Welcome back to our discussion of the mechanosensory systems which are part of our broader systems we have within our bodies for processing somatosensory information. All of this pertains to several foundational concepts in the field of neuroscience. Two in particular that I would highlight are, of course, our first one that should be very familiar to you now that the brain is the body's most complex organ. And also, that there are genetically determined circuits within the brain that are truly the foundation for the operations of the nervous system. And so we're going to focus, in this tutorial, on 1 such circuit. We'll call it a pathway. It's a pathway that allows mechanosensory signals to get from our surfaces where they're detected, such as our skin surfaces on the tips of our fingers, all the way up to the appropriate processing stations in the cerebral cortex. So, our learning objectives are for you to characterize and recognize, and you'll see these pairs of terms played out across several distinct mechanosensory pathways. First I'd like for you to be able to characterize the pathway known as, known as the dorsal-column medial lemniscal system. I'd like for you to be able to recognize components of this pathway when you actually confront them in images of the central nervous system. Be they views of the brain from the outside, histological sections through the brain or the spinal cord, or some other conceptualization of elements of these pathways. So here we're focusing on the dorsal-column medial lemniscal system which is long term but very descriptive. It tells you about one particular pathway. Another pathway is the corresponding mechanosensory pathway for the face, and this runs through the trigeminal nerve. And components of the trigeminal nuclear complex of the brain stem, so again I want you to be able to characterize the pathway and recognize the elements when we actually look inside the nervous system. And lastly I want to introduce you to a pathway that gets mechanosensory feedback about the movements of the body into the cerebellum. The cerebellum is a marvelous organ that facilitates agility. Be it agility of bodily movement or agility of thought and, and, we'll spend an entire set of tutorials thinking together about the cerebellum. For tutorial, I want to introduce the pathway. That gets information about movement from the spinal cord into the cerebellum. And as usual, I want you to be able to recognize components of that pathway, when you're confronted with them. And the recognition part is really key in this tutorial and it will be key for the rest of the course. Because I want you to begin to build up. A mental model of the nervous system and being able to recognize the components as they actually appear will be key for the success of your mental frame work or your mental model so that way you can consider what happens when elements of that model becomes damaged Or dysfunctional. And that will help you translate your foundational knowledge in medical neuroscience to the care and treatment of individuals with neurological injury and disease. Well let me introduce again the pathways and then we'll look at them one at a time. So for our conscious awareness of mechanosensory stimuli there are two pairs of pathways, one for the body below the back of the head including the back of the head. We call this the post-cranial body, "post" meaning posterior to the cranium, and there is a pathway that we call the dorsal column medial lemiscal system. It has elements of the dorsal columns of the spinal cord and the medial lemniscus, an important pathway in the brain stem. Now there's also a pathway that mediates pain and temperatures that we'll look at in a separate tutorial for the post cranial body. That pathway is called the anterolateral system. I include it here for completeness. And now for the face we have a corresponding pair of pathways. For mechanical, sensation, we have a pathway that runs through the division of the trigeminal complex of the brain stem, called the chief or principal sensory nucleus of the trigeminal complex, so that will be a pathway for Mechano-sensation. For pain and temperature, there's a pathway of the trigeminal nerve that runs through a different part of the trigeminal brain stem. A subdivision of that complex called the spinal trigeminal nucleus. Okay, well, let's begin by thinking together about this pathway for the mechanical stimulation for the post cranial body, called dorsal-column medial lemniscal system. Now, we have a view of the entire pathway taken from our companion textbook and this is probably really small on whatever size screen you're using to view this right now. So let me just give you the broad overview of this pathway and then we'll look at more magnified views of this figure so that you can see things more clearly. Well, I'll make a few points in general terms. This pathway from mechanosensation involves three neurons. As we go from skin to cortex. Okay? So I want you to focus on these three neurons. I want you to consider where are their cell bodies. I want you to consider the distribution of their axons. And thirdly I want you to be able to discuss and to illustrate the location of the synaptic connections from 1 axon to the dendrites of another neuron in the serial pathway. 'Kay so, so three things to focus on in this pathway and all the others we'll look at in the next few tutorials. That is the location of cell bodies, the distribution of the axons of those cell bodies, and then the locations of the synaptic junctions. So, let me just ask you, where would we find cell bodies? Hopefully your answer is, well somewhere in gray matter within the central nervous system or perhaps in a ganglion if we're talking about something outside the central nervous system. That's great if that was your answer. I might ask you where would we find axons? Well hopefully you would say in the central nervous system we'd find axons in white matter. And for the peripheral nervous system, we'd find axons and nerves, also a great answer. Well, what about synapses? Where would we find synapses? Well, maybe just a little bit more thought about that leads you to answer, well, gray matter. Synapses are the output zone of one neuron, but it's also the input zone of another. So we would find synapses in grey matter. Great. Well, let's