 Okay, let's turn to our last sensory system in the course. Any announcements from TAs? Are there any TAs? No, there are no TAs. I think you can pick up your midterms, but I don't know the details. I think they will be available in section. Okay, so let's go with that. Midterms will be available in section, i.e. tomorrow. There is the plot of the distribution of scores available as well, and of course you'll get your own score on the midterm once you get it back, and so you can compare yourself to the rest of the class and calibrate yourself and see where you fall. If you have questions about how you're doing, let your TAs or Henry or myself know that. Any other announcements? I don't want to be having any TAs. Any other announcements? Problem sets? Oh, John, any announcements? Okay. Midterms, they're getting in tomorrow in discussion section? Okay. Okay, so it's not a sensory system. The last sensory system after having gone through alfaction and vision and audition. I guess that's it. So we'll do the same thing that we did, very similar to the other sensory modalities. We're going to look, in this case it's a little more complicated because somatosensation is more than one thing, but we will start at the periphery. We will ask one of the receptors that transduce touch into electrical potential changes, action potentials in the case of the somatosensory system. Where do they project next? Where are the second-order neurons? Where are the third-order neurons? Eventually, how does that information end up in primary somatosensory cortex? And what are some basic principles? And as you will see, they're very similar to what we saw in the case of vision and audition. So we'll walk our way through that. And then we'll spend a little bit of time at the end also briefly talking about pain, which is similar, but also different from discriminative touch. So we come back to this picture here that you've seen many times from Sherrington's classification of different sensory modalities. So first off, it's worth pointing out that there are a number of sensory modalities that we don't have time to go into in the course. You should be vaguely aware of them, and if we've mentioned them briefly in passing, you might know a little bit more, but we won't expect you to know anything in detail. For instance, you heard about our sense of balance, the vestibular system, briefly, when we talked about the auditory system, but we wouldn't expect you to know where that projects centrally and so forth. There are other very interesting ones. There's taste, which we haven't spoken about. There's electro-sensation, which we don't have, but weakly electric fish do and use in order to find a way around and find other animals. And there's a magnetic sense that some birds, like homing pigeons, use, for instance, to navigate around that we also don't have and about which it's probably the only sensory modality about which we really don't know how it's transduced and how it works. It's clear that if there's something like it, that there is a magnetoreception, but we don't know a lot about how it works. Or even whether humans have it, there's some debate about that. At any rate, so with respect to somatosensation here, it's worth pointing out that it maps that sensory modality consists of multiple sensory modalities, and it maps onto three that are shown here in terms of scheme. One is proprioception, which is the sense that you have in the location of your limbs in space. So you know where your limbs are located in space, both by looking at them visually, but you also know in complete darkness where your hands and your feet, etc., and at least part of that comes from proprioception. Extroception is the main one we will focus on today and what people typically think of when they think of the somatosensory system. This is touch on the surface of your body. So that's an example of extraception. And then there's interception as well, which is typically not accessible consciously, but is also an example of somatosensation in that it is information about the body. So somatosensation, in general, and your book makes this point as well, consists of multiple modalities that range from extraceptive to proprioceptive to interceptive. Okay, so here's the question for you guys. Anybody want to take a stab by analogy to what we had with vision, what we had with audition, where we asked the question what is seeing, what is hearing, what is feeling? Anybody want to guess? What would be the default answer? You should know that by now, even though you might think it's wrong. What's the answer to this question? Somebody. Yes, Olivia once again. Okay, good. So that would be the analogy from vision and audition, right? It's active, touch things, and you want to know what they are and where they're located. And certainly that's one function of somatosensation and the function of discriminative touch to some extent. Now, you might think this doesn't apply to everything. So for instance, feeling pain doesn't quite seem to map onto that answer, and that shows the point that I was just making, that this doesn't have an answer because somatosensation is not one thing. It's multiple things. So take home message number one is somatosensation consists of multiple sensory modalities, some like discriminative touch, have an answer similar to the one that Olivia gave and some like pain don't. At this point, you want to make comparisons between all the different sensory modalities that you've heard about. We did a little bit of that last time and we'll do it now between the three that you've heard about, audition, vision, and touch. It's worth asking yourself how a faction might fit into this scheme as well. It's quite different from the others in many