Transcript for:
Understanding Spinal Nerves and Cord

okay so this lecture is going to focus in on the spinal nerves so we should have just covered the uh the brain and the cranial nerves and so now we want to we're still firmly within this central nervous system at least for uh most of this lecture but it this is going to build off of the uh the trailing end of the cranial the brain and cranial nerves lecture and talk about spinal nerves talk about the spinal cord in general its organization and whatnot and then how it interacts with the peripheral nervous system so the spinal cord is that vital link that we see between the brain and the rest of the body uh in it of itself the in in certain cases the spinal cord is actually a is is independent that is it functions independently of the brain to make certain decisions that that don't need to go to the brain so we'll we'll talk about those as we get there right but otherwise it is that link that brings information uh typically again we're talking about sensory information coming up into the brain right to be integrated to be processed to be uh used to respond to and then motor information leaving the brain through the spinal cord and out to the bral nervous system so that a response can be made a response in terms of muscle movement or glandular secretion now uh on the actual spinal cord we see that its external surface has two longitudinal depressions and I can point them out to you here so here is one of those depressions here and then we see that other depression here now this is the posterior side of the uh spinal cord you can tell because this is the spinal process or spinus process of the actual vertebrae and so obviously that has to stick out posteriorly and therefore that this depression here is the posterior median sulcus right that is the term posterior median sulcus the term dorsal is also used here so it can be called posterior median sulcus it can also be called dorsal median sulcus uh I will always use posterior so posterior median sulcus so we see that here and then this one here obviously by extension if that's posterior then this depression over on this side of the spinal uh spinal cord is going to be the anterior median fissure or sulcus you may use the term fissure or sulcus with respect to these terms interchangeably uh again I always refer to it as the sulkus but whatever so this is the anterior anterior median fissure or sulcus uh sometimes it's also referred to as the vental median F uh sulcus or fissure but I won't do that I'll refer to it as the anterior median sulcus now the function of the spinal cord again as we've talked about is that it's a pathway for sensory information coming into the central nervous system it's also a pathway for motor responses leaving the central nervous system it's also responsible for reflexes and so we're going to talk about reflexes and that's when we get kind of heavy into talking about the independence of the spinal cord in certain cases now we look at regions of the spinal cord we associate it with again those those types of vertebra that we see in the vertebral column and so the spinal cord has 31 pairs of spinal nerves that connect the central nervous system to all the periphery so that is the muscles The receptors the glands uh in the cervical region we have eight cervical spinal nerves right so C1 through C8 is the numbering I'm not going to ask you to number any of these I just want you to know that there are in fact eight of them and this region is continuous with the medulla of Lata the thoracic region has 12 thoracic spinal nerves so T1 through T12 so again 8 + 12 here that's 20 so far that we've covered the lumbar region has an additional five lumbar spinal nerves L1 through L5 so and then again we can see on on this figure C1 through C8 T1 through T12 and now we've already covered L1 through L5 and that leaves us with the sacral region this is an additional five spinal nerves that we see down here highlighted in pink S1 through S5 and then finally the coxal region right so the coxal region is the most inferior tip of the spinal cord and we see that that coxy geop pair right here at the very base now the something that's kind of interesting that I've always found kind of interesting is that the spinal cord itself is actually shorter than the vertebral Canal that it lies within so you you think about it there's this there's this cord that that sits inside this tube and the cord itself is is actually shorter than the entirety of the tube the canal that it sits in and as a result there that poses a little bit of an issue for movement so we're going to talk about that um there are specific structures on the spinal cord that we expect you to know uh the first one here is what we call the conus medelis right and this is the tapering cone like end of the spinal cord and so we see see this conus meelis right here now right off of the con uh the conus meelis this tapering and you see this kind kind of fray set of threads that are seem to be coming off and this very much looks if you kind of look at it minus the the fact that it looks like actual tissue it kind of looks a little bit like a horse's tail and that's an important analogy to make here because these set of axons that are running inferiorly out to the legs uh this is known as the C aquina literally H's tail and these are axons that run inferiorly off of the conus meelis uh to go down to the leg and then the uh the very uh very inferiormost portion of the pelvic uh region now within the C aquina right so within this horse tail like structure is a single and you can tell here it's a slightly different colored structure it's almost a little bit whiter or grayer here this is called the film terminali and the film terminali itself is simply a thin strand of pamer so it's an extension of that pamod that we've talked about uh in the brain in one of those meninges and this is actually the anchor so this actually anchors the Kus melis to the cockx at the very very base of the vertebral column and so this allows us now to make sure that when we say bend over right so we Bend forward right or we Bend backward doesn't really matter either way although it's obviously much more drastic if we Bend forward when we Bend forward that the spinal cord itself doesn't slip upward and forward in the spinal in the vertebral Canal because if that were to happen if you can imagine for a moment bending that that that tube and when you do so the spinal cord itself or that that cord on the inside of that tube actually changes position it would now begin to pull on all of the actual spinal nerves and this would be very bad this would be very very bad for us so that's what that film terminality does it serves as an anchor now there are meninges in the spinal uh cord as well and these are continuous with the cranial meninges for all in tons of purposes they are the exact same meninges and so this is the point that I want to uh kind of really drive home right now we're going to start from the outermost layer The Superficial most layer and work our way deep just like we did with the the meninges and the bur in the skull so obviously the very first layer of protection on the very outside of this whole structure is the ver the vertebra the