Neurology | Descending Tracts: Corticospinal Tract

iron innards in this video we are going to talk about descending tracks so what are descending tracks descending tracks they basically start for the most part we're gonna talk about in this video within the cortex we're gonna talk about corticospinal tracts so when we talk about descending tracks right so we have descending descending tracks we're gonna talk about two different types alright we're gonna talk about cortical spinal tracts and we're gonna talk about another one in separate videos which are going to be different subcortical sub cortical tracts we'll talk about these like rubra spinal vestibular

spinal we'll talk about the Ponto reticulospinal and the medullary reticulospinal but we're gonna focus right now on the corticospinal tracts and there's gonna be two types here one is you're gonna have what's called the lateral corticospinal corticospinal tract and the other one is we're gonna talk about the anterior or ventral cortical spinal tract we're going to talk about these two the more important one which is going to be controlling like your distal limb musculature is going to be the lateral corticospinal tract and the anterior is more for the axial musculature and we'll talk about that

now something else we have to understand with corticospinal tracts where do they start let's say here I draw a crude diagram really quick one I'm gonna have here central nervous system and then your spinal cord right from up at the top the cerebral cortex we're going to talk about this in more detail but for simplicity sake you have a bunch of neurons here and these neurons are called your upper motor neurons we're gonna abbreviate them for simplicity sake as you MN upper motor neurons from here they can send down fibers that move down through the

cerebral cortex through the brainstem Ensign apps on cell bodies that are located within the actual spinal cord specifically in the anterior the ventral gray horn from there these axons will come out and go and innervate a special muscle generally we're gonna be talking about skeletal muscle okay and it's gonna go and innervate a muscle now what is this neuron right here called located within the spinal cord we call this guy a lower motor neuron okay we'll abbreviate this as L M in so we have to understand that what we're basically gonna be talking about in

this video is the corticospinal tracts the upper motor neuron starts in the cortex moves its way down to a lower motor neuron located within the spinal cord now there is other cortical tracts then we're not going to discuss them in this video but you do want I do want you to understand that as these fibers can go down they can give off little connections to nuclei that are located within the brainstem different cranial nerves like maybe the the twelfth cranial nerve or the 11 to the 3rd of the fourth so you might have here maybe

third cranial nerve maybe fourth maybe the 11th cranial nerve maybe twelfth cranial nerve whatever from here these fibers can go out and innervate different muscles like maybe extraocular eye muscles maybe the trapezius maybe the tongue muscles it all depends but since this is going from the cortex to these cranial nerve nuclei we don't call these corticospinal we actually call these ones right here these fibers quartic Oh nuclear fibers okay and this is for cortex cerebral cortex sending descending fibers that go to the different cranial nerve nuclei that are gonna go and intervene skeletal muscles all

right but our main focus is gonna be on the cortical spinal tracts so let's go ahead and get started here before we do that though we have to understand a little bit about this cerebral cortex how does it know to send a command down how does it do them so if we take over here we're gonna have a section of the brain right so I'm taking a section of the brain where I'm doing like a sagittal section and we're looking at one half so over here you're gonna have temporal lobe right here's your lateral sulcus

or lateral fissure and then right here you're gonna have another sulcus or Fisher called the central sulcus and then over here you can have another one called the parietal occipital sulcus right so then you have occipital lobe to impor a lobe parietal lobe frontal lobe right now we already discussed in the ascending tracts that the parietal lobe that post central gyrus was associated with sensations in the anterior or frontal lobe anterior to that central sulcus there's a special lobe here so look at this you have a special gyrus just anterior to that central sulcus what

is that gyrus called they call this gyrus the pre central gyrus this precentral gyrus has a bunch of different cell bodies which are going to be important in being able to send down those descending tracts so it's the primary area of where the motor fibers begin those upper motor neurons so we actually call the precentral gyrus functionally we call it the primary motor cortex okay we call it the primary motor cortex so precentral gyrus is the structural gyrus the functional aspect of that gyrus is that it's called the primary motor cortex most of the actual

axons coming from this cortex controls voluntary or conscious awareness of the movement of our skeletal muscles okay but that's not the only area there's two other really important areas and there's another one that's really shocking that usually confuses people there's one right here there's a little bit anterior to this precentral gyrus there's another part here just anterior to it and this one is actually called the premotor cortex so this one right here is called the pre motor cortex this one is more associated with learned and repetitive or planned motor movements okay so it's associated with

learned or repetitive or planned motor movements and this one also has descending axons that can come down there's another one just a little bit superior to that so just a little bit superior to that one and this one right here is called the supplementary it's called the Supplemental motor area okay the Supplemental motor area is also associated with certain types of motor movements so so far we have three different areas we have the actual primary motor cortex on the precentral gyrus the Supplemental motor area and the premotor cortex all of these guys are gonna help

us to be able to initiate the movements there's one shocking one though this one usually confuses people there's another one and you guys have already learned this if you guys've watch the ascending tracts videos you know there's another area here this is behind the central sulcus this pink area you know the behind the central sulcus that was called the post central gyrus well the post central gyrus is where we had what's called the primary some Matt toe sensory cortex you know the primary somatosensory cortex a good chunk sometimes thirty to forty percent of the motor