respects. Good question for an exam or a problem set. So we had to slide up before. As was the case with vision and audition, there are different processing streams. In the case of touch, these are quite clearly distinguished already at the periphery, at transduction. So they're different receptors for pain, for temperature, for different types of touch, and those project differently centrally and specify different processing streams. So different processing streams specified at the periphery already. There are topographic maps. There is a somatotopic map of the surface of your body for touch, just like there was a retinotopic map of the receptor epithelium, the retina in the case of vision, and a tonotopic map of the receptor epithelium, the cochlea in the case of audition. So in all of these three cases, you have topographic maps that are also already specified at the periphery. There are maps there in terms of just the spatial relationships of the receptors on your body. As was the case with the other sensory modalities, there's distortion and magnification so that that part of your body about which you process the most information and where you have the highest spatial acuity has the greatest cortical representation, your face and your hands in particular. There's lots of inference involved to make inferences about touch from the receptor's comparisons like surround inhibition we will see, very similar to what you saw in the other sensory modalities that serves to provide contrast and sharpen the tuning of neurons. Just like in the other systems, there's feedback everywhere and you can imagine being touched. These two down here have been studied in great detail in somatosensory systems. They're very important plastic periods, not just in development, they've studied this for instance in people who have a hand or a finger amputated and you will find that the cortical territory in primary somatosensory cortex would normally represent that digit, gets taken over over time by representations of the adjacent fingers. There's lots of plasticity here and there's also interesting stories in comparison with other animals and it's important in social communication. You might not think so but if you think anything from just a handshake or a hug, like how you greet people in different cultures to close interactions with a loved one or between a mother and an infant and you see this in many animal species as well. So we have two cats at home as far as I can tell their main channel of social communication is touch. They come up to you and they rub and same with one another. So it served an important role. These aspects, the social communicative aspect of touch, its role in development have also been studied in relation to the what happens if you deprive mammals of touch. So touch is extremely important in the development of infant mammals. So they're always with their mother. They're typically being groomed by the mother. If you take that away, you end up with severe problems in the development of those animals and animals ranging from rat pups to monkeys and humans and of course you don't do experiments like that but people have observed this. So anyway, all the same themes that we had for the other sensory systems also apply to the somatosensory system and here we had a table making some comparisons between audition and vision before and now what we would like to do is to add in somatosensation to this table. Here it is. So let's just go through this. So you know from previous lectures the transduction is extremely fast in the auditory system and there's phase locking up to about 4 kHz or so so very high temporal fidelity by contrast to vision because at the transduction stage it depends on second messengers it's very sluggish in time and that's one reason why you can see movies it's fusion if you have temporal things coming quickly in the somatosensory system it varies and you can detect vibration or flutter that has fairly good temporal acuity not nearly as good as the auditory system but still it has better temporal resolution than the visual system by contrast there's some aspects of somatosensation like a dull aching pain that are very slow and that last for a very long time so there's a big range in line with the fact that the somatosensory system consists of all these different channels that I mentioned to you. So temporal acuity has a range spatial acuity same thing and as I mentioned it varies depending on where on the surface of your body you're talking you have much better spatial acuity like two-point discrimination on your hands and your face than you do on your back just like you have much better spatial acuity in your phobia than you do in the periphery in the case of vision. So feedback, remember there was feedback to the cochlea, massive feedback in the case of the auditory system none at all to the retina and there is feedback to the spinal cord of the somatosensory system. This is for instance involved in modulating ascending information about pain. So pain is very subjective and it may feel very bad or it may not feel so bad and at least part of that effect already takes place by top down modulation feedback down to neurons in the spinal cord that will modulate ascending pain information for example. It's somewhat active and then there is various specializations here. The receptor numbers are intermediate so if you look at receptor numbers in the auditory system remember this was really small in fact only about 3,000 inner hair cells per cochlea tiny compared to the huge number of photoreceptors in vision about 100 million. Sematosensory how many receptors are they located on the body surface about 100,000 or so intermediate and if you wanted to add olfaction to that that would be about 