bone themselves right and so the vertebral uh bones there are actually that first layer and just deep to the vertebral coms is a space right that little space between that and the first menines is called the epidural space and most of you I'm sure have heard about this many of you probably have some kind of experience with this one where or the other the epidural space right it's named there's a procedure that's named specifically for it the epidural so women that are in the process or in labor right can elect to get an epidural this is a shot that's given uh between the spaces of the vertebra and it's this shot is a local anesthetic uh at the uh um at a relatively low point on the spinal cord given in that epidural space and the idea is that it delivers this uh anesthetic to the the inferiormost cranial nerve so that we see a great deal less pain for the the woman giving birth right should she elect to get it now it's not required it's not necessary by any means but it can make the whole process much more comfortable and so again that's the term of the EP that's the significance of that epidural space now just deep to the epidural space we see the the duramater which we've already discussed same duramater that's on the brain deep to that is the subdural space again exact same structure deep to that is the arachnoid modder or arachnoid same structure here as well then the subarachnoid space and then finally the pamer so it's really only the vertebra and epidural space that differ here now there is gray matter in the spinal cord just like there is gray matter in the the brain and there is white matter in the spinal cord again just like there is white matter uh on the brain however in this case it's kind of interesting this is a very flip-flopped scenario so where on the brain the vast majority of the gray matter that we see is located around the external portions of the brain on the cortex of the cerebrum for example the cortex of the cerebellum well here the the actual gray matter in the spinal cord is actually innermost or interior most and we see that right here this is that gray matter structure right and it's shaped like an a like an H forgive me shaped like an H it's shaped like a butterfly it's kind of another uh more common term that's usually the way I refer to it as a butterfly because it is kind of shaped that way now this gray matter itself is actually divided into the following components so we see some areas that we actually actively identify now I want to make it a point to to again Orient you this is the anterior surface of the spinal cord as indicated by the anterior median fissure here and so if you see the the bottom of These Wings Are Much More rounded here and here and this very horn here or here they're called horns they're points basically here or here these are the anterior horns and then the structures just lateral to that here or here somewhat of the side of that lower portion of the wing here that is the lateral horn here and the lateral horn here and then finally on the posterior side of the spinal cord we have the very top tips of the Wings here and here and these are the posterior horns and these small space here the small strip of gray matter that it's connecting these two sides this is called the gray commisure right think of commisure like to commiserate to communicate that's what the commisure does it allows these two sides of the gray matter to communicate and right through the center of it is a is a very small Canal it's an open space it's a that continuance of those ventricles in the brain now now this is a small uh Canal of space here through the spinal cords called the central Canal this is also filled with cerebros spinal fluid now the white matter itself again and unfortunately in this view it's a little bit weird because this is an actual cross-section of spinal cord and it's been stained so that you can see the differences here so unfortunately here it doesn't appear white but it would be these much more purple areas here here all the way around here and then around here again this is all the white matter and this is external or superficial to the gray matter and the spinal cord so again complete reverse of the brain and the white matter on each side again is partitioned into three regions unfortunately as you would expect right posterior side is here right we can see posterior posterior side is here my apologies this is the posterior finicula now if we were to go to the very opposite side of the spinal cord we see this region here is actually the anterior finicula and then finally these lateral portions here and lateral portions here are called the lateral finicula right so lateral lateral posterior anterior right pretty simple again very similar to the uh the horns of the gray matter now the spinal nerves themselves are actually they're they're they're actual nerves right so these are these are bundles of thousands of motor and sensory axons all grouped together as you'd expect and they serve specific regions of the body that's the idea I mean you would expect that cervical um nerves are not going to be serving the legs because they're so high up on the body and that's exactly what we see so they're there to to serve reasonably close regions to where the nerve is actually located now the motor axons themselves are actually going to originate from the spinal cord and so if we if we look at it here so here is that whole spinal nerve here and then we've got these roots that we need to talk about here and again we need to make sure that we pay attention to where these roots lie on ter in terms of surface on the spinal cord so here is an anterior roote and we can tell that because this is the the anterior surface of the spinal cord and we can tell that because this is the body of the vertebra which again would be anterior on the body and here is the posterior root that we see here and these two are going to unite here within that uh invertebral invertebral foramen and become this spinal nerve that then radiates outward again it is both motor and sensory axons that exist here because we have to have sensory input coming into the central nervous system and by the same extension we also have to have motor responses coming out of the central nervous system so both of these axons must exist each spinal nerve itself is associated with the vertebrae of the same numbers that's why you see those numbering on those spinal nerves one of my favorite figures ever right big one because it's particularly useful but too because it's just probably the worst superhero costume ever um is one way to think about it uh these are dermatomes right and so these colored regions it's very interesting these colored regions are actually segments of skin that are associated by with a particular single spinal nerve and so we can see um where on the body we would expect to see sensory information going through a spe specific spinal nerve and then obviously motor information coming to that region from a very specific uh spinal nerve now all spinal nerves except for C1 are going to in inate a segment of the skin so each of these nerves is associated with a dermatome so here's a good example right we see C2 as the the ears here and then this very inferior surface