Now I want to turn to the topic of information coding. And one concept that we'll see played out in the somatic sensory system, in the visual system, in the auditory system, in our chemical sensory systems, and even in our vestibular system, is the notion of labeled lines. Now what we mean by labeled lines is the idea that the response of an axon in a sensory nerve is going to be shaped or dictated, if you will, by the kind of receptor that's elaborated at the end of that axon. And that sensory receptor is going to be specialized to a code, a certain quality of information. So what we're looking at here is a cross section through the skin and what we see is a rich variety of sensory specializiations. So what we're looking at are a variety of endings of the axons of our first order neurons. Many of these are going to be sensitive to different aspects of mechanical stimulation. But some of them are going to be sensitive to the potential threat to damage of the skin and relationship to injury, to temperature change to the presence of, of chemicals that might harm the skin. So, these are examples of what we have in mind by labelled line. And here is one example in particular, here is that Pacinian corpuscle that we illustrated schematically a moment ago. This structture is found deep in the skin, in the subcutaneous layers or near the deeper part of the dermis. And this is a structure that's especially sensitive to the vibration. Meanwhile, there is a type of receptor that is right where the epidermis, and the dermis, interdigitate. It's called the Meissner corpuscle. This structure is especially sensitive to light touch. The kind of touch that we might use as we use our skin surfaces on our fingertips to explore a sensory surface. So the selectivity of the different axons that terminate in these receptors is really determined by the specializations we see at their terminal endings. And the fact that different axons are associated with different types of receptors, implies that there are parallel channels that transmit various submodalities of sensation from the peripheral surface back toward the central nervous system. So another principle that we see illustrated here in this figure, is that of parallel processing. All of these afferent axons are transmitting information in parallel from their various locations in the skin into the central nervous system. in this case, either the spinal cord or the brain stem. Now let's consider how information is encoded in the firing patterns of the neurons. Then this brings us to an important phenomenon that's present across all of our sensory systems, and that is adaptation. What we find is that when a stimulus is applied to a sensory ending, there is typically a, a barrage of action potentials that we see at the onset of that stimulus. But notice that, over time, the numbers of action potentials typically diminishes. For some neurons the barrage of action potentials is really all that we see. And after the intial onset of the stimulus there's basically no fruther activity in that first order axon. Others continue to fire, perhaps with some regularity, but often at a diminshed rate to the abrupt onset of that stimulus. So these are examples of what we call adaptation. An adaptation can happen very quickly and dramatically, or it can occur more slowly and to a less significant extent. So some receptors adapt rapidly and other adapt rather slowly and this has consequence for the communication of information. From our sensory surfaces into our nervous system. You can imagine that the slowly adapting types of receptors might be very useful for telling us about the presistent presence of a stimulus. Whereas the rapidly adapted, adapting sorts of receptors might be quite good at telling us about the dynamic changes in the presence of the stimulus. Now, we've already considered the concept of threshold and let me just make on further comment about it now. And that is that while all sensory receptors have some kind of threshold for generating an action potential, that threshold might be different for different kinds of receptors. Based on the sensory transduction mechanism that's involved, the specializations that may surround that nerve ending. And also with respect to the location of that receptor within the sensory structure, be it the skin, or a muscle, or a joint, or wherever it may be. So sensory thresholds can be an interesting way that the nervous system can use to, be sensitive to the right kind of information, in just the right sort of way. So, as an example, our mechonsensors, those that are sensitive to mechanical stimulation, tend to be very sensitive, with a low threshold for activation. Whereas our pain fibers, those associated with the free nerve endings in the tissues of our body tend to have a higher threshold for activation. Now that's a good thing, is it not? Because it requires further stimulation of our sensory surfaces in order to elicit pain we wouldn't want a light gentle caress to be painful. and by modifying the sensory thresholds of different kinds of labeled lines we can achieve that goal. Lastly I want to consider with you the topic of receptive fields. Now this is a fascinating topic that we'll see over and over again as we consider our different sensory systems. And let's again stick, for now, with the, example of our somatic sensory system. What we mean by a receptive field is simply that region of the body. Or that specific area in the space around the body. That, when a stimulus is presented, activity can be modulated in a specific nerve cell. And here's an example of how this might work. Imagine that there's a micro-electrode inserted into the post central gyrus. in this case let's imagine that we have a primate as an animal model and we can record the activity of neurons in that post central gyrus. Now, if that should be the case, what we will discover is that that neuron requires stimulation of a very specific location in the body in order to have it's activity modulated. And the job of the experimenter, in this scenario, is to discover What is the surface of the body that must be stimulated in order to modulate the activity of that neuron in the pro central gyrus? And what the experimenter might discovere is that there is some patch of skin that when stimulated can modulate the firing pattern of that neuron. So what we see here is that during the period of stimulation there is an increase in the number of action potentials that that neuron fires compared to what we see before and after. And furthermore, what we find is that receptive fields tend to have a center surround organization. And here's what we mean by that. There tends to be a center of the receptive field, that when stimulated, gives rise to a robust, modulation of activity. often, it's a dramatic increase in activity, which is what we see here. But sometimes, it's actually a decrease in activity. That really doesn't matter, the important point is that There is a rapid modulation of nerve cell firing when the center of the receptive field is stimulated. Well, receptive fields are surrounded, typically, by an annular region that we call the surround. And the physiology of the center surround interaction Is antagonistic. That is, if the center of the receptive field leads to an increase in firing, then activation of the surrounding annulus typically leads to a decrement in firing. Which is what we find for the patch of skin surrounding this particular receptive field on the skin, for this cortical neuron in question. So notice how activation of the center leads to an increase in firing, whereas activation of the surround leads to a decrease in firing. We'll see the very same pattern of activation in the retina, in the visual parts