6 million olfactory receptor neurons. So there are big differences in the number of receptors with vision the highest addition, surprisingly small number and somatosensation and olfaction intermediate between these. Any questions about these comparisons between sensory systems? So again these kind of comparative questions are ones we like to ask on problem sets and exams so make sure that you know these you don't just want to know information about one sensory modality and all the little facts about that that you read in your book but you want to make sure that you can make comparisons between these as well. Okay so what happens in the somatosensory system as you go on up to cortex or remember that for all of the modalities the exception of olfaction there's an obligatory release with thalamus somatosensation is no exception so we have information from the skin as was the case in the auditory system from the skin there's information that goes to a variety of places in the spinal cord first and then on up the brain stem and only then to the thalamus so again if you wanted to make analogies you could say that the spinal cord levels of processing that you have in the somatosensory system just like the midbrain the brain stem nuclei that you have in the auditory system are sort of functionally serve a purpose similar to all the processing that you have in the retina in the case of vision and only after that do they give their information to the respective thalamic sensory nuclei the lateral geniculate nucleus for vision the medial geniculate nucleus for addition and then the ventral posterior lateral and ventral posterior medial nuclei for somatosensation and you can take a look at your book to figure out to make sure that you know the names of those and roughly where they're located but the main point is there are specific sensory nuclei in the thalamus for each of these three sensory modalities and then that information from the thalamus is relayed on up to primary cortex primary somatosensory cortex which you'll remember from the first lecture where that is in a minute ok so let's look at the pathway this is a very very simplified version we'll take a look in more detail in just a minute of the pathway for discriminative touch this is called the dorsal column medial lanniscal pathway and very the simplest possible version of it is outlined here there are more complex paths so this is not the only path forward but if you wanted to ask what's the fastest way of synapses to get from the skin to primary somatosensory cortex that's what this shows you so there are receptors in the skin for touch many different kinds that we'll take a look at the cell bodies of neurons for those are located outside of the central nervous system in the peripheral nervous system in dorsal root ganglia that's where all of the sensory the cell bodies of all the sensory neurons are located and then their axons project into the spinal cord the spinal cord they course on up through white matter tracts and then eventually they get to the brain stem they synapse there onto second order neurons so right away you can see that these first order neurons can be very long they can have receptors in the skin and then they can project all the way up to the brain stem with their axon so that's a pretty long neuron and only then do you have the second order neurons those then project on up to the thalamus and then the thalamic sensory nucleus projects on up to cortex each of these has receptive fields as I mentioned and so there is somatotopy that is specified already at the periphery and to some extent the magnification the overrepresentation in this somatotopy of certain parts of the body surface those parts that you can discriminate with the best like your fingers, like your hands and your face arises from the density of receptors on those portions of your body and so it's just schematized here you would have a spatially restricted receptive field of one of these dorsal root ganglia neurons here just because of where it sits spatially in the skin and then if you recorded from this you would find that this neuron would fire action potentials only if you touch the skin near that surface but not elsewhere one big distinction here between this sensory system somatosensory system and audition and vision that should be apparent already but just to make it explicit is because these first order neurons are neurons that have long processes, remember in vision you have very short photoreceptors in addition you have very short hair cells those had graded receptor potentials not action potentials here in the somatosensory system the first order neurons the receptor neurons already fire action potentials because they need to get stuff from the skin into the spinal cord that's a long distance that's a big difference between those three sensory modalities that wasn't in the table but that you should make sure that you know ok so the neural code that we spoke about before in the case of the auditory system also applies here the rate of how many action potentials a neuron fires encodes the intensity of the stimulus, the hurry of push on the piece of skin the greater the action potential rate of the neuron where they are located the particular axon that's coming into the brain tells the brain from where, where on the body it is encoding information so this is sort of place information and then the type of axon that is coming in whether it's coming in from a temperature sensor a pain sensor or a touch sensor that tells the brain what the kind of submodality of the somatosensory input is ok this is just so one important thing is that because there are these submodalities pain is not just very intense touch so it's not as if you have the same