of the Mand all the way up here and then back up to this other ear so it's a relatively small patch and then here's C3 just that collar on the neck and then C4 begins to show up a little bit where the just above where the clavical bone is and then we've got C5 which all of a sudden begins to extend all the way down the arms so it depends how much region is associated with a a nerve is dependent upon which nerve it is now this is collectively known all of these regions are all collectively known as a dermatome map now if you think about it so far I've only talked about 31 pairs of spinal nerves which an all total it's 31 pairs you've actually got 62 individual nerves coming off the the uh spinal cord or this yeah and so if you try to think about it our 62 nerves just by themselves without branching off anymore from that are those 62 nerves going to be able to hit every organ every portion of the body every muscle and all and so on and so forth well no obviously not our body is much bigger than what 62 nerves could possibly serve and so those cranial those sorry those spinal nerves must branch and that branching Network that we see is called a nerve plexus and it's a network of interweaving anterior and posterior I branches often referred to as Rami of spinal nerves the anterior Rami which again we're going to see some relatively complicated anterior Rami because the spinal cord is located in the posterior portion of the body so it's got more surface area to cover on the anterior side so the anterior RI the anterior branch is and are going to be much larger right uh and they're going to form nerve plexuses both in the left and right right side of the body as well as the anterior portions of the trunk upper and lower limbs as well posterior Ram small and they only really inate small patches of muscle or sorry small patches of skin and small muscles in the back they don't form plexuses because it's not that complicated remember the spinal cord is already located in the back of the back of the body and so therefore you don't need to Branch out that much because the distance isn't far from the spinal from the actual spinal cord itself now here are the principal plexuses and again I'm not really I don't really focus on these on an exam by any means but it's just kind of useful to see uh we see the principal plexuses being the cervical plexuses so we see some associated with the neck the brachial plexuses for those of you that remember brachial region is infected arm uh the lumbar region this is the lower back we're going to think of this area with the lumbar vertebra are and then finally the sacral plexus is that area near the sacrum so we're we've got a couple that are relatively high on the human body right we're talking about neck and upper arms and then we're talking about very low on the body so they're near the uh lower back and the sacrum now reflexes are cool I think reflexes are very cool I hope you find them the same way um they're used often enough as kind of a clinical means of figuring out if there is in fact something wrong with your peripheral nervous system and or a portion of your central nervous system right because they're very useful the thing about reflexes that that make them so useful for that purpose is that they're automatic in involuntary responses which means they should occur the same way every time we're going to talk about that reflexes have the same kinds of properties and and and as a result of that they become very very useful in diagnostic medical tests because they are in fact supposed to be the same thing every time right so think of this rapid so fast autonomic that is automatic uh involuntary reactions of uh of the muscle or glands to a stimulus again rapid automatic involuntary so we're not thinking about it it happens fast and automatically happens every time these are the similar properties that we should see okay let's talk about those properties break this down a little more so you get a better sense these are the properties that all reflexes must exhibit first in order for a reflex to occur there must be a stimulus that precedes the actual reflex that initiates the response so that is um for those of you that have been to the doctor right every one of us has has had this happen who has seen a medical professional if you sit down uh they have you sit down and then they go ahead and they'll use that silly little triangular-shaped rubber mallet and they'll whack your knee they'll hit your knee with it and you kick this is the patella uh they're checking that Patell ligament right and in this case if they don't hit your leg with that little Hammer you're not going to kick right because there's been no stimulus to respond at so that doctor has to hit your patellar ligament in order to start the reflex now once that stimulus occurs a rapid response then will ensue and that rapid response requires that only a few neurons be involved and the reason that only a neurons are involved is that it it truly minimizes what we call that synaptic delay the number of the delays resulting from the number of synapses that are along that pathway and then finally an automatic response that occurs the same way every time should be the ending portion of that reflex so in this case using this example that I've already used the kick right doctor hits you with that little Mallet that's the stimulus the response is that you get the stimulus coming in that rapid response is sent back out and the the automatic response is the kick and that kick happens the same way every time should happen the same way every time now the cool thing about a reflex is that it requires no intent so you don't have to intend to do the reflex it requires no pre-awareness you don't have to know that it's coming you don't even have to know that it's happening in order for the reflex to occur it is supposed to be involuntary and that's why it's so importantly used in diagnostic tests because it is voluntary involuntary you do not need intent you do not even need awareness of it going on in order for it to happen now reflexes should not be suppressed they should not be easily able to be suppressed Because by the time you're typically aware that it's happening it's already begun now awareness of that stimulus occurs after the reflex action has been completed so typically we were we we we feel the hit right after we've kicked right so after that reflex action has been completed then all of a sudden we actually feel that stimulus hitting us that's how fast it occur C now reflexes are used to test specific muscle groups they're test they're used to test specific spinal nerves or the segment of the spinal nerve so they're looking to see again is there damage to just muscle is it damaged to a specific spinal nerve is it damaged to a segment of the spinal cord Al together right that's the kind of information that they're looking to get at and so this is part of that diagnostic tests to help figure that out or elucidate that now a consistently abnormal reflex response on the part of a patient could potentially indicate that there's damage to either the nervous system or the muscles of that area so this is obviously again very important very key the idea that is this consistently abnormal in this