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Matthew Barry Jensen 创建




in this video I'm gonna talk about somatosensation in the peripheral nervous system somato sensation refers to senses of the body and that includes a whole bunch of different senses but I like to think about five senses in particular which turn out to be really useful for medical purposes because we can test them on examination the first of these we call position sense by which we mean the position of body parts relative to each other for example if you close your eyes and somebody lifts your arm above your head you can feel that your arm has been moved you don't actually have to see it to know that another sense of the body is vibration sense if we come into contact with anything that's vibrating we can feel that we often use little tuning forks when we're examining patients to see if they have normal ability to feel something vibrating the next sense is touch I'm gonna write that in a different color and I'll come back to why I'm using these different colours in a minute then we have pain and temperature temperature now let me just write a little R here to represent a receptor for some one of these types of somatosensation because to be able to sense anything you need a receptor or something that can detect that type of stimulus and there are many different types of somatosensory receptors and we lump them into a few different categories the first big category of somatosensory receptors respond to physical forces so we call those Meccano receptors because they respond to mechanical stimuli Meccano receptors and receptors in the category of mechanoreceptors can detect the position of body parts relative to each other and vibration and touch and then there are other receptors for the senses of pain and temperature which I'll just represent with a big R here so somatosensory receptors that can detect noxious stimuli that can create the experience of pain we call nociceptors nose scepters and they actually can detect a number of different types of stimuli that can give rise to the experience of pain and then somatosensory receptors that can detect temperature we call thermo receptors there are mo receptors now somatosensory receptors and all these different categories can be found in a number of places throughout the body one of the big places is in the skin so here's a drawing of the skin this is the various surface of the skin with a hair coming out of it and then here are the deeper layers of the skin and there are a number of neuron axons entering the skin here in a few different places that will have some of these different somatosensory receptors types so for example for mechanoreceptors in this drawing there are a couple of mechanoreceptors drawn and they tend to have these structures on the end so for instance right here this little structure on the end of this axon that's coming into the skin that's one type of mechanoreceptor close to the surface of the skin and here's another type of McCann or receptor with this structure here at the end of this axon that's coming into the skin so these are two types of Meccano receptors that can be found in the skin and there's lots of different types of mechanoreceptors in the skin that sense all sorts of mechanical stimuli to the skin but there are also somatosensory receptors in the deep tissues way below the skin here for example in this drawing here's a muscle this is one skeletal muscle and in this illustration they've magnified one somatosensory receptors that's in the muscle it's this whole structure right here and this particular somatosensory receptor is a Meccano receptor that detects stretch of skeletal muscle so in this this skeletal muscle is stretched this receptor can detect that and send that information back to the central nervous system through neurons and these types of receptors like this one and muscle along with other ones in tendons and in the capsules around joints are very important for position sense because they can send information back to the nervous system about the relative position of body parts whereas some of these mechanoreceptors in the skin or the tissues just closer to the skin are often more important for detecting things like vibration and touch nociceptors that detect pain and thermo receptors that detect temperature usually don't have these sorts of structures at the end of an axon that's going to carry that information back to the central nervous system instead they're usually like they've drawn here where there's an axon coming into the skin right here and at the end there is no structure the axon just ends in some uncovered terminals let me circle that here and these are usually called bare nerve endings because they are not covered by any kind of structure like these guys are and so nociceptors detecting noxious stimuli that can cause pain and thermoreceptors detecting temperature tend to be these bare nerve endings now once one of these somatosensory receptors detects the stimuli that it's specific for it's going to send that information back in axons of the peripheral nervous system and these are going to be axons carrying information from the periphery through nerves in the peripheral nervous system back into the central nervous system so these are a type of affair in tirana carrying information into the central nervous system and these afferent neurons carrying somatosensory information we call somatosensory neurons most of these neurons have their somas in ganglia close to either the spinal cord or if they're entering the brainstem their somas will be in ganglia close to the brainstem and the different types of somatosensory information tends to travel in different types of somatosensory neurons so that position sense information vibrations sense information and some of the touch information tends to travel in certain somatosensory neurons like I've drawn here in blue while pain information temperature information and the rest of the touch information tends to travel in different types of somatosensory neurons that also bring information back to the central nervous system one of the big differences between these different types of somatosensory neurons is how big their axon is and how much myelin there is on the axon so that this type of somatosensory neurons carrying information about position sends vibration sense in some touch sense tends to be in large diameter axons so let me just draw a couple of lines far apart to represent a large diameter axon and these axons tend to have thick myelin sheaths so the Schwann cells that are creating the myelin sheath on these somatosensory axons tend to be wrapped around with many many layers so that they're very thick so I'll just draw these orange circles to kind of represent the thick myelin sheath on this type of somatosensory axon whereas these other types of somatosensory neurons carrying the pain and temperature information as well as the rest of the touch information tend to have small diameter axons so let me just draw a much thinner axon here like this let me draw two of these actually they're both gonna have these thinner neuron axons with a smaller diameter and then they'll either have a thin myelin sheath so let me just write some thinner circles here to represent a thinner myelin sheath with less wrapping of Schwann cell membranes around the axon of these somatosensory neurons or they'll have no myelin sheath at all they will be unmyelinated axons and since axons with a larger diameter and a thicker myelin sheath conduct action potentials more rapidly these types of somatosensory neurons will conduct action potentials fast whereas the somatosensory neurons that have small diameter axons and thin myelin sheaths or no myelin sheath at all will conduct action potentials more slowly that information will get there but it'll take a longer time to get back from the receptor in the periphery to the central nervous system and the sense of touch is a funny one because it travels in both these rapid somatosensory neurons and these slower somatosensory neurons but what tends to happen is that very precise touch information that we call the fine touch sense tends to travel in the fast somatosensory neurons whereas less precise touch information that we usually call gross touch sense tends to travel on the slower somatosensory neurons so in later videos we'll get more into Samana sensation in terms of what happens to this information once it gets into the central nervous system but this is a general overview of how some a docent occurs in the peripheral nervous system