receptors and it's just after a threshold when they reach a certain rate of firing that you convert touch to pain instead what happens is that the touch neurons have lower thresholds and they're firing in a dynamic range up to a certain point of pressure and then if you really push hard where it starts getting painful you start recruiting different sets of receptors higher thresholds and those are the ones that convey pain to the brain of course these would overlap to some extent but it's important to note that they're separate sensory submodalities that span the dynamic range it's not all encoded just by differences in rate of a single one ok we'll take a look in detail at the different types of receptors here in a minute ok so for touch here are four sorry for the somatosensory system here are four main submodalities then to think about there's discriminative touch and that consists of not just one but several different kinds of receptors we'll take a look at them in a minute on your skin there's proprioception this is information that often has to do with somatosensory receptors that are located in joints that give you information about the orientation of limbs in space there's pain which is again consists of multiple different channels but is separate from these up here and projects to separate places to some extent centrally and there's temperature sensation so these are all different submodalities of the somatosensory system and as I mentioned each of these consists of different receptors subtypes so it's much more heterogeneous already at the transduction stage than was the case with vision or with audition and then you can add interception which is information about the internal state of all your body organs typically these up here to some extent to a large extent are modalities that contribute to your conscious experience of somatosensation this one here in general not your brain gets information from the body to interception most of that is used for homeostatic regulation and not part of your conscious experience in terms of just discriminative touch which is the one we're going to look at in the most detail there's these four different things that you can do behaviorally and they're subserved by different sets of receptors so you can localize fine touch in different parts of your body you can discriminate two very closely adjacent points this is just like spatial frequency tuning in the visual system that we looked at and you can get itching or vibration or flutter sensations again from specialized receptors you can do this here which is similar to the question that Olivia answered at the beginning this is this allows your stereognosis means figuring out the three-dimensional shape of objects by palpating them so you can if you close your eyes you can figure out what something is by just touching it that's complicated requires many receptors and a lot of inference centrally okay so this just mentions what I said let me finish with this figure for now so this illustrates for touch the main receptors that you want to know about and take a look at your book for more detail to these four Meissner corpuscles Merkle discs Pacinian corpuscles and Raffini endings and then there are also these free nerve endings that have to do mostly with temperature and pain sensation but so this gives you an overview of several not all but several of the different types of receptors in somatosensation and a little bit of information about where they're located so the skin is up here in addition to this we have hair follicles coming up that also have receptors on them where these are located and the particular specializations you'll see these all have weird little schematics on them the particular specializations of these end organs here determines what sub modality of touch it is that they are responsible for transducent so for instance Pacinian corpuscles way down here get highly spatially filtered input because they're fairly down far down over the skin so not great for two-point discrimination and they're interested in vibration and flutter kinds of sensations okay we're going to take a break here for like let's say seven minutes or maybe ten minutes and John's going to hand out a quick quiz to you and then we'll resume looking at the rest of this so don't turn it over yet let me know or I'll ask you in a minute if you all have one write your name on the first page as usual okay let's pick up where we left off okay so this table here just gives you a list of some of the sensations that are produced if you micro stimulate some of these receptors that we just saw let me just go back to the slide here some of these receptors that we saw here individually and so we would never get this normally of course so normally the sensation that you have of touch will activate always multiple of these receptors and so the conscious percept that you have is something very complicated that reflects the relative strength of input from all these different ones and just to say a little bit more here again these are all specialized in terms of the nerve endings here depending on what the specialization is it will transduce mechanical stimuli in a certain way and where it's located on the skin if you want the highest spatial resolution you need to have receptors that are closest to the surface of the skin as you might imagine so you want it really close up here so when you push down with a pin or something there would be some deflection that this receptor could detect this one down here for instance that I mentioned Pacinian corpuscle is poorly suited for good spatial resolution it cares about deeps or vibrating kinds of things coming in but it can't tell if that's coming from this location or an adjacent location because