patient and if so why would be something to look into now an IND idual can have a normal response that is quote unquote we think of as normal what we think of as be fitting kind of the norm of most people it could be hypoactive right so hypo means less than active right so this could be a lot less active than we expect so so there's a normal kick there's a kind of a subtle less than active kick and then there's a hyperactive kick there's that that that hyperactive reflect response that is far more it's far in excess of what we'd expect right now again doesn't mean that any one of these is bad it's just that a reflex response by an individual could be any one of these things and a hypoactive or hyperactive reflex response isn't necessarily bad it just is what it is it all depends on um if there is in fact something wrong with that particular reflex that's causing it to be hypoactive or hyperactive okay so we need in order to talk about this meaningfully we need to break down the components of a reflex arc right so this Arc means the basically we're talking about the wiring so starting with stim the receptor all the way through that sensory neuron and then going out into the motor neuron for response out to the affector this is the pathway the uh the arc so to speak this is the neural wiring of that single reflex now again going back to that notion of um what are the properties of all reflexes number one is there must be a stimulus stimulus to activate that uh response so there must be a stimulus that begins in order to cause a response in that reflex so we always start in the pns or the peripheral nervous system that receptor once the stimulus hits that that receptor uh from that sensory neuron that information is going to be communicated all the way to the central nervous system now maybe it's just spinal cord level here maybe it goes all the way to the brain but either way it's still going to the central nervous system now a motor neuron is going to carry the response out to the peripheral effector and this is essentially the end of the actual reflex arc the muscle or gland at the end of the arc so let's talk about some of these very specific types of reflex arcs so in the case of an ipsilateral reflex arc this is uh the type of reflex arc in which both The receptors so that is the the actual part of the body that's actually receiving the stimulus externally right and the affector organs are on the same side of the spinal cord so a good example of this and actually I hate I actually legitimately hate the example given your book I keep it in here specifically because I want to make a point I'm going to go back to this would be uh when you feel a pain on say your right arm right and you pull your arm away in response to get it away from that pain now you got to be careful a little bit with pain uh in in the example I just gave you it you know you you felt the pain that is the stimulus on the right arm and then moved your right arm away in order to pull it away from the the source of that pain right but unfortunately you have to be a bit careful with pain because this is not something that is actually a reflex because the problem with reflexes remember we talked about this when we first defined it is that a reflex is is actually uh where it occur is something where response occurs the same way every time and the problem is is that that doesn't necessarily isn't always the case with pain and it certainly isn't always the case across people uh people have a a great threshold in terms of between individual people a great threshold for pain where some people feel pain very very uh are very sensitive to pain and it's very intense for them and some people have a very dull sensation of pain and so they have a larger uh threshold for pain they can tolerate a great deal more pain but remember you're not supposed to also not supposed to be able to to suppress a reflex and the problem here is that in this particular case in the example that your book gives you that when the muscles in your left arm contract to pull your left hand away from a hot object so let's say that hot object hot object is a flame the problem here is is that you can Will and force yourself to hold your hand over a flame and in that direct pain so you can actually burn your hand by forcing your arm to stay where it is and that by definition breaks the idea of what a reflex is supposed to be you can't will yourself to not kick your leg when they tap your patellar ligament say in the medical in a doctor's office right it doesn't matter how much you want to will yourself to not move that leg you are still going to move that leg this is an inst in instance where unfortunately that is not the case however the Central Point here being when it's a ipsilateral reflex arc both the receptor and the affector organs are on the same side of the body I.E the same side of the spinal cord now a controlateral reflex arc is exactly the opposite thing right so we expect that the receptor organ is going to be on one side of the spinal cord that is one side of the body and the affector organ is going to be in the opposite side of the body so the best example is and this is one I know all too well unfortunately um the idea that if you let's say you step on a nail with your Left Foot Right so you're you're walking through say an area um like for me for example when I was in graduate school I did a lot of um sampling of snake species right and so I'd go out and in Meadows and prairies and whatnot and I would collect uh um snakes and and that's part of my research uh to get data from those individual snaks now sometimes you know when you're when you're walking through some of these areas you know some of them are nice relatively pristine natural areas and some of them are kind of run down some of them used to be all Old Boy Scout camps or old paintball gun uh uh um facilities and all kinds of things and so there's there could be trash land about there could be old boards laying about and those old boards might have nails to them and you might not necessarily know that they're there and so in in several instances with myself I can recall where I would actually step and in this case we're going to use my left foot as the example I'd step my left foot on a nail and I didn't know it of course until it pierced my my foot and then the pain came through and then you know that was always a fun uh a fun time but here here's the point when that occurred The Reflex was that my right leg stiffen up and the muscles contract in such a way to allow me to maintain my balance so that I could pull my left leg away from that that nail that that damage that I had just inflict inflicted upon myself right so here you see the affector affector is on the right side of the body that's the leg that's that's straightening it's helping to allow maintain your balance because it's the left leg that you're perceiving the pain from contralateral right so opposite sides of the body okay so those were two different types of reflex arcs but now we want to talk about about how those reflex arcs are composed in terms of the number of synapses so that is how many neurons in order is that reflex how many synapses are involved in that reflex we're going to start with the simplest