Welcome back. We're going to shift from talking about the nervous system in general to talking about the senses. So talking about the afferent pathways that are bringing information into the central nervous system. And so we're going to be applying a lot of the principles that we learned when we were talking about the nervous sys, system in general. In this session, we're going to be talking about the senses in general for a few minutes and then we're going to focus on vision. So we're going to start off talking about general principles about the senses. And we're also going to spend a little time talking about Somatosensation. Which, if you remember, is really going to be the senses of the body. So, these are going to be coming from receptors that are in our skin, in our joints, in our muscle and other connect, connective tissue. And we're going to sense pain, touch, vi, certain vibrations, pressures as well as temperature. And this will also allow us to be able to use proprioception which is where we can determine where our body is in space. And we can do that through some of these stretch for instance receptors in muscles or receptors in connective tissue and joints. And this will also involve vision as well in determining where we are in space, but we don't have to rely on vision. And then we're going to be focusing the vast majority of our time when we talk about the senses, talking about what we call special senses. Special because they are, they have a special organ that is dedicated to the detection of the stimulus. And so, the receptors in the special senses are combined to a specific organ, and they're going to be found in the head. So those inputs are going to be fed into cranial nerves, in contrast to most of our somatosensation which happens over the rest of the body. And then also don't forget however, that also in the process of sensing, our body is sensing things like the oxygen content of our blood, the PH of our blood and as was the osmolarity of our blood. And so these are referred to as visceral stimuli. But our body is going to integrate these signals and respond to those signals in a very similar mat, manner that it responds to other types of sensory information. Let's talk about some chen, general principles about the sensory portion of the nervous system, where we are going to have se, sensory information that is going to reach the brain and that's going to be sensation. That is going to be able to tell us what the stimulus is, where it's happening on the body, and how strong it is. Keep in mind that the language of the nervous system is going to be action potentials. And so, it's going to be action potential frequency that is going to tell us how strong the stimulus. So if the stimulus is stronger, there'll be more frequent action potentials. Another important element is the idea that sensory neurons are going respond to primarily one type of stimulus, and this is going to be important again since our only language is action potentials. In order for the central nervous system to interpret the firing of a neuron, it has to know what the stimulus is that it's responding to. So, if we have a, a heat sensing neuron that's firing frequently. Since that's the main stimulus that that neuron responds to, then the central nervous system knows. Well, then there's something hot on that part of the body. But if that neuron responded to two or three stimuli equally, the central nervous system would have no way of interpreting that action potential frequency, and knowing what it actually meant. And so that's going to be another common theme that we're going to be talking about. Just that these neurons are going to be specific for a specific type of stimulus. Then another process in sensing is going to be converting that into the perception. What does the sensation, how do we interpret it, and what does it mean? And then a third principle, that we're not going to focus on much, but keep in mind that it's going to also be a characteristic of these systems is, that very often they can under go adaptation. Which is when they have a decrease in sensitivity, where a, they'll have a decreased action potential frequency with the same stimulus. Some systems are going to do this more than others. This also gets into the idea of our ability to focus our attention on the sensory information that we find important. And so we actually filter out the vast majority of the sensory information that we collect. And you can think about that right now. If you're sitting on watching this video, you're able to focus on it even though you have many other visual clues. You might also have other noises that could distract you. You're sitting in a chair, so your touch receptors are being activated from sitting in the chair or from your clothes. But you're able to adapt or to filter out that information so that you're able to focus on what's the most important information at that time. Let's talk a little bit now about the circuitry that's going to be involved. So we know that these are going to be afferent pathways, and we're going to have this would be a afferent sensory neuron in red. Where let's say its going to be pain, a neuron that senses pain stimuli. It is going to have a unique morphology compared to the neurons that we've already talked about. So sensory neurons are going to be sitting in these structures called ganglia that are right outside of the central nervous system. So, it's going to be a collection of cell bodies outside of the central nervous system in the peripheral nervous system. So, that's what's shown right here. It's called the Dorsal Root Ganglion. So it's on the dorsal side of the spinal cord. And, this neuron has a unique morphology. It's called a pseudounipolar neuron, and you can think of it as