of the spatial filtering of the overlying tissue okay any questions about the basic scheme the fact that there are these four types of receptors that you want to know the names of that are up here and here are some of the sensations that would be evoked by micro-stimulating them so this tells you a little bit about what they do so if there's some kind of a flutter vibration tapping that's what these Meissner corpuscles transduce the Pacinian corpuscles are extremely good at picking up even fairly fast vibrations or your sense of tickle if something is vibrating like a tuning fork but they're poor at spatial resolution Merkle discs are better at spatial resolution but care more about sustained pressure so the way they're located on the skin how quickly they adapt and what the specialization of the receptor ending is in terms of its temporal resolution all of these contributes to the particular aspect of touch that they feed in. Raffini endings are concerned they're sponsored to stretch and they're concerned more with knowing where in space your limbs are located so if you move your fingers around on your hand knowing where they are that's what these signal and they do so together as an ensemble typically if you just stimulate one you actually evoke no conscious sensation and then there are these ones down here that you don't want to stimulate if possible that will evoke sensations that are related to pain and again different types of pain there's a sharp pain there's a duller burning kind of pain cramping pain so all the different flavors of touch and of pain temperature for that matter are conveyed by different kinds of receptors here this just says the same kind of thing why am I not moving forward here's an example you can actually see these barely with the unaided eye of a petrion core puzzle that we saw in the slide a couple of slides ago so this is like a layered onion there's lots of layers in here in which are interspersed free nerve endings and if there's some kind of shearing input like a vibration it will shear these layers and the free nerve endings get stimulated so this particular receptor specialization is specialized to transduce vibrating kinds of stimuli up to about 100 hertz and then there would be an axon from here that again fires action potentials that would go into the spinal cord from these receptors there are many nice things you can do in a lab in an experiment to take these receptors and record from them and you can shear them using all of these different kinds of devices that are shown here that will make these cells swell or you can pull them and you can measure how mechanical deformation results in changes in the receptor potential that is shown here so if you put a mechanical probe on and you poke one of these and you record the currents as a function of the strength of the deformation it's exactly as you would expect the current gets larger and larger the larger the deformation and then of course that will translate into different rates of action potentials that are then transmitted centrally so here's this kind of walks you through in a little more detail so you have the psychological properties that these particular receptors are responsible for and how does that work so it has to do with the threshold at which these neurons fire some like pain receptors would require a very high threshold they don't respond if you just touched lightly they respond if you have a pin prick or something that's really harmful coming in and they have different rates of adaptation so some are very fast some are very slow and that's schematized here so here are the different receptor subtypes listed at the top the kind of stimulus that would be best for activating that receptor subtype here's like a little picture of where they are and then this is the kind of response that you would elicit and these somewhat cryptic things here RA means rapidly adapting SA is slowly adapting and you can tell that by just looking at the discharge rate of these receptor neurons LT is low threshold which is what all of these are and HT on the right is the only one that's high threshold the one denoted by a hammer so that's your pain receptors and here's just the strength of the stimulus here's what the spatial receptive fields are like and this is the percept that would be evoked if you stimulated these so it's a nice comprehensive table just walk your way through this and make sure that you know it and this will give you an idea of how where these are located in the skin and what kind of specializations they have relates to the kind of action potential pattern that they transmit and how that is related to the particular psychological aspect of touch or pain that they contribute to somatosensiton any questions about this scheme again the most important thing to know is there are submodalities and these are subserved by particular very specialized receptors and the kind of specialization where they're located in the skin whether they're high threshold, low threshold fast adapting, slow adapting accounts for their particular properties good in terms of more central processing you have very similar in many respects very similar computational themes as what we saw in the visual system, in the auditory system sharpening to sharpen contrast by center surround mechanisms that depend on both feed forward and feedback inhibition from adjacent receptors I think I can probably just skip through these you can take a look at them this will look extremely similar identical to the picture that we had up in the visual system where we were talking about spatial frequency tuning so remember that the basic idea is that you have some receptor that in isolation would not be particularly sharpened by a broader inhibitory