of all reflexes because that kind of makes sense right we're going to start with one in which no interneurons are involved so we're talking about a mono synaptic monos synaptic reflexes have clearly one synapse mono for one synaptic for synapse so these are this is a situation where you have a single sensory neuron and a single motor neuron and there is no interneurons in between so you have literally one synapse and the best example of that one is a stretch reflex it's where um it regulates skeletal muscle length so a stimulus comes in where it it it detects the stretching of a muscle and that muscle then in turn reflexively contracts to keep the stretch from damaging the muscle because you don't want to tear the muscle obviously it's just too too much stretch and so a good example of that a specific example of that is the patellar or sometimes referred to as the kneejerk reflex this is monosynaptic this is also a stretch reflex right because they tap that patellar ligament the the muscle itself actually stretches and by virtue of that the actual quadriceps muscles the the rectus Fus and those vastus muscles there they're going to contract produce a noticeable kick that is the reason reflex now poly synaptic reflexes are exactly what they sound like if it's not one synapse it's multiple synapses in that reflex arc this is meant to be intentionally vague because poly synaptic means many but we don't know how many we're talking about is it is it two is it three is it four is it five is it seven this name would not imply that but nonetheless it is important to talk about them so this is the vague way in which we talk about poly syneptic reflexes as it says are more complex they're definitely going to be more complex than a monosynaptic reflex right and they're going to involve interneurons by definition because the second it's it's more than one synapse you have got to have an inter neuron in between the sensory and motor neuron now because this reflex arc has more components it's going to have a more prolonged delay between the actual stimulant and the response that result the response that comes about from it and that's again it's just a byproduct of the architecture of the number of synapses in between more synapses the longer of a delay there is between the electrical to chemical to electrical translation for each synapse and those each delay is kind of compounded so if it's 5 milliseconds for one and 5 milliseconds for the second poly synaptic reflex now all a sudden your synaptic delay is 10 milliseconds over long overall versus if it was just a monos synaptic it would be five milliseconds if that kind of math makes sense okay so we're out of reflexes now and we're going to start talking about the notion of a pathway right so how the body actually communicates or how the central nervous system communicate communicates with the body and these are either sensory or their motor they're not both so keep that in mind right now this is a very clear cut case where it's either sensory or motor and in these Pathways process processing and integration occur continuously this is a continuous process uh whereby we are going to process information we're going to integrated it together and see if it needs to go even further up the pathway or not so pathways are going to travel through white matter of the spinal cord and connect various uh central nervous system regions with the peripheral nervous peripheral nerves that bring information in or take information away now each pathway consists of what we call a tract and a nucleus a tract is a group of axons that that are going to travel together in the central nervous system now each tract might actually work with multiple nuclei within the central nervous system so that's important to keep in mind it might actually go between multiple nuclei for processing and integration a nucleus again we've talked about this but it's good to just Define it again to make sure that we remember this is a collection of neuron cell bodies located within the central nervous system so there's an asending pathway and an ascending pathway is information coming from the periphery up to the central nervous system and by default this must be sensory because sensory information comes into the central nervous system and then there's descending Pathways and descending pathways are by default motor they have to be because information leaving the brain out to the actual periphery this is not sensory information this is motor information the sensory information has ascended up into the ner central nervous system and now the motor response is descending out from that central nervous system and often the pathway itself is actually going to cross from one side of the body to the opposite side of the body and this is this process is actually called decussation This is a situation where say the left side of the brain processes information from the right side of the body and vice versa and as it turns out this is incredibly common about 90% of all Pathways decate so again this is very important not only is it common it's it's very much the standard now Pathways themselves are composed of a series of two or three neurons themselves that work together so let's go through that now when we talk about this we have to link up this idea of somatotopy which is where we have a precise correspondence between a specific area of the body and a specific area of the CNS and if you think about this a lot of this refers back to that that wonderful figure that I I I kind of like to joke about but it demonstrates the primary motor cortex and the primary somat sensory cortex of the brain because the primary somat sensory cortex right that is going that is a clear demonstration of a very specific area in the central nervous area the in the central nervous system that corrons very specifically to a specific part of the body so that's a good example of somatotopic right we have the same kind of thing with the motor uh cortex now all pathways are paired they must have paired tracks because you've got a right and a left side of the body and so you must be able to control and sense both now a pathway on the left side of the central nervous system has a matching track on the right side and vice versa again as we'd expect now sensory Pathways have primary and secondary neurons and occasionally have what we call tertiary neurons which are upper level neurons after the fact so so sensory Pathways may have two three or more um neurons typically three right at most three motor pathways on the other hand are going to be very specific these use two so motor pathways will use an upper motor neuron and then a lower motor neuron and the cell bodies of the nuclei are associated with each pathway so there so you have the cell bodies that are located within the nuclei that are associated with each pathway so it's got to be it's it has very specific nuclei that are associated with these Pathways as a result so if we delve into sensory Pathways specifically everything starts with a receptor and this is something we want to keep in mind when we think about the sensor the sensory aspect of nervous information receptors are going to detect that stimulant and they're going to detect or they're going to conduct