Okay, so, we've been talking about our first order neurons and their organization in the periphery and just a little bit about their organization as they enter the central nervous system. We're going to devote an entire tutorial to following the pathway by which the first order neuron eventually communicates with higher structure in the fore-brain. So, we won't look at those details now, but I do want to jump to identify what are those centers, that are involved in processing mechanism sensation in the for-brain and give you just a little bit about the physiology. So let's begin with talking about the somatic sensory division of the thalamus. So, there's a brief tutorial that talks about the thalamus to get you oriented to this part of the forebrain. For now I'll just remind you that the thalamus is derived from the diencephalon which is part of the forebrain that the prosencephalon and it's the more dorsal posterior part of the diencephalon with a hypothalamus being more ventral interior. Well the thalamus as my other tutorial goes into it more detailed is divided into a number of nucleoli and those nucleoli relate to a particular part of the cerebral cortex. So if we were to look at the cerebral cortex we could identify just about any region that we may decide to poke at, and ask, what is the thalamic nucleus that projects to this region of the cerebral cortex? And, and we'll have a different answer for most different sites in the cerebral mantle that we might inquire about. Well, if we focus on the postcentral gyrus, which is right here that's, as we'll talk about in just a moment, is our primary somatic sensory cortex. There's a particular part of the thalamus that maintains interconnections with this postcentral gyrus. That part is exactly the part of the thalamus that receives the ascending information about mechano-sensation. That part is what we might call the somatic sensory thalamus. And we have some anatomical names to give it. We call it the ventral posterior complex of the thalamus. And that ventral posterior complex is divided into two main nuclei. There is a division called the ventral posterior lateral nucleus, and a division called the ventral posterior medial nucleus. And the division there bascially reflects the terminations of our, pairs of pathways that are representing the body from the posterior part of the head on down. And then a different pathway conveying mechanosensation for the face. So for the body below the back of the head, we have signals arriving in the ventral posterior lateral nucleus. Where our signals concerning mechanosensory activation of facial structures arrive in our ventral posterior medial nucleus. So now we're ready to consider what happens to the somatic sensory information. As the thalamus communicates it up to the level of the cerebral cortex, and that gets us into our postcentral gyrus. So here, once again, is the postcentral gyrus in my model of the human brain here. And this is where we're going to find the region of cortex that we call the primary somatic sensory cortex. The reason why it's primary is that it's the first part of the cortex to receive input about mechanosensory information. And in anatomical terms, it's the part of the cortex that receives input from the somatosensory thalamus. That is, the ventral posterior lateral and ventral posterior medial nuclei . And as we'll see, the ventral posterior lateral nucleus projects from the para-central lobule in the mid line of the hemisphere, along this post central gyrus as it extends across the dorsal and lateral aspects of the cerebral hemisphere. Until we get to about two thirds the distance between the mid line and the lateral fissure. When we get to about this region now the rest of the postcentral gyrus is receiving input not from the VPL but from the VPM, the ventral posterior medial nucleus. This explains why, in the mapping of the body that we're, we will see in just a moment, there's a disjunction between the representation of the hand and the representation of the face. It's basically the boundary between the projection zone from the VPL and the VPM. Okay, well let's look at this now in a bit more detail. So here we have an illustration from your text which shows us our primary semantic sensory cortex which we call s1 for short. So it's found in the post central gyrus. And it's comprised of four cytoarchitect tectoniclly distinct regions of the human brain that Korbinian Brodmann named over 100 years ago. Areas two, one, and areas three and workers after Brodmann subdivided area summary into an area 3A in an area 3B. So from posterior to interior, they're areas 2, 1, 3b, then 3a right at the depth of the central sulcus. So in each of these cortical areas we have a complete representation of the opposite side of the body, and that representation Is beautifully mapped out. We call this somatotopy which means body mapping. And the way the body mapping works is that the distal part of the lower extremity is represented in the midline. Right at the medial termination of the post-central gyrus in this region that we call the paracentral lobule. So this is essentially where we find the representation of the contralateral foot. And as we progress over the dorsal rim of the hemisphere And on to the dorsal lateral surface of the postcentral gyrus, the body map progresses from the foot up the lower extremity, and then into the region of the trunk. And then from the region of the trunk, we encounter the shoulder, and then we get into the upper extremity. And where the central sulcus has a fairly distinctive curve in it, that's the region where we find an expansive representation of the contralateral hand. Now, notice we have not yet encountered the face. That's because as I just mentioned. The face is conveyed by inputs from the ventral posterior medial nucleus of the thalamus, which terminates in this inferior segment of the postcentral gyrus. So just below this S-shaped bend in the central sulcus we have a junction between. The representation of the hand and the representation of the face. The illustration that we have in front of us shows us the detail of that body mapping and it truly is an exquisite mapping of the body. With the progression from lower extremity through the upper extremity. And then, a disjunction, where we then have a break. And the representation of the face in the inferior third or so of the post central gyrus. Now, there's a wonderful story about, a famous neurosurgeon at the Montreal Neurological Institute, Wilder Penfield and his assistant. And, there's some discussion as to how much of this is really true or not. But evidently the assistant is the one who suggested to Dr. Pennfield that this distorted representation of the human body be represented in a simple