input from adjacent neighbors and that's just shown here you have the same thing on the skin so if these project centrally as is shown in this kind of complex scheme up here showing feed forward and feedback inhibition you would sharpen the spatial receptive field of this sensory neuron here shown in the middle because it is inhibited by its neighbors so it's very similar to mechanisms that you find in the other sensory systems and this shows the same kind of thing so any questions about that? Centrosurround inhibition okay so let's walk our way through the two main pathways that you need to know and memorize are these two shown here so make sure you understand this slide and where these white matter tracks go because the details of them make very specific predictions about what would happen, what deficits you would have if you had a lesion like a transection of the spinal cord at a particular location so let's just go through these the one we talked about so far is the one here shown on the left this is called the dorsal column medial omniscal pathway this is for discriminative touch everything we've spoken about mostly so far you have receptors here in the skin these dorsal dorsal root neurons here they project into the spinal cord they do a lot of stuff in the spinal cord so we're not saying much about what happens in the spinal cord here but there's a lot of processing that happens within the dorsal horn of the spinal cord there's different layers there different kinds of inputs there's lots of processing we don't have time to go into it but these first order neurons do make synapses in the spinal cord some of them go up, some of them go down there's lots of interneurons, lots of processing we don't talk about that because we don't have time so what they do is they don't synapse in the spinal cord where they enter but instead they run in white matter tracks on up the spinal cord for a long distance and these white matter tracks are called the dorsal columns so just again to reorient here's the spinal cord up here is dorsal down here is ventral sensory inputs come in dorsally and the outputs to nerves motor neurons are down here in the ventral part inputs at the top, outputs at the bottom so these big white matter tracks here are the dorsal columns and at least a good portion of the sensory input that comes in from these first order neurons their axons run all the way up the spinal cord until they make up here in the medulla they synapse onto these second order neurons whose cell bodies sit in these dorsal column nuclei and only then do they cross to the other side of the brain and then they go on up to the thalamus BPL BPM and then they go to somatosensory cortex so if you had a stroke and you had a lesion up here in primary somatosensory cortex I would have you tell me that you would then lack touch sensation would be unable to have numbness or wouldn't be able to feel something on the opposite side of the body corresponding to that part of the body that is topographically topographically represented at wherever the lesion is in somatosensory cortex take a look at that in a minute but remember the body surface is represented topographically on primary somatosensory cortex but if this is right somatosensory cortex you have a big lesion you would be unable to feel stuff on the left side of your body by contrast if you have a spinal cord lesion here on the left side of the spinal cord on the left you would lack touch sensation below the level of that lesion also on the same left side of the body okay does that make sense to people so walk your way through this it's fairly straightforward once you know it but you need to know it depending on the level at which you have a lesion whether it's at a particular point in the spinal cord or whether it's further on up here that will have consequences on which side of the body you will have deficits on that's for touch that's the dorsal column that these white matter tracts medial lemniscus which is this track that crosses pathway that's what it's called for touch for pain it looks different and so for pain what happens is that the input is similar so we have these free nerve endings that are very high threshold and they encode pain sensation and again the cell body information comes into the dorsal spinal cord so all of that looks similar to touch so far but then they don't run on up in these white matter bundles all the way up here to synapse instead they cross already at that same level in the spinal cord so they've crossed they don't need to cross again and then they run up the spinal cord and then they go eventually into many places but amongst them primary somatosensory cortex okay you had a lesion in primary somatosensory cortex you would lose touch and pain sensation on the opposite side of the body but if you had a lesion in the spinal cord the consequences for touch and for pain would map to different sites of the body so if you have a lesion in the spinal cord here say then you would lose pain sensation on the opposite side of the body but you would lose touch sensation on the same side of the body about that. So walk your way through this. These things are important to know, and certainly if you ever want to become a neurologist, they're essential to know. And they relate to where these pathways course. Okay. Let's take a look at somatotopy in a little more detail. So this is for the touch system now. Again, the dorsal column medial and niscal system. The different nerves with this picture on up, I think the first lecture or so, second lecture, so that where nerves come in and give information about the body surface into the spinal cord relates to that segment of the body that they innervate. So somatotopy is defined by part where