fibers that are coming down are going to be coming from the primary somatosensory cortex that's unbelievable so we have four areas that I want you guys to remember that it can be helping to control the motor movement these descending fibers what are they primary somatosensory cortex is one the second one is going to be the primary motor cortex and the precentral gyrus this is going to be the second one the third one is going to be the Supplemental motor area which is going to be just a little bit anterior to the precentral gyrus and the

fourth and final one is going to be the premotor cortex which is just a little bit anti hearing a little inferior to the precentral gyrus okay so we know the areas that are gonna help to be able to allow for the fibers to come down but how do we know when to do that well let's say that a thought comes into my mind where does the thought usually come let's say that I'm thinking about performing a movement I'm thinking about flexing the biceps right in order for that to happen there's a special area in the

and the frontal lobe and it's let's say it's just right here they call this the pre frontal cortex so there's just a little bit in the frontal lobe you have what's called the pre frontal cortex and let's say that this is where the thought comes from the thought of being able to cause flexion at the elbow joint by contracting the biceps brachii right I I had conscious control over that the thought arose in the prefrontal cortex from that what could happen it can send signals to these different motor areas when it sends signals to these

different motor areas what can happen it can stimulate these guys to send down the descending fibers but you know there's something really important there's a special checking mechanism because once we have the thought that comes into our mind to move our biceps we have to make sure that that movement that flexion isn't super like low and intensity or too high in intensity because I want to make sure that that motor movement is just perfect it's smooth and it's perfectly executed so in order for me to do that there's special structures located deep within the cerebrum

you know they call them basal nuclei so you have a bunch of different structures here we're just gonna draw them right here assume that they're right here these are called your basal nuclei and your basal nuclei like the put'em in and the Globus Politis and the caudate nucleus all those structures the thalamus sub thalamus substantia which are associated with these basal nuclei they're gonna get information so the primary motor area the supplementary motor area the promoter area and even the primary somatosensory cortex are going to come and tell the basal nuclei of what the motor

movement that they want to perform is the basal nuclei sifts through that information of the planned motor movement and guess what it does it sends up its little modified blueprint of the movement so it says hey there's a couple of things that I think you should fix cerebral cortex before you send this information down so what do we have so fourth thought getting ready to execute the movement before we do that we send it to the basal nuclei the basal nuclei kind of modify that prevent overshooting or undershooting of the motor movements send it back

up to the cerebral cortex but then there's one more checking mechanism you know just below a cerebrum you have another structure here and this is called your cerebellum right we'll talk about these things a little bit more but just to get the basic points here your cerebellum also has to communicate with the cerebral cortex so from the cerebral cortex from these motor fibers they have to send information to the cerebral cortex let the cerebral cortex know hey this is the planned motor movement that we want to do you know what's so cool about the cerebral

cortex though it's just unbelievable you know there's a part here right we have the brainstem midbrain pons medulla spinal cord right what's gonna draw a double cerebellum here but we have this whole part here the spinal right what's really interesting about this is that the cerebellum not only gets a planned program of what movement that the cerebral cortex wants to do it also is getting proprioceptive information if you guys haven't ready to go watch the actual spinocerebellar tracks because it's also getting information from appropriate receptors so let's pretend here's our muscle it could be from

the muscles you could have these those different muscle spindles remember we had the nuclear bag fibers and nuclear chain fibers that respond to stretching of the muscle and he had the Golgi tendon organs and they were responding to the stretch of the tendon and all these guys were sending information in right into the spinal cord and then up into the cerebellum letting the cerebellum know of the position of our muscles or tendons our joints our ligaments all that stuff so the cerebellum says okay this is the plan motor movement this is where the position of

the muscles the tendons the joint capsules all the ligaments are now you know what I'm gonna do I'm gonna send this final blueprint back up to the cerebral cortex let them know we now have a perfectly planned motor movement set and ready to go so the thought originated got sent to this motor area they court they coordinated this information with the basal nuclei and they coordinated with the cerebellum who's also getting information from our profile receptors so in that the final pathway that we're gonna send down is completely checked it's a perfectly executed movement let's

go ahead and follow it okay so if we start up here in the cerebral cortex we said that right here we're taking a section we're doing a coronal section here right and you can see here that we have the parietal lobes and we got your temporal lobe the midbrain the pons medulla and cross-sections of the spinal cord here in the actual Pryde lobe where we're gonna say that there's a bunch of different types of nuclei here okay bunch of different types of motor neurons that we're going to talk about here and these motor neurons their

cell bodies are located in a special lamina or action of the street record you know the cerebral cortex is like many different layers six different lamina sections partitions right in the fifth lamina of the cerebral cortex you find these special cell bodies okay and these guys these these cell bodies they're actually called these they're called pyramidal cells pyramidal cells now some of them are a little bit bigger than others pretty big and those are called your cells of bats so you might have in these pyramidal cells you might have special ones called cells of bats

