This is the lecture video for the special senses. In lecture what we'll be doing is looking at the introduction to the sensory receptors, including the tactile receptors, we'll examine the sense of gustation and the sense of olfaction, and we'll save for lab the topics of vision and the anatomy of the eye and the anatomy of the inner ear which is responsible for the senses of equilibrium and for hearing. We'll first get into some of the common terminology for the general and special senses. Our bodies are constantly exposed to sensory information that are called stimuli, that's the plural of stimulus. Sensation is the activation of receptors for stimuli, and our conscious awareness of these stimuli is called sensation. Stimuli are detected by sensory receptors in our body. Sensation is the activation of those sensory receptor cells at the level of the stimulus, and perception is the central processing of sensory stimuli into a meaningful... into a meaningful pattern. Perception is dependent on sensation, but not all sensations are perceived. Let me give you an example. We don't often perceive stimuli that remain relatively constant over prolonged periods of time. For instance, if you're in a room with a ticking clock, at first you perceive that sound, but after a while you no longer perceive that sound, even though that stimulus is still occurring. Another factor that affects sensation and perception is attention. Attention plays an important role in what's perceived and what's sensed. Imagine that you're at a party, full of music, chatter, laughter. If you get involved in an interesting conversation with a friend, then you'll probably tune out all the background noise. If somebody asked you what song had just played, you would probably be unable to answer that question, even though those sound waves were still traveling through the air, activating those receptors, you weren't perceiving it, you weren't paying attention. These sensory receptors are located throughout the body, and there are receptors for general senses, which are located all over the body. These are receptors for things like temperature, pain, touch, stretch, and pressure. We have receptors that are specialized for each one of those general senses. There are special senses that take place in specialized organs; these are senses such as gustation, olfaction, vision, equilibrium, and hearing. Here's the basic model of how the sensory system works. There's some stimulus, something that you detect in the external environment, this causes a change in a receptor. That receptor can be as simple as the dendrites of a neuron or something similar. Remember those are the receptive parts. These sensory neurons send then a signal to the central nervous system, where it causes a change in a relay neuron, and that might cause you to do some action, and that would be caused by an effector, which is stimulated by a motor neuron to give a response. So when we're looking at the senses, we're looking at this afferent branch up here. We'll look at some properties of sensory receptors. Receptors exhibit adaptation: that is, with prolonged exposure to a constant stimulus, they decrease action potential firing. Tonic receptors are those that receive and process stimuli constantly, at a constant rate. With continued exposure, sensitivity to the stimulus remains constant. For instance, balance receptors in the ear would be constantly firing, receptors for proprioception, that is, where your body is in space, or your receptors for pain, these all show limited adaptation; they're tonic receptors. Phasic receptors quickly detect a new stimulus or a change in a stimulus that's already been applied. For instance, tactile receptors in the skin, and with a new or a changed stimulus, they'll produce a response, but with continued exposure, sensitivity to the stimulus diminishes, resulting in adaptation. For instance, deep pressure receptors exhibit this phasic receptor type, they exhibit adaptation, they help you forget that you're sitting in a chair right no,w or mechanical receptors in the skin help you forget that you had socks on until just now. You can also get used to smells! Imagine that you're in Pike's Place Market in Seattle, which is where they're always throwing the fish over people's heads! You would probably have, at least at first, a really strong perception of a fishy smell, but if you worked there, you would probably get accustomed to it, you would become adapted to it, and you would no longer perceive that stimulus, that smell of fish. But adaptation isn't of the entire sense, it's just to the one stimulus. Even if you got used to that smell of fish, if somebody walked by smelling like Axe body spray (tm) you'd still be able to smell that! So adaptation is just to one stimulus. Receptors can be classified in a few different ways. We have general sense receptors that are located throughout the skin and throughout the organs, and the special sense receptors are housed in complex organs, which are in the head. Why do we have so many special senses in the head? It's so we can see where we're going. There are three criteria that are used to describe receptors: that is, their distribution, the origin of the stimulus, and the modality of the stimulus. We'll look at receptor distribution first. There are sensory receptors can be classified according to their distribution. The general sense receptors are found throughout the body. They can be somatic, found in the skin, the mucous membranes, lining body cavities, joints, muscles, and tendons, where they monitor a variety of stimuli, including texture, pressure, pain, vibration, stretch, whatever. Then visceral receptors are in the walls of internal organs and blood vessels. These are the same receptors, but they're located inside of the body. They would detect stretch, temperature, pain, things like that. Specialized sense receptors are special sense receptors. They're complex sense organs located in the head that are responsible for smell, taste, vision, hearing, and equilibrium. If we categorize receptors based on the stimulus origin, they can be exteroreceptors that detect stimuli from an external environment. This includes receptors in the skin and the special sense organs. The interoreceptors detect stimuli in internal organs, they include stretch receptors in smooth muscle as well as receptors for pain, pressure, temperature, and chemical changes, and the proprioceptors are sort of a mix of exteroreceptors, outside world, and interoreceptors, inside world. It's a stimulus pertaining to body position. These are found in muscles, tendons, and joints. This is how you know that your elbow is bent and how much it's bent, is proprioception. Lastly, receptors can be