory fiber that's really distinct this axon enters the dorsal column of the spinal cord the white matter that we call the dorsal column. Now we contrast that with the pain and temperature fiber, its central process enters the dorsal horn and makes a synaptic connection right away. So here we have a synapse with a dorsal horn neuron. And that dorsal horn neuron then grows out an axon that does something pretty interesting that we'll talk about later. Okay, so one clear difference between these two labeled lines is that the mechanosensory fiber grows along the course of the spinal cord. Whereas the pain and temperature fiber makes a synaptic connection upon entrance into the spinal cord at the level of entrance, Okay. So, keep that thought in mind and we'll see how that plays when we talk in detail about the pathways. Okay, a little bit more about these first order neurons. These first order neurons can be organized in various ways and this table from our reading from chapter nine gives you probably more information then you need to know, but at least it makes clear a few basic principles. And that is that there are different. Morphological categories of first order axons that relate to their functional properties. For example, you'll notice that the receptors that are going to be sensitive to the movements of our body, we call those proprioceptors. One example would be a muscle spindle. These receptors are supplied by really large, well...well myelinated axons. There's a name for them, they're called Group 1A, Group 2 axons. Now, contrast that to the receptor endings that are going to be sensitive to pain and temperature. Those who'd be down here. So these have a different morphological ending. We'll see that in just a moment. They're called free nerve endings. And these free nerve endings are formed at the ends of our smallest axons, our smallest sensory axons that we have. In peripheral nerves. Some of which are very lightly myelinated, we call those A-delta, and others are barely myelinated or completely unmyelinated and those are called class C fibers. So group 1, group 2 fibers, these are very large fibers. Group C is our smallest and A delta is very close to that. There is a group of mechanosensory fibers that are sort of in the middle but they're much more like the group 1 fibers. These are sensitive to various aspects of mechanosensation in our skin surfaces. And we'll talk about these receptors in just a moment. These axons are also pretty hefty with lots of myelin around them. They're just not the fastest of our axons with respect to conduction velocity that we have. So we see how these fall out here on this chart.