being just one long axon with a cell body sticking out of the side of it. Where the beginning of the axon is out here in the skin, in this case, and if the stimulus is strong enough, it's going to send action potentials, down the axon, as this arrow is showing. It's going whiz by the cell body and then move down the other part of the axon to enter the central nervous system as it's entering the spinal cord, here. So it's really like a single axon, as we know that most a, neurons have. And it's going to carry information from the periphery into the central nervous system, where it will synapse with neurons that are going to be the ones that are going to integrate the information. And well see concrete examples of this circuitry once we talk about the somatic nervous system. So then the central nervous system is going to integrate the information and then cause some sort of response. Which will have the first neuron in the central nervous system, and then send out the signal through the efferent pathways. So, keep in mind that we're going to have a stimulus. It is going to start a graded potential. It's going to initiate a graded potential that, if it's strong enough, will cause action potentials. So, see grade potential, you can think of it as a receptor potential, and as we've seen in the nervous system lectures, if it's strong enough, it'll initiate an action potential. And again, a single neuron, sensory neuron, is going to respond primarily to a single type of stimulus, which is going to be important for interpreting the action potentials from those neurons. Let's just briefly talk about somatose, somatosensation again, where it's going to include things, senses like the sense of touch, and being able to sense pressure. This is going to be detected by afferent neurons that have mechanoreceptors in their endings. And it's the morphology of those endings and where they're located in the skin, or in other things like in muscle that can sense stretch. Things like that. The morphology is going to determine what specific stimuli they respond to. So for instance, there are free nerve endings that are really just the end, bare end of an axon with mechanoreceptors. They're often pretty close to the surface of the skin. They're going to respond to specific types of touch compared to mechanoreceptor neurons that have a big capsule with layers of cells and fluid around that nerve ending that also contains mechanoreceptors. So because of that encapsulated nerve ending and the characteristics of the tissue surrounding it, that's going to determine what it responds to. It might be a s, a vibration of certain frequency for example. That those encapsulated nerve endings are going to respond to. And then also based on where they are, for instance, how deep they're in the skin, that will also determine what kind of, stimulus they will respond to. So keep in mind, we're not going to go into details, that we've got many different types of these touch and pressure neurons that have different morphologies and different locations that let us detect the whole range of textures and touches that we can sense. We're also going to have the system for proprioception. This is where those stretch receptors in the muscle, and in tendons and joints. So again they're going to be mechanoreceptors that are going to be channels that are gated by stretched or defamation of the membrane. That are going to be very important in proprioception and as well as the visual system. Being able to see where your body is in space, as well. Not only feeling where it is in space. And then your vestibular system, which we'll also be talking about. The organs in your inner ear that give you your sense of balance. That will also be important for proprioception. We're going to have neurons that can detect different temperatures. So we'll have different types of thermoreceptors. Some that are going to be activated at warm temperatures, some at hot temperatures, some at cold temperatures. And the reason why they respond to different temperatures is because they express different types of thermoreceptor molecules that are gon, going to be ion channels that are activated at a certain temperature. back of the retina, and interestingly the light has about these thermoreceptors is that, often, they also respond to chemicals, or to different molecules. So one example would be cold thermoreceptors, cold sensing thermoreceptors, that also bind menthol. Menthol can also open them and menthol makes, for instance if you put it on your skin, feel cold. And that's because it's acting in the same way that cold does to open cold sensitive ion channels. The same thing similarly happens for capsaicin which is what makes chili peppers hot. So capsaicin is going to bind to hot sensitive, heat sensitive thermal receptors and that's why when you eat a red chili pepper or a hot chili pepper, it's a sensation that's almost indistinguishable from thermal heat because it's acting on the same protein molecule and causing the same response. And that, that's also true for ethanol, which can cause a burning sensation as well. Then pain is going to be sensed using no nociceptors which are going to most often respond to chemicals that are going to be released by damaged cells or by immune cells that are responding to damaged cells. And in that way we can know that there's been some sort of wound or tissue damage and that we should feel pain. We're going to now move into the first of the special senses that we're going to consider: vision. And obviously when we're looking at something we are using the visual system to determine the shape and color of objects. And we'll be able to detect their movement as well, and what we're detecting are going to be photons of light. And that they're going to have