classified according to the type of stimulating agent that stimulates those receptors. Chemoreceptors detect specific molecules that are dissolved in fluid. Your senses of smell and taste rely on chemoreceptors. Thermoreceptors detect changes in temperature. These are located throughout the skin where they can detect hot and cold. Photoreceptors respond to light; they're found in the eye, where they detect changes in the intensity, color, and position of light. Mechanoreceptors are receptors that detect changes in the shape of the plasma membrane of a cell. These are found in the skin and in the inner ear where they detect touch, pressure, vibration, and stretch. A subcategory of these would be baroreceptors, which detect pressure changes within body structures. The last type is the nociceptor, which is the pain receptors throughout the body. They detect chemical damage, they detect painful stimuli. We'll take a look at some of these tactile receptors. Tactile receptors are the most numerous type of sensory receptor. They could be exteroreceptors, detecting pressure from the outside world, or interoreceptors. They are mechanoreceptors, they react to touch, pressure, and vibration. They're a general sense receptor, they're located in the dermis, and in the subcutaneous layer, and there are two types. There are unencapsulated tactile receptors; these are simply the dendritic endings of neurons that are present in the skin. They would respond to a very light touch. Or, they can be encapsulated. These are more complex dendritic endings that are wrapped in connective tissue or wrapped in glial cells. It takes more touch pressure or vibration to stimulate these encapsulated dendritic endings. We looked at some of these when we looked at the integumentary system. We looked at some encapsulated tactile receptors. We looked at the tactile corpuscle, also known as the Meissner's corpuscle, which is right here in those dermal papillae, really close to the surface. We also looked at the lamellated corpuscles, or Pacinian corpuscles, that are located deep in the dermis down here by that subcutaneous layer. Also present inside of the skin are unencapsulated tactile receptors, including free nerve endings, which are essentially just dendritic endings, remember those receptive parts of a neuron. There are tactile discs right at the border of the epidermis and the dermis, and the root hair plexus is nerve endings that are wrapped around the root of a hair. These allow you to detect even lighter touches than some of these more superficial unencapsulated receptors do. For instance, if there's a bug up here walking on your arm, it would be bending those hairs and causing a change in the root hair plexus, so these hairs and the associated root hair plexus lets you detect a very light touch or something that's just passing over the surface of the skin. Proprioception involves the muscle spindle and the Golgi tendon organ. Proprioception is the sense of movement and the sense of body position. In order for the nervous system to effectively control skeletal movements, it has to receive continuous sensory feedback from the muscles. This feedback is proprioception, and it consists of sensory detection of muscle tension, and this is via the Golgi tendon organ, which detects stretch and tension at tendons, and changes in muscle length which is accomplished via the muscle spindles that are present in them, wrapped around the muscle fibers. The muscle spindle determines the length of a muscle or, in other words, how contracted it is. It is a proprioceptor, it's a sense organ that's receiving information from muscle, it's sensing stretch and the speed of the stretch. When you stretch and you receive that message that you're done stretching, you can't stretch anymore, you're at the end point of the stretch, that's the spindle sending a reflex arc signal to your spinal column telling you not to stretch any farther or you'll hurt yourself. The Golgi tendon organ determines how much load is on a particular limb. It's a proprioceptor, it's a sense organ that's receiving information from the tendon, and it's sensing tension. When you lift weights, the Golgi tendon organ is the sense organ that tells you how much tension the muscle is exerting. If there's too much tension, then the Golgi tendon organ will inhibit the muscle from creating any force, protecting you from injuring yourself. If we look at the visceral senses, these are mechanoreceptors, they're interoreceptors, and they're the same types of receptors that are found in the skin. We can have free nerve endings, they're going to have Meissner's corpuscles or uh, or Pacinian corpuscles. These are also present inside of the body. For instance, if you were to swallow an entire unchewed burrito like this person has done, here there are senses that are sensors that are present within the lining of the stomach that would let you know that there's pressure, you're feeling a feeling of fullness. The perception from these visceral senses can be referred to the surface. They're referred to...what does it mean for it to be referred to the surface? It's where you feel pain or pressure on the surface of the skin when actually this is pain that's coming from inside of the body. This is called referred pain. It's most famous for, you know, if you are having a heart attack, sometimes people say that they have pain that's felt in the neck, the shoulders, the back, the left arm, rather than in the thorax at the chest which is the actual sight of injury. Another example of this would be the ice cream headache or "brain freeze" and that's when we have the vagus nerve or the trigeminal nerve in the throat or in the palate, they're transmitting pain signals, but it's actually because of the rapid cooling and rewarming of the capillaries in the sinuses. Why is it that we get pain that's referred to the surface when it's actually occurring inside of the body? There's a hypothesis that suggests that we have receptors in the skin and receptors in the viscera and that they converge at the same spot on the spinal cord. Here we have afferent sensory pseudo-unipolar neurons that are converging at the same spot on the spinal cord, and that they're then taking the same path up to the brain to the thalamus, where they'll be passed on to higher order areas of the cortex, and that because they're taking this same route, that the brain gets a little bit confused. We get wires that get crossed. If there's a signal that's being sent along the same path, it's much more likely you've experienced a lot more pain from your skin than from your heart in your life, so your brain interprets a signal coming from that particular route as coming from the skin, because of this convergent pattern.