keep those thoughts in mind as we look at this pathway in some detail. So, this paths, pathway begins with a first order neuron. Which is a dorsal root ganglion neuron. Okay. So here it is right there in the cervical enlargement of the spinal cord. And here's the dorsal root ganglion in the lumbar enlargement of the spinal cord. These are the first order neurons. So they have a peripheral process as we saw in the last tutorial. Specialized to establish some labeled lines some particular aspect in mechano-sensation. And then the central process enters the spinal cord. That central process enters the dorsal column and ascends, it runs the longitudinal length of the spinal cord until we hit a set of nuclei on top of the dorsal column and those nuclei exist in the caudal part of the medulla. Specifically in the dorsal part of the tegmentum. We call them dorsal column nuclei, or DCN for short. That's what we have at this level. So that's where we find the synapses for the next neuron in the pathway, so those neurons then grow axons that do something really important. They cross the mid-line and as they do they, establish the location of the decussation of this pathway. So for every pathway we consider, we want to remember that for the cerebral hemispheres there's a principle of contra-lateral representation that we must solve. So in the case of the mechanosensory pathway, contralateral representation is established in the caudal medulla with the outgrowth of the second order axon. It sweeps across the tegmentum of the medulla, and ascends the rest of the way through the brain stem. And synapses on the next neuron in the pathway which is in the thalamus, specifically the ventral posterior lateral nucleus of the thalamus and that's what we have here, our third neuron. So the dorsal column nuclei was our second order neuron, and our third order neuron is in the ventral posterior lateral nucleus. So that's our third-order neuron. And from there, the thalamic cells provide input on up to the post-central gyrus. Once we reach the post-central gyrus, we basically abandon trying to order the neurons, because we recognize that When the thalamus projects to the cortex we're providing input to a vast network involving, hundreds a millions of neurons that potentially could be modulated by this sensory stream. So, at that point, given that scale, it just doesn't service well to continue the numbering scheme that Has served us well along the pathway from skin up through the thalamus. Alright, well, let's have a bit of a closer look. And we'll focus in on the first-order neuron making a connection to the second-order neuron. So again, our first-order neuron is a dorsal-root ganglion cell. Now we can see a little bit more clearly, I, I trust. An important distinction between the afferents/g that arise in a cervical enlargement and those that arise in the lumbar enlargement. The first order axons from the lower part of the spinal cord enter the dorsal column near the more medial aspect of. The dorsal column on one side of the spinal cord. As we contribute axons from increasingly superior directions, those axons add to the lateral aspect of that dorsal column. Consequently, when we reach the cervical cord, the input, which is significant of course, because we're talking about mechanosensory signals from the upper extremity. They simply add to the lateral side to the dorsal column. So there's a medial to lateral progression as axons contribute. As a result of this progression in the cervical spinal cord we recognize a division of the dorsal column into two parts. There is a medial gracile tract, and a lateral cuneate tract. We'll see them shortly in Sylvius. These two pathways run the length of the spinal cord, the gracile tract runs the entire length, the cuneate tract begins to form in the upper thoracic cord and then runs on up through the entire cervical enlargement. And these tracts meet a nucleus at the top of the column there was 1 nucleus on top each tract there's a nucleus called the gracile nucleus that sits on top the gracile tract and then a nucleus called the cuneate nucleus. That sits on top of the cuneate, tract. Okay? So that's where we find our first synapse in this pathway. And the second order neuron, then, which is part of this dorsal column nuclear complex grows an axon that sweeps across the mid-line. And as these axons sweep across the mid-line, I'll just draw a bunch of them here, we give that a particular name. We call that the internal arcuate fibers. It's simply the second order axons as they cross the midline. Now once they cross the midline, now they make a sharp turn. And project in the superior direction. It's really the very same axons, OK. We just give them different terms and different locations. When they're crossing the mid-line, they're the internal arcuate fibers. After they've crossed the mid-line, they form a distinctive medial ribbon of white matter that we call the medial lemniscus, or ML for short. We'll let's see the medial lemiscus as we blow up the top half of this figure from our book. So now, we have an indication of at least two axons that are ascending from the dorsal column nuclei To the ventral posterior lateral nucleus of the thalamus, the vpl. And it's this fiber bundle that we call the medial lemniscus. Okay? So, again, these are second order axons. They make synaptic connections in the thalamus. In the vpl, in particular. And then, from there, the axons project into the post central gyrus. And they do so in sematotopic order. For example, the fiber that is projecting here into the para-central lobule is a fiber that was activated by the gracile tract. And that fiber Is communicating somatic sensory signals from the lower extremity. The thalamic neuron from the VPL that terminates here near the middle of the precentral gyrus is being driven by an axon that entered the spinal cord in the cuneate tract. So this is an input that's being driven by what we did with our contra-lateral arm or hand. Okay. So the exquisite body map that we talked about in the previous tutorial is established by the systematic ordering of axons from the thalamus projecting into the precentral gyrus. Okay, well I hope that was clear. I took some time to go through this quite slowly. We won't take quite so much time as we talk through the next two pathways.