different wavelengths and energies and that is going to be what we detect when we're detecting color, and we'll talk about how this happens. We can think of our eye as being very similar to a camera. Where we're going to be focusing light in the back of the eye onto the retina. And that we're going to have a lens to help us do that. And a pupil, which is like the aperture that determines how much light enters. And that it's going to involve three main steps, which is going to be that we are going to detect light that is reflected from objects, and then that light that's being reflected is going to be focused by the lens onto the retina. And then the retina will contain photo receptors that will convert the signal of the light to an electrical signal. And then that will be processed by the central nervous system. So let's look at how the eye is built, where the light is first going to hit the cornea, the outer covering, main layer of the outer por, portion of the eye. And most of the focusing is actually going to occur at the cornea, because the light is going from being, traveling through air, to traveling through a tissue, which contains a lot of water. And so, that is going to cause a refraction, or a bending, of the light that is going to accomplish most of the fac, focusing that ha, occurs in the eye. So that is an, a major role of the cornea. Then behind, after then, the light travels through the pupil. Behind the pupil is going to be the lens. Which is also going to help focus the light. It's going to be more of a fine tuning that is going to allow us to focus on things that are near versus those that are far. And we'll see how the lens is going to change shape in order to accomplish that. And then it's going to be the retina, once we focus light onto the back of it. Then it's going to contain the photoreceptor cells, rods, and cones. So let's look at how the lens changes shape. It's, that's what represented here in blue, is the lens. And, under kind of basal conditions, it is surrounded by muscle. These muscles are going to run circumferentially. As you can see in this diagram, around the lens. And they're going to be attached to the lens through zonular fibers, which are going to be little ligaments, little pieces of connective tissue that attach the lens to the muscle. If the ciliary muscles, these muscles surrounding the lens, contract, that means that they are going to get shorter, which means that the diameter of the ring that they're forming, is going to be smaller. So it's going to move in this direction. That is going to release some of the pulling from the zonular fibers and allow the lens to become more sphyrical. Less flat and more spherical. So when it's contracted, there's less tension on the zonular fibers. The lens wants to be round, and so that makes it more round, and that allows us to focus on an object that's nearer to us. In contrast, when the muscle relaxes, then that means that the muscle cells are going to get longer, which means that the ring that they are forming gets bigger and that's actually going to pull more on those zonular fibers and produce more tension. So it gets confusing because we're talking about muscle cells that relax and then that produces more tension in those ligaments. And that's just because those cells are now longer and so our diameter of the circle formed by the ciliary muscles is now bigger. And so now we have more tension and that makes the lens that wants to be spherical become more flat. And that allows us to focus on distant objects. Once light comes through the lens and then is focused on the back of the retina It's going to, it, come in contact or hit the photoreceptor cells that are sitting in the back of the retina. And then it's going to undergo the process of phototransduction, the process of converting the energy that was in that photon to an electrical signal. And so that's going to happen through some sort of photo pigment, which is what's shown here in orange. It's going to be a membrane bound protein. It's going to be a G protein coupled receptor, more specifically. And it's also going to be bound to a small molecule called retinal, which is what's shown in here, here. Which is a form of vitamin A, or based from vitamin A. And retinal is going to change it's confirmation when it is, bound, when it is struck by a photon. So that's whats shown here in the first step one, where when the retinal in the photo pigment is exposed to light, it's going to change its confirmation. That's going to cause a change in confirmation of the photo pigment protein. Which is a G protein coupled receptor, so that's going to start a signal transduction cascade. I'm not worried that you remember the details. So you don't know, need to worry about the details of these single transduction steps, but what you do need to know is that that change in confirmation of the photopigment protein is going to cause a decrease in cyclic GMP Which then causes a cyclic GMP-gated cation channel to close. So this is almost anti-intuitive, where when light hits it, you're closing an ion channel. So that means that at rest, when there isn't much light around, that this cation channel is open, which is letting cations enter the cell. Which means that at rest, the photoreceptor cell is depolarized. Again, anti-intuitive, that at rest the cell is depolarized. So that means that when you close that channel, that now the, the membrane potential is going to become hyperpolarized. So we have a graded potential but it's a hyperpolarization of the cell and that reduces neurotransmitter secretion. So it's kind of very anti-intuitive that you reduce neurotransmitter secretion from these photoreceptor cells when they're exposed to light. But that's how it works. It works, so we let's all, there's