all right so we're going to start up now with our first video on sensory Pathways and nerves so we're still in the peripheral nervous system and so we're going to take a look at a few of the pathways that are used to communicate sensory information from the peripheral nervous system to the autonomic nervous system so that's kind of the uh the big picture topic for this chapter is communicating sensory information from peripheral to the central nervous system okay so the first uh type of sensor receptor that we're going to see in this part of the unit is uh sensory receptors that are involved in touch so exterior receptors are one type of sensory receptor um usually in animals that have exterior receptors uh they're more functional in either water or on land right so in the case of humans obviously it would be on land more so than in the water but they are specialized to perform their respective functions in the different kinds of environments and then intro receptors are a bit simpler and they're going to be involved in looking at conditions inside the body as opposed to extra receptors which look at conditions outside the body so in terms of sensory receptors there are three main categories there are mechanoreceptors these are ones that are going to be set up to detect um tactile stimulus so pressure and pain and touch and those kinds of things and we will take a look to see different subcategories of those as well as how they function we also have chemoreceptors which are going to be involved in detecting the chemical nature of various substances whether it's pH whether it's osmolarity whether it's taste or or olfaction those kinds of things are also going to be linked to chemo receptors and then electromagnetic receptors would be like our visual receptors now in other animals other kinds of animals have different types of electromagnetic receptors in our case hours are set up to detect the visible part of the electromagnetic spectrum all right so our our eyes are only set up to see um red orange yellow green blue indigo and violet although other species may be able to see into the infrared in some cases as well as into the UV parts of the spectrum all right but we start getting into some very very different kinds of animals for those that can do those sorts of functions okay so the overall process here with the sensory receptors is to essentially go through four steps the first step is going to be some type of stimulation of the sensory cells or sensory receptors there is going to be a transduction that occurs all right so anytime we see the term transduction now we know that means a change in signal so we're going from a mechanical stimulus to a neurological impulse for example that would be transduction we also then have transmission which is the actual like projecting of the signal from point A to point B not just converting it from one side to another but actually sending it through the nervous system that would be a third function and then finally interpretation would be the file function so this is where the brain perceives the stimulus as the appropriate type perhaps related to some sort of associative uh function as well and potentially you do something about it right so we can also have the the response to the stimulus that occurs as well Okay so we've already seen back when we covered the central nervous system the idea of graded potentials and gated ion channels and those kinds of things so there's not a whole lot new here at a cellular level at least in terms of the mechanoreceptors especially um all these sets of resells are stimulated when their membranes are distorted in some way in the case of mechanoreceptors at least so if there are other kinds of sensory cells they might open or close depending on other types of stimuli but in any case what they're going to do is change in some way in response to a stimulus now when that occurs that is going to cause a depolarization of that cell to a point where we might actually Reach action potential we might actually reach the threshold so the depolarization of the receptor cells are going to effectively create an epsp um and therefore become or cause the cell to become more likely to to reach action potential right so receptor potential um this is going to be created when molecules that are outside of the cell bind to receptors and as such those receptors help to create a difference in charge across the membrane okay so receptor potential is a lot like graded potential right there's a degree of sensing of magnitude here so if you have a larger receptor potential that's going to be interpreted as a more intense stimulus okay so more stimulus means more depolarization more depolarization means greater likelihood of reaching action potential and actually firing in some way um so unlike Action potentials and more so like graded potentials receptor potentials are able to stack on top of each other they are also able to cancel each other out all right so just like we saw before with the various graded potentials back in the CNS okay um the intra receptors uh in the skin we'll start with the cutaneous receptors first so these are receptors in the skin these are the ones that are sensing changes in mechanical stimuli that occur between the cells and the outermost surface of the body and the external environment okay so these particular mechanoreceptors could be things like pain receptors various types of thermoreceptors tactile receptors pressure receptors all of these are going to be located in the skin all right so we'll start off with the nociceptors nociceptors are otherwise known as pain receptors and the impulses they transmit are going to be interpreted as such okay so these are going to be sensory receptors that are going to be looking for damage that has occurred they're also going to be looking for noxious substances like capsaicin for example capsaicin is the protein which gives Peppers the spiciness the more capsaicin present the more intense the um I guess the flavor of the food is going to be perceived as being so a lot of these especially nociceptors are just free nerve endings that are located near the surface of the skin there's not really any sort of encapsulation it's just a nerve ending embedded within the skin and if there is damage or some sort of perturbation to that area of the skin that's going to cause these cells to start to fire right okay so there is what is referred to as a transient receptor potential ION channel um these again are essentially parts of cells that are able to respond to substances chemicals capsaicin in this case and so encapsation is presence there is going to be a transient receptor potential present as well so let's break that term down and see what it means transient means again it's there briefly and then it's gone and then receptor potential refers to an actual molecule bound to a receptor on the outer surface of the cell all right so if you have that um molecules bound to receptors on the outer surface of the cell then you're going to potentially have an influx of ions which is going to cause depolarization which could eventually cause action potential to occur