cartoon. And so that's the cartoon that we see here in panel c. This is called the little man, or the homunculus. And this is a illustration you've probably seen before. It's intended to communicate the idea that, in the post central gyrii, taking the two hemispheres together We have a complete representation of the body, but that representation is distorted. Those body parts that provide us with the greatest spacial acuity and the greatest sensitivity to mechano-sensory stimuli are hugely represented in the brain. And they're represented in these distinctive features of the postcentral gyrus. The hands are represented where the central sulcus has that S shape where the two gyri interdigitate against each other. And then the face is represented in that inferior segment of the postcentral gyrus. Whereas the remaining parts of the body are not nearly as magnified. We call this phenomenon cortical magnification, which has to do with how much more does cortical circuitry magnify the representation of the body that's otherwise related to the density of peripheral receptors, the density of neurons at all the antecedent stations in the somatic sensory pathway. Well the up shot here is that the homunculus makes very clear that the important somatic sensory surfaces for us as humans would be our hands and our face especially the facial structures around the mouth and the tips of our fingers. Now before we leave the primary somatosensory cortex, I do just want to comment on the functions of these four paralell body maps. You, you may find that curious, and frankly, a lot of us find it somewhat curious. But there do seem to be distinctive receptive field properties that are represented In these four different maps of the body. One might suggest that area three, being subdivided into three a and three b is truly the primary somatic sensory cortex, from a physiological perspective. Whereas areas one and two seem to process a more higher. Order aspect of somatic sensation. Area 3a is especially concerned with our proprioceptors. Those receptors that are found in deeper structures, such as our muscle spindles, and our golgi tendon organs and our joint receptors. Where as area 3B is more concerned with cutaneous receptors. So area 3B is more cutaneous, is going to be more concerned with like touch for example the then the movements of our muscles and joints. Now once we get into areas wanted to wear encountering more complex aspects of somatic sensory processing. Area one for example is going to respond to complex stimuli activating multiple skin surfaces. For example, over multiple fingertips and perhaps responding selectively to a particular pattern of motion. Our fingers passing over a counter surface from left to right, for example. Area two seems to be concerned more so with the shapes of objects that we encounter. So it suggests there is some kind of computation of lower level signals into more higher order signals that begin to build up a sense of structure. In our somatic sensory interactions with the world. Okay, and so finally let's consider what happens to this information beyond the primary somatic sensory cortex. What we discovered is that the primary somatic sensory cortex being at this level here. Has extensive interconnections with other parts of the brain. There is something that we recognize as a secondary somatic sensory cortex. That is in the inferior and posterior aspect of the parietal lobe perhaps also involving the posterior aspect of the insula. And this secondary somatosensory cortex receives input from our primary areas. And from there, that information is further elaborated in a hgher order sort of way, and passed into structures in the medial part of the temporal lobe that are going to be important for making memories. And for interacting with our emotional networks. So, that involves the hippocampus and the amygdala respectively. Areas one and two, likewise project into our secondary somatic sensory cortex, but area two in particular, the one concerned with more complex geometrical aspects of somatic sensation projects into parts of the parietal lobe that sit just posterior to it in the superior parietal labial and these parts of the parietal cortex seem to be especially concerned with our orientation relative to the environment. So there's a, a spatial mapping that's going on here in this part of the brain. And that mapping is used to drive our motor system to interact with the world in a appropriate way so, for example imagine a visually guided reach that is a means of directing or somatic sensory surfaces towards a particular goal. Well that's going to be integrated through parietal and frontal networks, most likely involving the superior parietal lobule region. Okay. Well, this has been a very quick sense of cortical processing in the somatic sensory system. There are so many big questions that we have yet to address. Such as how does perception actually work. How is it that we can have for example the inner grated sensation that one might experience when holding a snow ball or an ice cube or maybe immersing a finger in a cup of tea. Consider those experiences were combing a sensation of shape with structure with a sense of texture and a sense of temperature and just maybe a sense of pain as well. I've been emphasizing the concept of label lines and parallel processing But at some point in our perception we unite these labeled lines into a solitary percept. And it's still not clear to neuroscientists how that unity of representation occurs. There may not be an anatomical explanation for that perceptual sense of unity. There may be other physiological means to build up that percept and this is all under active investigation. So hopefully this tutorial has given you a framework for understanding how mechanicization works and will also help you to put the anatomical pieces together when we look in a different tutorial in detail at the organization of the somatic sensory pathways. So I'll see you then when we consider that interestting anatomy that gets us from skin to cortex. Well, finally, before I leave you to this tutorial, I want to suggest a, study question that begins to apply this knowledge in the clinical domain. I want you to consider what would happen If a patient were to suffer a stroke involving the middle cerebral artery. And I want you to think about such a stroke occurring in the right hemisphere. Okay? Now, I've given you a bunch of options to think through, and they all begin with this word, anesthesia. And anesthesia implies lack of somatic sensory sensation, particularly in the domain of mechano-sensation. So consider these options, and go ahead and indicate in one form or another, what you consider to be the best choice among this set of options. I'll see you next time.
