Hi everyone. Welcome to this tutorial on the general principles of sensory systems. We are at a bit of a pivot point in the course where our focus has been on gross brain anatomy. As well as the principles of neurophysiology that underline neural signaling and even a discussion of neuroplasticity. Now we're going to begin to bring these concepts together. On the one hand, the discussion of anatomy, in the other, the discussion of cellular physiology and we want to bring them together. As we discuss the organization and function of our sensory motor systems. So this is the beginning of that pivot and what I'd like for you to be able to focus on in this tutorial are some of the general principles that will help us understand the organization of our sensory systems which is what we'll discuss first and then we'll move on and discuss the motor systems. So this topic pertains once again to our key concept or foundational core concept in the field of neuroscience pertaining to the complexity of the brain. And this tutorial I trust will help break down some of that complexity for you as we consider how sensory systems are organized, and how they function. And this will allow us to maybe reflect for just a moment on this incredible capacity we have to explore our world. Even the world with inside our own brain. You know, I'm reminded of a student I once had some years ago who I will not name in case you're out there somewhere but he showed up one day to the lab and he said, you know I just don't think the brain should be allowed to study itself. And I thought, what a profound observation you know, the kidney doesn't get to study itself nor does the liver, but somehow the brain has the credentials that allow it to study itself. And I think that is an interesting thought to consider. Well let's move on and consider what we have to learn from this tutorial today. So I have several learning objectives for you, I want you to be able to account for the generation of action potentials in peripheral axons in response to somatic sensory information. Now, the somatic sensory system will be the first of the sensory systems that we consider, so I thinks it's useful to see how this general problem of action potential generation in a sensory system is realized in this one domain of sensation, that of somatic sensation. I want you to be able to account for differences in mechanosensory discrimination across the surface of the body. Again, we're going to take somatic sensation as our model paradigm, but the basic principles will apply to our other sensory systems as well. And I want to consider in fairly general and broad terms, how information is coded in the nervous system. So, I'd hope that you'd be able to have a discussion of the important factors that influence information coding. And one very important concept in sensory physiology is that of the receptive field. So I want you to have a thorough understanding of what the receptive field is. And one way to ensure yourself of that is to have a discussion with someone about it. Okay, what I'd like to do next is to consider some general organizational principles that will apply to our consideration of our sensory systems. And we'll see how they played out specifically in different systems over the next several tutorials, but today I'd like to keep it fairly general. So let's begin with one principal, and that is the principle of neuronal pathways. So let me illustrate for you, in very simple terms, what I mean by a neuronal pathway. Let's imagine that there's some kind of sensory surface out here perhaps this is the skin. And the skin is innervated by nerve fibers and those nerve fibers are grown by cells in this case cells that reside in the dorsal root ganglion so an axon extends out and there's some kind of specialized receptor structure as the axons reaches the skin surface. So this is what we call the peripheral axon of this neuron. So this neuron can be considered a first order neuron in this pathway. Well there is a central process that enters the nervous system. So let's say this is now into the central nervous system. And that first order neuron synapses on a second order neuron. And that second order neuron projects some distance through the central nervous system.to typically a third neuron. We'll call that a third order neuron. And then, that third order neuron provides input to a vast network of interconnected cells. And some processing station within the brain, perhaps this processing station is the cerebral cortex. So, we might have some, some complicated network that allows all this to function. So we might call this now the cortex. So the neural pathway would be the first order neuron, the second order neuron, the third order neuron and then once we get into our complex network that is being modulating by this ascending input, we sort of give up on our numbering scheme. So, we'll see how that works? But the pathway, in essence, is a series of neurons that are connected. But the pathway is essentially a set of neurons that are connected in a serial fashion. And this allows neurosignals to be generated at some receptor surface and then be transmitted into the nervous system ultimately to be processed at the level of the cerebral cortex. Now, I drew this pathway in the horizontal dimension across the screen just to make use of the space that I have, but I want to make an important point about how we talk about our pathways in the brain. So, let me clear that and redraw this in the vertical dimension. So often we have some receptor connected to our first order neuron, which projects to a second order neuron. But now we often think about this as ascending into the nervous system and here's our complicated network again representing the cerebral cortex. So here's our first order neuron, our second order neuron, our third order neuron and then the cortex. Now, this we might consider to be a sensory pathway and the information is essentially flowing in the ascending direction. Well, this cortical network also has a motor component to it where these sensory signals are eventually integrated within some kind of a network that coordinates and produces an output. And that output eventually reaches our motor neurons and behaviors result. So we can imagine that there's some output neuron here that is going to grow an axon which will probably synapse on some kind of local circuit cell. That modulates the output of a series of neurons that ultimately supply some kind of defector such as a skeletal muscle. So this now is an output pathway which can be conceptualized as descending. From a higher level from the cerebral cortex for example, down to a lower level maybe in the spinal cord where we have a motor neuron leading to the contraction of a skeletal muscle. Okay, so pathways can be either ascending or descending. Ascending typically implies a sensory function, descending