that nerve impulse to the CNS now sensory pathway centers within either the spinal cord or within the brain stem process they're going to filter incoming sensory information and this is vital this is so important because they're going to determine whether the incoming sensory stimulus should be transmitted to the cerebrum or terminated and this is very important uh for those of you again I I like to use this example uh because it's just it's just such a an a great clear contrasting um example between adults and newborn children right so for those of you that have had a newborn child or have have spent any kind of time with a newborn child you watch a newborn child and they're often they often look kind of over stimulated just like it's just too much right that's because there is for a newborn child everything is new that they're experiencing you know every slight and subtle Breeze of of air or draft of air is a brand new stimulus because those kinds of things didn't happen in utero and so as a result they don't experience they're experiencing the world an entirely different world and all these new stimuli in a very different way for the first time ever and so for them they just kind of look generally sort of quot like frazzled or sort of um they just look like overloaded because they are but adults don't often every every one of us probably know somebody that generally always looks a little frazzled but generally speaking normally speaking most of us have learned that there's a lot of things that happen to us or around us that just simply don't matter they just simply don't matter and this is what I'm talking about the filtering the progressive filtering because more than 99% of all incoming impulses are not going to reach the cerebral cortex right and as a result are not going to become a part of our conscious awareness we don't care we don't care if the air temperature changes from 71.1 to 71.2 degrees fah it's completely negligible the likelihood that you're going to care about a tenth of a degree change in temperature is very unlikely right but again this is all conditional and so we're going to come back to the idea that what is important is always conditional depends can it depends on the circumstance okay so that primary neuron often also referred to as a first order neuron uh and it depends on how you think some people like that term first because it tells them hey this is the first one some people like the term primary because that seems to rein force that more it doesn't matter which term you use primary neuron or first order neuron this is the receptor neuron so the dendrites here are a part of the receptor that D that detects that specific stimulus and then that stimulus will be carried from the cell body that resides in what we call a posterior root ganglia right a gangan uh is a collection of cell bodies in the peripheral nervous system on a spinal nerve right or we see it uh in a sensory ganglia of the cranial nerves right so those that impulse is going to be sent along to the cell body that's in the posterior root ganglia or in the sensory ganglia now the axon of the primary neuron is going to project to a secondary neuron within the central nervous system and at this point that secondary neuron is by definition an inter neuron the cell body resides within either the posterior horn of the spinal cord so we're talking about the cell body of the secondary neuron here or that cell body is within a brain stem nucleus the axon of that secondary neuron is going to project to the thalamus right remember this is that portion of the Dian seyon we saw it it's kind of in the center of the brain if you want to think of it as the center of the brain um and here is where it could potentially synapse with a tertiary neuron so that's what we need to keep in mind that tertiary or third order neuron this is also an interneuron because from here it's going to be translated into motor response once we've passed through this tertiary neuron and it's gone from this point up into the actual um cerebrum so the the tertiary neuron its cell body resides within the thalamus and the thalamus is that central processing and coding uh Center for almost all sensory neuron or sensory information and it's at this point where uh the ultimate decision is going to be made as to whether this is going to be sent up into the cerebral cortex or it's going to be terminated here because it does not need to be conscious we don't need to consciously think of this particular set of stimuli so a good example of that here is the posterior finicula medial lemniscal pathway no I'm just going to make it clear right now I am not going to ask you about the posterior finicula medial lisal pathway on any exam that I give I give this example because it puts a name to a particular pathway and so you can actually use this as an example to to locate the primary secondary and tertiary neurons accordingly right so we're going to go through this as an example of such but I'm not going to ever ask you about this specific pathway so you see this this primary neuron here right is it's Aon is entering the the actual central nervous system and at this point actually turn terminates here and the medulla of Lata and this is where the secondary neuron picks up the secondary neuron is highlighted in blue and this secondary neuron here is going to pass all the way through the mezan seylon and then up into the actual Thalamus here right now the green neuron is that tertiary neuron notice how it brings information from the uh theth up to the in this case primary somat sensory cortex and the specific location of the primary somat sensory cortex because this particular pathway uh is concerned with proprioceptive information for limb positions you know where your limbs are located a specific limb in particular it talks about or it helps us discern discriminative touch so um you know pain and such and so forth right pressure V vibration Sensations now in the motor pathways we're going to we're going to see the descending Pathways uh in the brain and the spinal cord that specifically control activities of the skeletal muscle and these are formed from participation by both the cerebral nuclei and the cerebellum as well as the descending projection tracks in motor neurons and specifically the descending Mo uh descending projection tracks are motor pathways that originate from the cerebral cortex and brain stem and depending on what response is being sent this may be either a direct or an indirect pathway so the cortical spinal tract is a good example of an actual motor pathway uh but again same thing applies uh as compared to the uh posterior finicula I will not directly ask you about the cortical spinal tract but I put it here more as a a figure and an example that you can go through that you can look at uh and and compare uh you know your upper and lower motor neurons and see where these things start where they end and whatnot it's a good example now this is where things get really interesting because when it comes down to it when you when you bring the cerebrum into the mix this is where all those the where the real decision making or where the real thoughts conscious thought is coming into play here in terms of what are we going