other neurons involved. So that's what this image shows. Where actually at the back of the retina, the very back, is where you have the photoreceptor cells. So, in this case we have six rods, and then there are other cells called bipolar cells and a ganglion cell that are also present there. So we have layers of neurons at the back of the retina, and interestingly the light has to travel through those other layers of neurons, as shown right here, to get to the photoreceptor cells. But again it works. The rods or the photoreceptor cells and the bipolar cells are going to generate graded potentials, that if they're strong enough will then cause the ganglion cells to generate an action potential. So that's something that's a little different. We've got many layers of neurons that are eventually going to lead to an action potential. The other thing that's important about this diagram is to see that in this case we have rods. And under those into one ganglion cell. And so this is going to allow for a low resolution aspect of the vision. Because we are summing six different rod cells and converting it into one signal. So that's going to determine the re, re, resolution. And in this case, it's going to be a low resolution, because we're taking six rods and they're feeding into one ganglion cell. That's going to be in contrast to cones, our color sensing photo receptor cells, where it's usually going to be a ratio of one to one. So each cone is, only one cone is giving signals to only one ganglion cell. And so that's going to be a, a high resolution system versus this system which is low resolution, but somewhat highly sensitive, because if a photon hits either of these six rods, it, it can send a signal to the ganglion cell. And so we're going to see that in this next figure where we're looking at different levels of light, and which photoreceptor cells are going to tend to be active in those conditions. So when we are at very low light. For instance, the amount of light when you have just stars out. Then we are not going to have color vision. Because, at this point, we are only going to be activating rods. They're going to be very sensitive, but they are going to be that low resolution sight because of that convergence onto, of six rods for instance, onto one ganglion cell. So rods are going to be used for night vision. They're not responsible of colored vision, so we can't see colors when we're in very low light, but and it's going to be low resolution but fairly high sensitivity. Because they can sense light even though its very dark in starlight. Once we're hitting moonlight, then we're at least above the threshold for cones. So at that point at least cones can sense some of that light, however color vision is still not going to be very good at moonlight. It's once we get closer to indoor lighting, where now we're starting to have enough light that we're getting good color vision because we're activating the cones more efficiently. And we're going to be really relying on the cones. And remember we said because the cone is going to be in a one to one ratio with the bipolar and ganglion cells, now we're going to have high resolution vision. When we get to this higher lev, amount of light. So how do we detect different colors? And if you think about it, it really is amazing how many different colors we can discern. And if we had to have a cone for each color that we could see, that would be an extremely inefficient system. And so luckily, we can see all the colors that we see with only three different types of cones. So these are, this is graph, a graph showing the amount of activation of these different types of cones based on the wavelength of light. So this is our curve for rhodopsin. That's not going to detect color, just detecting light, and usually at low levels of light. And then we have three different types of color sensing cones, blue cones, green cones, and red cones. So what the nervous system does is it looks at the level of activation or the amount of activation of each cone and that level of activation for each cone corresponds with different colors. So for instance if we look at yellow, right here. Then yellow is when the red cone is basically maximally activated and the green cone is about half activated. That's what our central nervous system is going to interpret as being that the object is yellow. Versus when we're over here in the blue range. And we've got basically we're only relying on our blue cones. And how much they're activated to determine the different shades of blue. Or we can look here in the green range where the green cones are maximally activated, but then the red cone is also fairly highly activated and that corresponds to this green color. So in this way, with just these three different cone types, we can detect an amazing number of colors based on this system. And this is going to be very similar in the nose with our sense of smell, where we only have a few hundred types of receptors that can bind odorants, but yet we can discern thousands, 10,000 different odors, and it's going to be through a similar sort of process. So, we've talked about just sensory receptors in general. Where they're going to detect a change in the environment, which is going to be a stimulus. And convert that energy from that stimulus into a recep, a graded potential. Which will lead to an action potential. And that's going to be a process called con, transduction. We talked a little bit about somatosensory pathways that are going to be pain, temperature, touch, vibration, as well as proprioception. And then we talked about how the visual system is going to detect the shape and color of objects and that we're going to have rods and cones, that are then going to send signals into the central nervous system.