and for this message to be set on to the CNS all right the next one we're going to look at are thermoreceptors and so these are going to pertain to both cold and heat receptors once again these are going to be located as free nerve endings in the skin and they are most sensitive to changes in temperature that might occur so again just like with the mechanoreceptors we saw before the thermoreceptors contain transient receptor potential ion channels that are responsive to hot and cold so again transient receptor potential ion channels what this means is that these are going to open up when there is a depolarization or need for depolarization coming from the rest of the cell and when these open up we have positively charged ions flooding into the cell reversal of charge action potential and then recovery like we've seen before um so cold receptors actually we have a bit more of these than we do heat receptors in our skin they are going to be activated cold cold receptors are activated under cold temperatures warm receptors are inactivated under cold temperatures and the opposite is true for warm receptors so activated under warm conditions and inactivated under cooler conditions so we have a few more of these than we do the heat receptors and they're also located more superficially in the skin which means they're going to be more easily stimulated as well as opposed to the heat receptors that are located more deep in the skin all right so the next thing we'll take a look at uh would be a couple of these somatosensory systems and from there we're going to go into some specifics and talk about some uh particular types of somatosensory systems and essentially how they're wired kind of the map of how they play out in the body more or less okay so again we're back to mechanoreceptors but we're past the nociceptors and our thermoreceptors and so now we're moving into touch receptors okay so touch receptors um are going to Fire and Fire more frequently and more effectively when they're being stimulated by some type of membrane distortion so again there's a physical pressure a physical Force presence that is going to be pushing in on the plasma membrane of these Sensory neurons distorting it and that's going to cause ion channels to open thereby causing cations to flood into the cell and cause that reversal of polarity like we've seen before okay so there's there's two types of mechanoreceptors and these the the names of these two types of mechanoreceptors actually by the way are going to be very very similar in concept to the same types of of responses when we talk about muscles right some muscles are more phasic where they're contracted and then released some muscles are more phase air I'm sorry more um sorry more tonic meaning that they're continuously activated okay so they're firing all the time if they're continuously activated so a few examples here um the meissner corpuscles and vaccinian corpuscles we'll take a look at both of these These are hair follicle receptors so if you've ever uh you know has your arm out the window driving down the road and you feel the hair on your arm moving um those would be misner and paxtonian corpuscles that are detecting that change in uh in pressure and touch um tonic receptors though are activated all the time the phasic ones are only activated under stimulation tonic ones are active all the time so the refini and the merkel's discs those are examples of tonic mechanoreceptors that are only um only turned off under very specific conditions otherwise they're going to be activated most of the time okay so mice nurse corpuscles these are again these are in the phasic so they're they're firing when there's a stimulus presence and then when the stimulus goes away they stop firing as opposed to Tonic where they're just firing all the time so these uh encapsulated nerve endings measures corpuscles are for fine touch in the skin so these sensory cells are located again fairly superficially in the skin but each one of these has a small receptive field which means that every single one of these meissner's corpuscles has a relatively small area of skin for which it's responsible it also means that these myosinos corpuscles are able to distinguish touch sensation very specifically so even if you have two pressure stimuli and relatively close proximity they may actually activate different meissner's corpuscles right because the the sensory fields of these receptors do not do not extend very far past the actual cells themselves now Miracles discs and merkel's cells on on the other hand are going to be for a different type of pressure go back a second here so the merkel's discs down here are going to be more for pressure as opposed to um General sort of more low-level tactile stimulation Myrtle's discs take quite a bit more to stimulate them okay so just a quick look here at our meister's corpuscles and our merkel's discs we see here are layers of skin right so stratum basil dermis below that hypodermis below that we have a Merkel cell which is able to detect pressure and notice too how deep in the skin this is so in order to have enough Distortion to actually trigger this cell to do something there has to be a fair amount of distortion for that to happen otherwise the Distortion of membranes is going to stop before it ever actually reaches the Merkel cell and again Merkel cells have relatively small sensory Fields or sorry meister's corpuscles have relatively small sensory Fields so you can discriminate fine touch very well on your fingertips because each one of these has a small area for which it's responsible and therefore if it's a larger area of skin affected you're actually going to be using several of your meister's corpuscles in the process all right vaccine Corp results are next these are also mechanoreceptors but these have what are referred to as encapsulated nerve endings so now we're not looking so much at a free nerve ending like we've seen before now we're looking at nerve endings that have a capsule around them these also tend to be a bit deeper in the skin as were the ones the Maestro score vessels on the previous slide they have a larger receptive field however so you're going to tend to generally have fewer of these than you will the meissner's core muscles because each one of these can handle a larger area of skin when it comes to perception of touch alright so we see here um meissner's corpuscles I mean you might have like one of them for an entire finger or part of a hand because they have relatively large sensory Fields you can exert pressure over here and still stimulate this cell to pass on that sensory information to the brain if you're careful how you do it um okay so this slide actually I kind of like this one a lot because it actually shows the layers of skin and it shows the types of sensory cells in the skin it shows what they look like sort of and it also shows you um uh so level of the skin shape um so it's just kind of a nice comparison of the different types of touch receptors so pexania we talked about graffitis we haven't talked about yet Miracles are for light touch remember they're way way up in the epidermis