these come in and so you can map the different nerves, the different cervical, thoracic, lumbar, cervical nerves onto these so-called dermatomes, which are the regions of the body that are inhibited by those nerves. So if you cut one of these nerves, if you have a pinched nerve at one particular location, you will have some pain or numbness or some altered somatosensory input from a particular segment of your body because that's the segment of your body that's innervated by the receptors running, whose axons run in that nerve. Okay. Is that clear to people? You will notice that the face isn't colored here and that's because input, a touch input from the face doesn't go into the spinal cord. So every part of your body, all the somatosensory input goes into these spinal nerves here, all the way on up here, but then it stops and actually the first step, actually C1, doesn't give you input from the face but it gives you input from the meninges, those membranes that cover the brain, which have pain receptors, unlike the brain itself, but not the face. The face touch information comes in through the trigeminal nerve, which is one of the cranial nerves. So even if you have a very high section spinal cord transection, so if your spinal cord is cut way up here, you're going to be in a wheelchair and you're not going to be able to feel anything on all of your body with the exception of your face because the face information doesn't have to go through the spinal nerves, it comes in through the trigeminal nerve. As I mentioned, the somatotopy is relayed on up, so it comes, there's particular regions on your skin that are innervated by particular spinal nerves, as those enter the white matter here in the dorsal columns, that somatotopic arrangement is preserved. And of course, as you start down in your legs and you keep going on up the spinal cord, you're going to add more and more axons as these upper parts of your body contribute information. So these dorsal columns get thicker and thicker, the higher on up you go, and they preserve the somatotopic mapping so that the arm is more lateral, and the lower levels like the legs are most medial where they come in first. And then this just shows the rest, they're going up and then you have a somatotopic representation on the body. As I mentioned, the head gets information from the trigeminal nerve. So this is just the scheme again. Let's take a look at cortex quickly. So you remember from one of the very first lectures that somatosensory cortex is in the anterior parietal lobe in the post-central gyrus. Here it is. And this is called Brodman's area. There's actually several psychoarchitectonically distinct regions, each of which has a separate map of the body surface. Brodman's areas one, two, and three are the same as the post-central sulcus, which is the same as the primary somatosensory system, just like we have the calcron sulcus being the same as primary visual cortex being the same as Brodman's area 17. So that's where it is. And then just like with the other sensory systems, you have higher order somatosensory cortices that represent more complex aspects of touch, like particular psychological aspects of touch, and they begin to combine it with other sensory information like vision. So if you put electrodes into primary somatosensory cortex, they only care about touch. If you put electrodes back here in the posterior parietal cortex, they can be driven by touch, by vision, and they have more complicated properties related to attention, for instance. Here is the somatotopic representation of your body surface on primary somatosensory cortex. So it looks just like this. And it shows you also we can just plot the magnification of somatotopic representation onto a small so-called homunculus that represents the body surface that would look like this. So what this represents the larger these regions are, the more the size of these different body parts corresponds to the size of their cortical representation in primary somatosensory cortex. So this just makes the point that I made earlier, the receptor density and your ability for two point discrimination are all greatest on certain parts of your body as you would imagine the hands and your lips and your face and you and other animals, many other animals would explore the world primarily with that. So if you want to, you know, figure out something or you want to read Braille, you're not going to rub your torso over that, but you're going to use your fingertips because they have much better resolution. If you look in other animals, so the lots as you might imagine of specializations relating to the particular behavior and the way in which particular different animal species explore the world through touch that map onto this. Probably the best studied one is in many animals, but not in you, they have whiskers on their face. So in rodents, for instance, they have whiskers and they explore the world, not exactly on their face, but very close to their face with those whiskers. And as you would imagine, those whiskers are over represented in somatosensory cortex. In fact, they're very specialized and there's a part of somatosensory cortex for mapping the whiskers in rodents that is called barrel cortex because each whisker is represented by one little barrel and people have studied those in great detail in that particular model system of somatosensory processing. There's lots of plasticity. Roughly it's as you would imagine and as I briefly mentioned to you, people have done this experimentally in monkeys and they've also looked non-experimentally in humans who because of an accident had an amputated finger or arm or something like that. And what you find is that normally there is somatotopic representation up