to do what decision are we going to make and so when it comes down to it several regions of the brain are going to participate in the control of these motor activities and that in of itself is particularly important because these motor programs require conscious directions from the frontal loes so specifically the frontal lobes are there to to help us to determine whether or not we should or should not do this action uh and again the frontal loes are are a a major player in the in inhibition of certain activities so specifically it allows us to make those good those good conscious decision- making as whether good conscious decision making as to whether or not we should actually do something so should we do this should we not do this and I think we can all think of a good example uh of of a time in our adult life when we desperately wanted to do something that was a really bad idea and we held ourselves back because again this frontal lobe was participating these frontal lobes are participating in in reminding us that it's not a good idea to make this decision um now movement is initiated when commands are received from the primary motor cortex right so we're talking about again that motor associ motor association areas within the primary motor cortex so as we talked talked again about with respect to that um that primary Mor C cortex just anterior to the central sulcus and so those primary those motor association areas are going to send out the initial decision to do something or to not do something in terms of a response now notice we've got participation on the frontal lobe telling us whether or not we should do this and then the motor Association telling us where we should do it from and then we we're going to take the cerebellum into account as well because the cerebellum is going to be incred incredibly important in coordinating movements because a lot of the the the responses that we we are going to do are going to require some very specific and exact timing uh for these different muscles when do I contract when I relax and whatnot in order to properly do whatever task it is that we were going to do and so you see from this figure here there's a lot of back and forth and and and backwards flow almost like kind of a reverberating circuit as we talked about before back in neuronal pools or circuits and the idea behind this is very simple I mean often enough we we will ponder what it is we're going to do and the example I like to use and I know this is kind of a silly funny example but it's a it's a nice current example of our current times socially uh texting is f is fantastic for this because there's nothing like receiving a text and looking at that text and I'm sure we've all had similar experiences like this where you look at a text and you just don't know how to interpret that is that person being rude is that person being uh nice or they just being uh um is it just a quick response and they didn't really think much about it and so you have to wonder well what did that person mean what did that you know what do they mean by that text was it you know and if it's rude then all of a sudden your decisions are going to be very different you know if if they're sending a a rude text then you'll likely respond in kind or respond with a uh some kind of blow you off kind of text um but if they're responding in a nice way then you'll obviously not want to respond that way and so you might go back and forth on your decisions as to what you how you're going to respond before you ultimately respond and that's where a lot of this information comes from we're going to continue to go back and forth as to what it is we're going to do before that decision is ultimately made now now where in where information is processed and where motor controls are depend entirely on how complex that that system is simple reflexes right are going to be simple and so they require the lowest level of motor control in respect here so simple reflexes that stimulate motor neurons are going to represent low control the nuclei controlling these reflexes are typically either in the spinal cord or the brain stem um brain stem nuclei typically participate in more complex reflexes but the highly variable the really complex voluntary motor patterns not patters by the way that is a that is a mistake there uh motor patterns are controlled by the cerebral cortex and so again highly variable complex voluntary patterns these are going to be universally controlled by the cerebral cortex because they're very old because they're complex now this is kind of a fun example because I loved using this example here's War's area which is often considered to be the association area for um uh formulating what you're going to say and then the motor speech area is what you actually say how you actually say what it is you're going to say and the example I always use uh um was was my stepdaughter years ago um who used to consistently wake up in the middle of the night and she she'd wake up and she'd stand right next to the bed and she would just kind of stand there like a zombie which is always in and of itself very creepy and I'd wake up and then freak out you know momentarily you know everybody how you if you see something in the middle of the night or when you just wake up it's a you know it makes you jump or startle and so after that whole response um she would look at me dead in the face and say the most ridiculous things like there's bees in the bed or um um you know there's a bear in the driveway or these just ridiculous kinds of very nonsensical sentences things that didn't make sense things I know that she didn't mean and at first I didn't quite get it I didn't understand what was going on there and then as time went on I recognized that it that there was really only one thing that she meant every single time this happened she just had to use the bathroom the problem was was that wari War's area was likely telling her the exact right thing to say but the motor speech area this the part this is the association area where this is all about what to say but then the motor speech area is is how it's actually said and there was some miscommunication between these two areas in the brain and so when she really meant to say I've got to use the bathroom what she invariably always said was something that couldn't have been further from that and that's because there was some miscommunication there as she was so exhausted probably sleepwalking or whatnot uh between these two association areas as to what it is that she was going to say okay now uh this stuff is always interesting to me again a lot of things are but but I love this left brain right brain notion like somebody's left brained or somebody's right brained and we we determine that by things that they're best at or worst at and and that makes sense and yet in a lot of ways it doesn't make sense I'm going to come back to Breaking that down a little bit because nobody is left brained nobody is right brained we are globally brained this is a a whole brain function but some of us might be stronger in certain areas weaker in certain areas for obvious reasons um each hemisphere is specialized for certain tasks and so that that I that correlates really nicely with the idea of