Welcome back. We're going to shift from talking about the nervous system in general to talking about the senses. We will talking about the afferent pathways that brings information into the central nervous system. We will apply a lot of the principles that we learned when we were talking about the nervous system in general. In this session, we will discuss the senses in general for a few minutes and then we focus on vision. We will start off talking about general principals about the senses. And we're also going to spend a little time talking about somatosensation, which, if you remember, is the senses of the body. These come from receptors that are in our skin, in our joints, in our muscle, and other connective tissue. We are going to sense pain, touch, certain vibrations, pressures, as well as temperature. This will also allow us to be able to use proprioception, which is where we can determine where our body is in space. We can do that through some of these stretch, for instance, receptors in the muscles or receptors in connective tissue and joints. This will also involve vision, as well, in determining where we are in space, but we don't have to rely on vision. Then we will focus the vast majority of our time when we talk about the senses, dealing with the special senses. Special because they have a special organ that is dedicated to the detection of the stimulus. The receptors and the special senses are confined to a specific organ. They are 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. Then also don't forget that in the process of sensing, our body is detecting things like the oxygen content of our blood, the pH of our blood, and as well as the osmolarity of our blood. These are referred to as visceral stimuli. Our body is going to integrate these signals and respond to those signals in a very similar manner that it responds to other types of sensory information. Let's talk about some general principles about the sensory portion of the nervous system. Here we are going to have sensory information that reaches 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 the action potentials. So it's going to be action potential frequency that is going to tell us how strong the stimulus is. If the stimulus is stronger, they'll be more frequent action potentials. Another important element is the idea that sensory neurons are going to respond to primarily one type of stimulus. This is gonna 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 heat sensing neuron that's firing frequently, since that's the main stimulus that that neuron responds to, then the central nervous system knows, that 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. So that's gonna be another common theme that we're gonna be talking about., These neurons are gonna be specific for a specific type of stimulus. Then, another process in sensing is going to be converting that into the perception. What causes this sensation, how do we interpret it, and what does it mean. Then a third principle that we're not gonna 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 undergo adaptation. This occurs when they have a decrease in sensitivity. They'll have decreased action potential frequency with the same stimulus. Some systems are gonna 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. We actually filter out the vast majority of the sensory information that we collect. You can think about that right now, if you're sitting and 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 gonna be involved. So we know that these are going to be afferent pathways and we're going to have an afferent sensory neuron shown here in red. Let's say, it's a pain neuron that senses pain stimuli. It is going to have a unique morphology compared to the neurons that we've already talked about. Sensory neurons are going to be sitting in these structures called ganglia, that are right outside of the central nervous system. These are a collection of neuronal cell bodies outside of the central nervous system in the peripheral nervous system. That's what's shown right here. It's call the dorsal root ganglion. It's on the dorsal side of the spinal cord. This neuron has a unique morphology. It's called a pseudounipolar neuron. 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. If the stimulus is strong enough, it's going to send action potentials down the axon, as this arrow is showing. The action potentials 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 neurons have, and it's going to carry information from the periphery into the central nervous system where it will synapse with neurons that can integrate the information. We'll see concrete examples of this circuitry once we talk about the somatic nervous system. Then the central nervous system will integrate the information and cause some sort of response. This first neuron will be in the central nervous system. It will then send out the signal through the efferent pathways. So, keep in mind that we're gonna have a stimulus. It is going to start a graded potential. It's gonna initiate a graded potential that if it's strong enough will cause action potentials. So, the graded 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 will initiate an action potential. Again a single neuron, sensory neuron, is going to respond primarily to a single type of stimulus, which is gonna be important for interpreting the action potentials from those neurons. Let's just briefly talk about somatosensation again. It's going to include senses like the sense of touch and be able to sense pressure. This is going to be detected by afferent neurons that have mechanoreceptors in their endings. 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 bare end of an axon with mechano receptors. They're often pretty close to the surface of the skin. They're gonna 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 tissues surrounding it, that's going to determine what it responds to. It might be a vibration of a certain frequency, for example. That those encapsulated nerve endings are gonna respond to. Then also based on where they are, for instance, how deep they are in the skin, will determine what kind of stimulus they will respond to. So keep in mind we're not gonna go into the details. Just know that thers are many different types of touch and pressure neurons that have different morphologies and different locations. THey 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 the stretch receptors in muscle and in tendons, and in joints. Again, they're gonna be mechanoreceptors. They are channels that are going to be gated by stretch or deformation of the membrane. These are very important in proprioception. Here we use as well the visual system, so that we are able to see where our body is in space. Not only feeling where it is in space. Your vestibular system, which we will also be talking about, the organs in your inner ear give you your sense of balance. This will also be important for proprioception. We're gonna 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 going to be ion channels that are activated at a certain temperature. The interesting thing about these thermo receptors is that often they also respond to chemicals or to different molecules. So one example would be cold thermoreceptors. Cold sensing thermo receptors also bind menthol. Menthol can open them.For instance if you put menthol on your skin it feels cold because it's acting in the same way that cold does to open cold sensitive ion channels. The same thing happens for capsaicin which is what makes chili peppers hot. Capsaicin binds to heat sensitive thermoreceptors. That's why when you eat a red chili pepper or hot chili pepper it's a sensation that's almost indistinguishable from thermal heat. It acts on the same protein molecule and causes the same response. That's also true for ethanol which can cause a burning sensation as well. Then pain is going to be sensed using nociceptors, which most often respond to chemicals released by damaged cells or by immune cells that are responding to damaged cells. 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 now going to move into the first of the special senses that we'll consider, vision. 