here, for instance, that would represent a digit that then was subsequently removed amputated either experimentally or through accident. And what happens over time, so this happens in various stages over time, is that other cortical territories from the adjacent digits take over that tissue that used to represent the digit that is now no longer there. There's lots of work on the mechanisms by which this happens and the mechanisms occur at multiple stages of processing. Some at lower levels and some at cortical levels. Some are relatively fast and some take years. But that's what happens. We're going to run out of time here in a second. So let me skip these. This is not really too important. Let me finish with pain. This, again, just take a look at this. This just schematizes or summarizes the two main pathways that we already spoke about that you want to know about. The one for touch on here on the right, the one for pain shown here on the left. And pain is much more complicated than touch because it projects to many more different central targets. It's much more distributed in its representation. And that's shown here. So if you have an injury here, you have pain information coming in. Remember that this crosses already in the spinal cord, unlike information for touch. And then they're ascending white matter pathways here, the antrolateral system, on the opposite side of the spinal cord from where the input came about the pain. There's lots of processing internal to the spinal cord, as I mentioned. So there's lots. It's not as if the spinal cord is just a relay. It's very complicated, lots of processing in the spinal cord that we don't have time to go into, but that's what's schematized by these little layers here. And then it projects to many places. So pain information projects to places in the brainstem, in the thalamus, and multiple cortical targets. One of those is primary somatosensory cortex. So you do get pain information to somatosensory cortex. It's topographic. And so in general, you know roughly where the pain is located on the body surface. But in addition, it feels bad. And it may feel very bad or not so bad, the different flavors of pain. And those emotional and motivational components are represented not in primary somatosensory cortex, but in other regions that are shown up here, like anterior singlet cortex and insular cortex. So this, to some extent, relates to the theme that we saw already reiterated in the first slide, that there are different processing streams. Not only are there all these different submodalities that we talked about in these two big streams for touch and for pain, but in addition for paying different aspects of pain, like where it is and how bad it is, are represented in different cortical targets. In addition, one thing just to mention, to close on, is that there is massive feedback down to lower levels. It does a lot of complicated things, but in the case of pain, it's very important. It's shown on the right here. So there's feedback from many regions, and in particular these regions in the periactal gray, that project back down all the way down to the spinal cord. And so they are in a position to gait the ascending information about pain. Some of the neurotransmitters that these neurons use are endogenous peptides that bind to the same receptors that opiates like morphine bind to, and indeed they serve the same function, their analgesic. And so whether or not a stimulus is painful can be psychologically modulated. You can have a very painful stimulus, like say childbirth, that cannot feel so painful because it occurs in a very happy context. People have done studies where you can hypnotize people, so they feel less pain. In all those cases, there is descending information from higher brain regions that reaches all the way down to the spinal cord and that modulates how pain is transmitted in the spinal cord and passed on up. This is just a picture of some of the the precursor proteins from which these endogenous peptides that I alluded to, these endogenous opioids, are made. There's a whole bunch of them. They're all derived from very large initial transcripts. These are enkephalins, endorphins, and again they bind to opiate receptors. In the last slide, just to make a point about how complex pain is, it has multiple channels, multiple cortical targets, and many different flavors of pain, and that shouldn't surprise you because there are a zillion different things that can induce pain. And so here's a free nerve ending amnosusceptor, and if you ask what makes this thing fire, well there are many different things. So there could be some noxious chemical, there could be inflammation and the local release of histamine, there could be pressure and shearing, so you want mechanically gated channels, like this trip channel shown here. All of these different transduction mechanisms are going to be represented in this nosusceptor and across different nosusceptors. So it's extremely diverse. So again, it's very different from what you would find in the rest of somatosensation, and it's very different from what you saw with photoreception or auditory hair cells. There are many many different kinds of receptors that care about chemicals, that care about inflammation, that care about mechanical shearing. All of those things can signal harm, local tissue harm, so the distal causes are very diverse, but they all are bad, and so you all want to transduce them into pain. So that's what's illustrated in this last slide. Okay, so that's the end of sensory systems. Think about them, maybe go back and review what you've heard about division, olfaction, auditions, myosensations, you can make comparisons, and finally we're going to move on to talk about learning and memory.