of the thalamus and in the early parts of the primary visual cortex when we consider the physiology of the visual system. Allow me to make a few additional points about receptive fields before we conclude this tutorial. Well, we've already looked into the dermis and appreciated the variety of receptor endings that we find there over different parts of the dermis. I'd like to point out that some of these receptors are really quite spatially limited in their extent. Whereas others, because of their size and their organization, tend to extend over a greater extent. Well, as it turns out, there is, in fact, a variety of sizes of physiological receptive fields. So for example, we can imagine that for this Meissner corpuscle, perhaps we have to be right on top of it in order to stimulate it. Whereas this Pacinian corpuscle, perhaps, can be activated through deformation of skin over a much broader area. So the size, the orientation, the distribution of the peripheral sensory structure will have a lot to do with defining the size of the receptive field for the first order neuron. But we can actually record a receptive field for any neurons as we saw in the previous slide, that receptive field was defined for a quartical neuron. So what we'll find is that at each level along a sensory pathway There will be a variety of receptive field sizes and shapes that can be identified. For the first order neuron, the size is often related to these geometrical factors that I've just outlined for you. But when it come to neurons within the central nervous system, the second order neurons, the third order neurons, the neurons in cortical networks. Principles of convergence have much to do with the finding the size of those receptive fields. So for neurons in the cerebral cortex for example, what really determines the size of the receptive fields will be the degree of convergence. And for neurons in the cortex large receptive fields are created by a high degree of convergence. That is, many neurons at lower levels of the sensory pathway converging onto the same cortical neuron. whereas, smaller receptive fields are created by minimal patterns of convergence. That is, fewer numbers of cells at antecedent levels of a neural pathway converging as information is transmitted onto the level of the cerebral cortex. So at the level of the cortex, we see more of a direct relationship between receptive field size and the degree of convergence. The more there is convergence, the larger we would expect to see the receptive field. Now let's look at one final example from the somatic sensory system, and how this plays out. Imagine we're going to do an an experiment to asses two point discrimination. And I would encourage you to do this with a friend or a family member. All you need is, let's say a paper clip or maybe a little piece of wire that can somehow be bent into a shape something like this. And you can vary the distance between the ends of this wire in such a way that maybe the wire can be bent so that these points of contact can be spatially varied. Well what you can do is you can ask a friend, or have them ask you to close your eyes while some modest part of your skin surface is explored with this kind of homemade caliper. Well if you do that you can ask your friend or your friend can ask you with eyes closed. Simply tell me whether you feel one point of contact or two. So one or two, that's the question. And what you can find is what is the minimal separation of the tips of these calipers that will allow your test subjects to report two points of contact. So, this is called two-point discrimination. And you can imagine how this might work. In order for our subject to report two points of contact, we must stimulate at least two receptive feelings on the skins surface. So, this is illustrated for the finger tip and in the patch of skin on the forearm. And what is shown in this figure is what's actually observed in most people. That is the receptive fields are smaller, in the fingertips where spacial acuity is quite high compared to the forearm where spacial acuity is much lower. And this means that two-point discrimination will be better. In the digit surfaces compared to the skin on the forearm. That is we can detect smaller seperations on our fingertips compared to our forearm. So from a neurophysiological perspective let's consider these three examples. So here in green we have two points of contact that are coming together quite closely. Within the receptive fields of neuron B. And what we see there is that with the green contact, neuron B is activated to a significant extent. But these points of contact only touch the margins of neurons A and C. So these neurons fire at not a very high rate. So, for the set of primary afferents coming from this patch of skin, it looks like only one neuron was activated. And we might imagine that we would interpret that as one point of contact when, in fact, there were two points of contact. Now, let's look at the points of contact shown here in red, a little bit further separation. So, we are now getting closer to the center of activating neurons a and c. But we're not yet so far to the extreme of the receptive field of neuron b that it's not going to be activated. So what we have as a result is activation of three receptive fields to a pretty similar extent. Well, this might also feel like just one point of contact, because we're not adequately differentiating the responses of one receptive field, or one neuron from another. That only happens once we expand our caliper to the points of contact illustrated here in blue. we are very near the center of the receptive fields of neurons a and neurons c and we're just at the margin of the receptive field of neuron b. So we would expect a robust response from neurons A and C, minimal response in neuron B. So in the context of a neural structrue that's representing the information coming from these three parallel channels, we see clear differentiation of activity between two of these three neurons. With a separation between those most active neurons. This will allow us to make a judgement of two points of contact rather than one. So, we think this is the basis of two point discrimination and indeed if we survey the body surfaces we find that there is a tremendous range of sensitivity. so what we're looking at is a plot of the two-point discrimination threshold across the left and right sides of the subject. And what we find is, is really a high sensitivity around the fingertips and some of the oral facial structures. And not so much sensitivity in some of the More proximal parts of our arms and our legs. Well, these concepts are illustrated here with respect to the somatic sensory system, but we'll see how they play out in other domains. I hope you appreciate that what we're talking about here with two point discrimination Is really the phenomenon of acuity. So what we're talking about here is spatial acuity. But the same principles will apply, let's say, in the visual system, where we have very small receptive fields, and a high density of receptors in the central part of the retinas in our eyes. That's a region of greatest spatial acuity. Whereas in the peripheral parts of our retina we have larger receptors that are much less dense in their distribution. Consequently we have much larger receptive fields in the neurons that are receiving input from that receptor surface. So acuity is an important concept in sensory systems and we'll return to that as we go along in our studies. Well, now that I've given you an overview of some of the important principles of organization and function in sensory systems. I'll just leave you with the study question, and give you an opportunity to consider that on your own.