somebody being left brained or right brained but again you can't be one of the two left hemisphere is typically referred to as a categorical hemisphere this is the one that's great with categorization with symbolization uh and specifically it it's got some very specific functions to it for one thing wary area and motor speech area the two areas that I just got done talking about these are both located specifically on the left hemisphere right so it's specialized for language abilities but on top of that it is that hemisphere that is particularly good at performing sequential and analytical reasoning tasks so those tasks that we often associate with Science and Mathematics and in addition to that it seems to be very quantitative in the in the respect that it appears to directly uh or um directly partition information into smaller fragments of for analysis and the best way that I can I I can make an example of this is to to ask you know what is what is 60 by 73 you know what what is the sum of 60 plus 73 now it doesn't really matter which numbers I pick here but I like to pick a couple of double digit numbers just because it helps make things a little more complex right now if I ask you add 60 to 73 and I give you a minute I guarantee you that there's going to be several different ways that people come to the right answer in this one some people are going to directly add 60 to 73 and get 133 but some people are going to add 60 plus 60 they're going to get 120 then they're going to add that other 13 from 73 and they're going to get 133 overall some people are going to add 60 and 70 and get 130 and then they're going to add the extra three and get 133 so this is all that partitioning that I was talking about right and that is kind of key here to the understanding of of left hemisphere uh and and and its direct kind of Specialties so to speak now again nobody is left brained or right brained and I'm going to give you a really good example of this right so so I'm a biologist right scientists are often considered to be left hemisphere people right just by sheer nature of the work that we do and work what we gravitate towards and yet I I'm relatively terrible at speaking other languages right I I've got English down I've got um I've got truck stop English down or or Bears game English down depending on how you think of it where it's it's more foulmouthed kind of stuff than I've got um but regular English but that's about it right I I don't speak a ton of languages but I'm very good at math and science so if I if you were truly a left hemisphere person you would imagine that that person would be good at everything the left hemisphere does and they would have a hard time with the things the right hemisphere does but that's it's not a it's not a functional way to look at the way the brain actually works might be stronger in certain areas but again the brain is global so you're using both hemispheres and so I'm G to I'm going to come back to this when I talk about the right hemisphere right um because the right hemisphere is what we think of as the not the categorical but rather the representational right and so this one represents kind of visual spatial relationships and analyses of this which sounds really complicated but it's really not uh you know I'd asked the question in a traditional lecture how many of you have ever moved and moved from one home to the next and you you know you you've put all your things and and you and you've organized them in that new home and for the first few nights that you get up maybe in the middle of the night right youve for whatever reason and you get up and you're you're exhaust Ed and you're half asleep and whereas in your normal home you would you would know exactly where everything was and so you could walk around pieces of furniture your bed for example and you knew where the door was for the bathroom where the light was going to be and all of those things but in this new home because those memories haven't been concreted yet they haven't uh they haven't been really truly instilled and stored in your brain yet those first few nights are like dangerous because you're running into things easily because you're not paying attention because you don't know where these things are because those memories haven't been instilled yet those visual special relationships aren't there whereas for those of you that have been have lived in a place in a while just like I have in my home if I asked you right now to picture what your living room looks like and where everything is I bet you you could close your eyes and form that mental image and see where everything is and know about how far each thing is from each other I know I could do the same thing about my home now again I would often be considered a left brain person and yet that is a clearly a right brain our right hemisphere task other skills other functions involved with the right hemisphere imagination Insight Musical and artistic skills right perception of patterns and spatial relationships as goes back to the idea of visual spatial relationships we just talked about comparisons of sights sounds smells tastes um we can't just be one hemisphere the next and I I guarantee you that all of you can find at least one task in each hemisphere that you are particularly strong at or you would say you're reasonably good at now both cerebral hemispheres must remain in constant communication because if they did it'd be like two entirely separate siblings never communicating and doing vastly different things trying to make the same household work just doesn't work right so there's there's constant communication through the commisures and in this case the commisure we're talking about here is the Corpus colossum these are hundreds of millions of axons that that pass between both of the hemispheres and allow for these two uh hemispheres to coordinate and to communicate this figure again is weird I'll tell you why one here's the right hemisphere here's the left hemisphere and right between it is the corpus colossum now there is never this much space between the two hemispheres this in this image these hemispheres have been spread apart which is a very very bad idea in somebody's actual brain but it's been spread apart so you can see these structures so just keep that in mind now the things I want to point out again here's that Corpus colossum these are those axons that project between the two hemispheres to make sure that they can constantly communicate the other aspect that I wanted to show you here is if you look at the purple shading versus the Green shading and you look at both eyes you'll notice that each eye has both a green and a purple field and these relate to Pathways that decate so we're talking specifically about the pathways that pass from one side of the body to the opposite side of the brain so for example here the um the right visual field here will get sent from this right eye and it will decate over to the left hemisphere for interpretation by the left acial lobe now it will also send information from the left visual field here over to the right a sypal for for interpretation and so with the left eye so both eyes have deating Pathways and it's an excellent way to look at decas and how those Pathways work