um free intervenings like heat and cold are a bit deeper in the epidermis even approaching the dermis meissners are for light touch again those are located closer to the surface of the skin which makes sense because they also have relatively low or small Fields as well sensory Fields as well so these are kind of our main players when it comes to touch meisners again a little more superficial strong touch over here kraus's receptors are for cold again deeper in the skin so we actually have two different kinds of receptors for cold here free nerve endings as well as krauss's cells okay so Merkel cells uh remember these are tonic Merkel cells are tonic is is contracted all the time or firing all the time and the information they're giving us is information about pressure and duration of pressure from the environment meister's corpuscles are more fine touch right so these are more in areas where there is less hair in this on the skin and raffini corpuscles are again near the surface of the skin um they are also sensitive to touch um fine touch especially but also duration and then paxilian corpuscles are very well encapsulated um they are surrounding essentially this nerve so that when these membranes are distorted and this membranes potential is reached or threshold is reached that information from these distorted membranes can be passed along the nerve to eventually get to the brain so those are kind of our our rundown on our mechanoreceptors right so we have meister's and merkels those are both relatively small sensory Fields they are both mechanoreceptors and they both allow for fine discrimination of touch inium corpuscles are a different type of mechanoreceptor they have a larger sensory field and they're involved more so for course discrimination so maybe not quite as as fine touch sensory function but more um um sensory of more intense kinds of pressure as opposed to light pressure with the paxinian core puzzles because remember they're encapsulated right so in order to get them to be distorted to actually cause this to fire there has to be a fair amount of um pressure on this to make that happen added to that this is also fairly deep in the skin so um not only do you have to exert enough pressure to to distort the membrane you also have to exert enough pressure to to reach the cell in the first place fine touch is not going to cut it all right so we have those free nerve endings again we talked about nociceptors those are pain receptors green Urban endings in the skin so again mechanical thermal chemical polymodal which just means they have multiple functions um pain can originate from different types of stimuli and these different types of free nerve endings allow us to discriminate among those different types of stimuli again are these stretch receptors in the skin so those are important for that reason krause's bulb again those are cold receptors also in the skin all right so we've talked a little bit about these um joint receptors and muscle receptors back in one of our previous chapters we're definitely going to see it again coming up soon in the muscle chapter but remember there are also receptors throughout the body located within muscles within tendons within ligaments that are there to give our body a better sense for how much stretch is occurring and if there's too much it might be time to kind of like back down the muscle power a little bit and reduce the stretching and reduce the um possibility of something being damaged sorry about that there for a second these these stretch receptors reduce the possibility of a tendon being stretched to the point where it actually breaks or tears same thing with ligaments um we have uh receptors in our joints as well so we can detect or feel when movement is not really in the direction that our joint wants us to be wants it to be it's so uncomfortable if not painful um so various structures associated with joints also have receptors to give us a sense for how much contraction is taking place how intense it is how much stretching is happening and these kinds of things um we also have once again refinis and paksinian corpuscles free nerve endings uh similar to what we saw on the skin for uh sensory perception there we have similar receptors in muscles and joints for the same kinds of Sensations now proprioceptors we did talk about a little bit before when we covered the cerebellum but proprioceptors are kind of a special type of sensory cell that's able to allow you to detect the position of your body in space without ever actually having to see where your body parts are in space all right so the example we did this before but the example where we close our note close our nose close our eyes and touch the tip of our nose with our finger it's not that hard because of the proprioceptors in our skin that are feeding information to our cerebellum and they're saying we're right about here and so the cerebellum says okay keep going a little bit further and your finger is going to land on the nose Okay so proprioception very important for understanding or or determining the body's position in space we have talked about our muscle spindles before right so again we want to protect the muscles by not allowing them to contract so hard that they could actually create an injury tear a strain that kind of a thing so muscle spindles are going to help us to limit the amount of power generated by muscles so that tendons and ligaments are not damaged and also so the muscles not damaged we have Golgi tendon organs as well we talked about those same things a second ago they are looking for muscle tension and so if muscle muscle tension is too great these sensory cells are going to allow for the reduction of that power um Okay so we've talked about this before as well a bit uh the the sort of differences in transmission speed in either large versus small axons and then myelinated versus unmyelinated axons so there really shouldn't be anything too new on this slide as we've seen before larger axons are going to allow for faster transmission it's like a larger cable allows for more electricity to flow through it larger axon more signal right now as it turns out in the case of vertebrates in humans in general or it's specifically a type of vertebrate there is not so much a really really large diameter axons as there are myelinated axons and so the vast majority of sensory cells in our body are going to have myelinated axons which allow for a more efficient and faster speed of transmission of signal okay so more myelination means more insulation which means that especially if you have different thicknesses of myelin sheaths on different neurons they may actually be transmitting at different speeds because the one that is more heavily myelinated is going to um yeah it's going to have a faster transmission speed it's also going to be less prone to injury and damage and debris and those kinds of things um okay so I think we're going to stop here on this section we'll come back we'll pick up with dermatomes this is more of a clinical application so we'll pick up with this and then we're going to start getting into some of those signaling pathways