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. What we're detecting are be photons of light. They're have different wavelengths and energies. That is how we detect color. We'll talk about how this happens. We can think of our eye as being very similar to a camera. Where we focus light in the back of the eye on to the retina. We have a lens to help us do that and a pupil which is like the aperture that determines how much light enters. Vision involves three main steps: we detect light that is reflected from objects. Then that light that's being reflected is focused by the lens onto the retina. And then the retina which contain photoreceptors, will convert the signal of the light to an electrical signal. Then that will be processed by the central nervous system. Let's look at how the eye is built. The light is first going to hit the cornea, the outer covering, main layer of the outer portion of the eye. Most of the focusing is actually going to occur at the cornea. The light is going from traveling through air to traveling through a tissue which contains a lot of water. That is causes a refraction or a bending of the light. That is going to accomplish most of the focusing that occurs in the eye. That is a major role of the cornea. Then, the light travels through the pupil. Behind the pupil is the lens, which is also going to help focus the light. It is going to fine tune the focus. It is going to allow us to focus on things that are near versus those that are far. We'll see how the lens is going to change shape in order to accomplish that. Then there is the retina. Once we focus light onto the retina this contains the photoreceptor cells, rods and cones. Let's look at how the lens changes shape. That's what's represented here in blue, the lens. Under basal conditions, it is surrounded by muscle. These muscles are run circumferentially, as you can see in this diagram, around the lens. They are attached to the lens through zonular fibers, which are little ligaments, little pieces of connective tissue that attach the lens to the muscle. If these ciliary muscles, these muscles surrounding the lens, contract, that means that they get shorter, then the diameter of the ring that they're forming is going to be smaller. It moves in this direction. That is going to release some of the pulling from the zonular fibers and allow the lens to become more spherical. Less flat and more more spherical. So when it's contracted, there's less tension on the zonular fibers. The lens wants to be round, and that makes it more round. That allows us to focus on an object that's near to us. In contrast, when the muscle relaxes, then that means that the muscle cells are longer. Which means that the ring that they're forming gets bigger. That's actually going to pull more on the zonular fibers and produce more tension. It gets confusing because we're talking about muscle cells that relax, and that produces more tension in those ligaments. That's just because those cells are now longer, so the diameter of the circle formed by the ciliary muscles is now bigger. So now we have more tension. That makes the lens that wants to be spherical become more flat. Flattening the lens allows us to focus on distant objects. Once light comes through the lens, it then is focused on the back of the retina. It's going to come in contact, or hit, the photoreceptor cells that are sitting in the back of the retina. This starts the process of phototransduction. The process of converting the energy that was in that photon to an electrical signal. That's going to happen through some sort of photopigment, which is what's shown here in orange. It's gonna be a membrane bound protein, it's gonna be a G-protein coupled receptor more specifically. It's also going to be bound to a small molecule called Retinal, which is what's shown in here, Retinal is a form of vitamin A, or is based from vitamin A. Retinal is going to change its confirmation when it is struck by a photon. That's what's shown here in this first Step 1. When the Retinal in the photopigment is exposed to light, it's going to change its confirmation. That's going to cause a change in confirmation of the photopigment protein, which is a G-protein coupled receptor. That's going to start a signal transduction cascade. I'm not worried that you remember the details. You don't need to worry about the details of these signal transduction steps. What you do need to know is that that change in conformation of the photopigment protein is going to cause a decrease in cyclic GMP. Which then causes a cyclic GMP-gated cation channel to close. This is almost anti-intuitive. Where when light hits it, you're closing an ion channel. 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, So 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 membrane potential is going to become hyperpolarized. We have a graded potential, but it's a hyperpolarization of the cell. It reduces neurotransmitter secretion. It's very anti-intuitive that you reduce neurotransmitter secretion from these photoreceptor cells when they're exposed to light, but that's how it works. [LAUGH] It works so, there are other neurons involved. That's what this image shows. 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 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 in this case six rods that are feeding 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 resolution. In this case, it's going to be a low resolution because were taking six rods and they're feeding into one ganglion cell. That's gonna be in contrast to cones, our color sensing photoreceptor 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 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 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. 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 a six rods, for instance, onto one ganglion cell. So rods are going to be used for night vision. They're not responsible for color vision. We can't see colors when we're at very low light. But 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 at least we're above the threshold for cones. So at that point, these cones can send some of that light. However, color vision is still not going to be very good 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. We're going to be really relying on the cones. Remember we said because the cone is gonna 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 amount of light. So how do we detect different colors? If you think about it, it really is amazing how many different colors we can discern. If we had to have a cone for each color that we could see that would be an extremely inefficient system. So luckily we can see all the colors that we see with only three different types of cones. This is a graph showing the amount of activation of these different types of cones based on the wavelength of light. This is our curve for rhodopsin that's not going to detect color, just detecting light and usually at low levels of light. 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 amount of activation to 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 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 basically we're only relying on our blue cones and how much their 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. 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, ten thousand different odors. It's gonna be through a similar sort of process. We've talked about just sensory receptors in general, where they're going to detect a change in the environment, which is gonna be a stimulus. And convert the energy from that stimulus into graded potential, which will eventually lead to an action potential. And that's gonna be a process called transduction. We talked a little bit about somatosensory pathways that are going to be pain, temperature, touch, vibration, as well as proprioception reception. And then we talked about how the visual systems is going to detect the shape and color of objects. And that we're going to have rods and cones, that are send signals into the central nervous system.
