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in this video I'm gonna talk about the somatosensory tracts somatosensory which are the senses of the body and tracts which are collections of axons traveling together through the central nervous system and the different types of somatosensory information tend to travel in different pathways as they move through the central nervous system and in general the different types of somatosensory count into two big categories the first includes position sense position sense vibration sense vibration and fine touch sense or very precise touch sense information fine touch and the other big grouping of types of somatosensory information that tend to travel together includes the sense of pain pain sense temperature sense temperature and what we can call gross touch sense or less precise touch sense information gross touch now recall that somatosensory information from most of the body is going to travel back to the central nervous system through nerves in the peripheral nervous system and then spinal nerves that'll enter the spinal cord so for example in this category let's say we have a receptor that carries some position sense information so let me just draw like an R here in the arm for a receptor for position sense and then that information is gonna travel in nerves of the peripheral nervous system and then spinal nerves to enter the spinal cord and deliver that information into the spinal cord and the same thing with this other big category of types of somatosensory so let's say we had some sort of receptor here down in the leg I'll just write an R for a receptor that can detect noxious stimuli if it can cause the experience of pain then that information can travel through nerves of the peripheral nervous system and then spinal nerves to enter the spinal cord and deliver that information into the central nervous system now the same is true for somatosensory information from the face and other parts of the head but that information will enter usually the brain stem through cranial nerves instead of entering the spinal cord through spinal nerves so for instance if we have a receptor that can detect vibration somewhere here on the face that information could travel through a cranial nerve into the brain stem and if we had another receptor here in the face that could detect say temperature one of these other types of somatosensory at also could travel through a cranial nerve and enter the brainstem but now what happens once this information is delivered into either the spinal cord or into the brainstem so I've taken the brain and the spinal cord and I've drawn a little bigger illustration over here and we're still looking at it from the front but I've kind of cut into the brain here so we're see on the inside of the brain and the inside of the spinal cord and first let's consider this category of different types of somatosensory that includes pain sense and temperature sense and gross touch sense so let's say like I've drawn here we we have some of that coming in from the leg on one side let me just draw an arrow showing that information entering the spinal cord down low here inside the spinal cord neuron axons are gonna carry that information up to the brain in one of these somatosensory tracts one of the tracks that's specific to this category of types of somatosensory evoked a low one important detail is that it actually crosses to the other side and then goes up through the spinal cord and up through the brainstem and then it comes to a place kind of deep down in the cerebrum we'll talk about a little bit more in another another one of these videos but notice that it's on the other side it's actually coming into the cerebral hemisphere on the other side from the part of the body that the receptor was on and the same is going to be true for this other category of types of somatosensory and vibration and fine touch sense so let's say just like I drew some of that information coming in from an arm on this side let me just draw an arrow here showing that that's also coming in the spinal cord but a little higher up on the spinal cord now there'll be a different tract that's gonna carry that information up and it's gonna cross in a little different place a little higher up in the brainstem and then it's also going to keep going up and it's gonna come to the same place deep down and one of the cerebral hemispheres on the other side from where that information came in from the body now the pain temperature and gross touch sense from the face and some other parts of the head can come into the brainstem through cranial nerves and then that information is actually going to take kind of a funny pathway that actually goes down first and then crosses and then goes up to about the same place or pretty near the place that this information from the rest of the body came from and lastly position sends vibration sense and fine touch sense from the face and some other parts of the head will also come into the brainstem through cranial nerves and it will also cross over to the other side and go to about the same place deep in the cerebral Hemisphere now in this place deep in the cerebral Hemisphere all these different types of somatosensory information are going to come back together they're going to be very close to each other now and then they're gonna stay together pretty close as they send that information on to areas of the cerebral cortex areas on the outside of the cerebral Hemisphere that will do more processing of that information so because the somatosensory tracts carrying these types of sensory information through the central nervous system have this sort of Anatomy this sort of structure one of the big consequences of this is that if there's some sort of injury to one of the cerebral hemispheres let me just draw a big orange arrow through this cerebral hemisphere to indicate that there's a injury or some kind of disease has affected this side of the brain then what we often see with an injury to one side of the brain is the other side of the body can have somatosensory loss because all of these pathways carrying somatosensory information crossed from one side over to the other side so if we're looking from the front and this is the left cerebral hemisphere that has an injury we could see loss of somatosensory neurons somatization on the right side of the face and the rest of the body depending on how much brain tissue is injured and how much of these somatosensory pathways are affected and I think you could also see that since all these different pathways have slightly different parts of the central nervous system that they travel to abnormalities and some different parts the brain stem or the spinal cord could affect some parts of somatosensory not others depending on where the area of abnormality is so we'll save a lot of the details of the exact anatomy and the places these cross for other videos but I just wanted to introduce this idea of these somatosensory tracts and why we often see injury to one side of the brain causing somatosensory loss or abnormalities on the other side of the body