implies a motor function. In our nervous systems that are bilaterally symmetrical there's a very interesting principle of organization in play, and that that is a principle of contralateral representation. So, let me illustrate that for you. Imagine that we have some body part in question here, maybe it's an upper extremity, and that upper extremity is going to be represented in the contralateral, or the opposite side, of the cerebral hemisphere. So this is the cortex, let's say this is the post central gyrus, which is where we find our somatic sensory cortex. So this body part will find its representation in the somatic sensory cortex, something like this. Now, contralateral representation implies that there must be some sort of crossing of the midline. Somehow this information has to get from one side of the body to the other in order for the ascending sensory pathway to relay it to the appropriate processing station at the level of the cortex. So as we work through our studies of our sensory pathways, please pay attention to this issue of decussations. So decussations simply means midline crossing. So we're talking about where in the nervous system does this pathway cross the mid-line? So that the principle of contralateral representation can be understood. Now this is not just an academic exercise for future clinicians, this is a critical point. One of your challenges in the examination of your patients will be to understand, to have a mental model of what's wrong with the nervous system. There may be a focal lesion somewhere in this nervous system. And understanding where sensory and motor pathways cross the midline, will be critical for your proper understanding of what level of the nervous system has been impaired. Now this issues of decussation, it's not just for sensory systems, it's also for motor systems. Now if we get just in front of the post central gyrus, we may be in the pre central gyrus which is where we have our motor control networks and there may be a neuron there in the pre central gyrus that gives rise to a descending axon. And that descending axon has to confront the same challenge of contralateral representation. At some point, there has to be a crossing of the midline, a decussation and as it turns out the decussations. For sensory and motor pathways, are not in exactly the same place. Even within our somatic sensory systems, different pathways cross the midline at different locations. And that will be critical for understanding the neurological signs and symptoms of injury and disease, that you may observe in your patients. Now let's turn to a new topic, the topic of sensory transduction. By sensory transduction what I mean is the process that allows for the energy that's present in the environment. Either in physical or chemical form, to be converted into electrical signals within nerve cells. And sensory transduction is a problem for each of the sensory systems that must be solved in some ingenious way. And in fact it often is and many times there are highly conserved mechanisms that are employed across animals with nervous systems, including us. And sometimes those methods seem quite surprising. so I hope you have some fun discovering how sensory transduction works in each of the sensory systems that we will consider in this course. Let's consider one example from the somatic sensory system. Well, we know that the somatic sensory system is concerned with Touch with physical contact is one modality of stimulation. And it must happen that physical contact leads to the generation of eletrical signals. So, sensory transduction would be part of that story in addressing the question, well. How does this mechanical contact lead to the generation of electrical signals? Here's how this works for the receptors that are sensitive to mechanical deformation of the body surfaces. Imagine that we have some kind of receptor ending at the end of our first order neuron This happens to be a beautifully laminated structure called a Pacinian corpuscle. And this Pacinian corpuscle contains a nerve ending that has cations selected channels. And these ion channels are sensitive to stretching forces that are applied to the membrane. So when there's some kind of mechanical interaction with the structures containing these pacinian corpuscles, these ion channels will open. Not because they're voltage gated or because they are activated upon by neurotransmitters as we saw in our previous tutorials on neuro signalling. But rather, because the mechanical forces open these channels, so here's what we observe. When a physical force is applied to this tissue, these ion channels will open, this will allow our cations to enter. In this particular case, the principal permiant ion is a sodium ion. And so, as I think should now be familiar to you, as sodium rushes in to a neuronal cell it carries with it its positive charge. And that leads to the depolarization of the sensory ending of this axon. Let's have a physiological perspective on this process now. Imagine that a fairly weak stimulus is applied. And that weak stimulus will lead to the influx of relatively small number of sodium ions. So one can imagine a fairly weak depolarization of this receptor. Now, we have a term that we use to apply this kind of depolarization. We call this a receptor potential, because we're talking about a change in membrane potential, in a receptor ending, of a primary sensory axon. Now, if the strength of this stimulus is increased, what we would expect to see has a greater response and indeed, that's what we see. So with the stronger stimulus, we see an increase in the amplitude of this receptor potential. But notice that, we don't yet generate an electrical signal that can propagate, as long as this potential stays below threshold, for generating an action potential. But with a stronger stimulus, it very well may be that this receptor potential is now sufficient magnitude to achieve the threshold for generating an action potential. Once that threshold is achieved, an action potential fires. Now notice what's going on here, the generator potential is a function of the stretch activated ion channels. But once that membrane is depolarized to achieve the threshold for generating an action potential, now our friends the voltage gated sodium and potassium channels are activated and an action potential is generated. So different kinds of ion channels contribute in different phases to the transmission of information along the axon of the primary sensory neuron. Now in the case of our sub-threshold potential receptor potential that fail to elicit an action potential. We're never going to know about that contact through this sensory ending because there has been no action potential. There's been no signal transmitted into the central nervous system that requires a receptor potential of significant amplitude to achieve threshold.