and these are the really big ones and they're heavily myelinated too but for simplicity sake just remember that these cells that are located within the fifth lamina of the cerebral cortex are called your pyramidal cells some of them that a little bit bigger than others are called the cells of Betz and they're heavily myelinated now what do these guys do okay we had the thought from the prefrontal cortex it's being organized we had all the coordination in the modifications from the basal nuclei and the cerebellum we're ready to send the motor movement down okay from

here these guys give off their axons these pyramidal cells and there are some of them which are a little bit bigger than others again are called the cells of that's very very high heavily myelinated these guys come down in like a fan basically like a radiating crown what do you call this they call it the corona radiata now the corona radiata then actually condenses and compacts and moves through a special white matter structure which is called the internal capsule so you know this is right here's our internal capsule in the actual posterior limb of the

internal capsule these fibers funnel down they kind of move down through the internal capsule and as they funnel down through the internal capsule they get really really condensed and there's a reason why that's clinically important because if you remember we talked about this in the ascending track videos there's an artery called the lenticular straight artery and sometimes and people who have chronic hypertension that artery can actually get blocked and it can affect the blood flow to this area and it can cause a lot of different types of problems possibly paralysis of the muscles because this

is gonna go down to the muscles okay so now these fibers are coming down same way what are we having over here you have in the pyramidal cells I'm not going to draw as many as I did on the left side but we're gonna have same thing here and then what are these guys doing they're getting off their axons and their axons are coming together from this corona radiata they're condensing and then what are they doing they're running through the posterior limb of the internal capsule and now as they run through the internal capsule they

come down and they're gonna start descending and as they start descending they go through a part of the brainstem well part of the brainstem are we getting ready to enter into we're getting ready to enter into what's called the midbrain so now we're at the midbrain right here okay to understand this a little bit better of how they're coursing through let's come over here to the right because I want to show you as a special structure that it actually enters through because all this stuff is is clinically relevant because remember any problem in which there's

a change in the normal physiology can lead to pathology so we have to we have to know some of these things here so here's our midbrain right and here are these little stalks of white matter they're called your cerebral peduncles so they're called your cerebral peduncles all right now the cerebral peduncles has this part here where these fibers these motor fibers move through they really condense and they start moving through this area that area of the cerebral peduncles is called the crust cerebri okay so those fibers that are coming from the actual cerebral cortex they're

coming down through the internal capsule they're gonna start moving through the crust cerebri and they're gonna start funneling down through these cerebral peduncles and they're gonna enter into the pons so that's how this is working so I just want you to understand that that where are they entering they're entering into the actual crust cerebri of the cerebral peduncles and then moving continuously in fearly now we get to an important point here remember I told you that the cerebellum is constantly coordinating with the actual cerebral cortex to make sure that there's a proper planned movement but

you see these nuclei here that are located within the pons these are called your pontine nuclei sample all right so these are called your pontine nuclei they're constantly getting informed from the cerebral cortex when they're constantly informed from the cerebral cortex of what the planned motor movement is guess what they do they take that information so let's say that the information is coming down from here right from these fibers and these guys can take this information of what all this motor movement is and guess what they do they're really weird they say I'm gonna take

this to the opposite cerebellum so they take this information and they go through the middle cerebellar peduncles and alert the contralateral cerebellum so again over here it might get some stimulation from the cerebral cortex and then guess what they do they come over here they come to the opposite side contralateral side and they go into the actual cerebellum through the middle cerebellar peduncles and alert the cerebral cortex of this movement right so you have to understand here that these pontine nuclei they're constantly interacting with the cerebellum through the actual middle what are these structures right

here these white matter fibers here that is connecting the pons to the cerebellum they call this the middle cerebellar peduncle and what's important is that the cerebral cortex is constantly coming down giving information to these pontine nuclei which send the information to the cerebellum let the cerebellum know of what the actual planned motor movement is so the cerebellum can modify it but we want this let this actual motor movement to come down so what happens is it hacks to actually kind of like scatter some of these fibers kind of scatter around the pontine nuclei but

eventually they come back into a condensed form so some of them are gonna scatter across the actual pontine nuclei but again afterwards they're gonna start condensing now these fibers once they start condensing we've already gone past the actual pontine nuclei where this Ponto cerebellum connection is active right now they're gonna move through this next part what's the next part the medulla now let's come over here to the other diagram over here for a second so I want to show you something with the medulla in the medulla right here let's make a section right here and

then let's do it like this like this if we kind of do like this you'll notice here that we have kind of a special structure this is actually called a pyramids these are your pyramids and over here you're gonna have the olives so here's your two pyramids and here's the two olives what happens is if we follow those fibers those corticospinal fibers right over here right what they do is they start moving through these actual pyramids so some of the actually start moving through the parents that really get condensed there as they start moving through

the pyramids kind of towards the distal part of the pyramids some of the fibers will cross to the opposite side and move down so let's say let's follow these ones if we follow these guys they're gonna come down they're going to enter into the pyramids as they enter into the pyramids they're gonna start really really condensing and some of the fibers about 80% of the fibers will cross over at the distal half of the param and go down a small percentage 15 to 20 percent of them will stay uncrossed and they're going to the ventral

part of the spinal cord so if you look here what's happening at the pyramids the fibers are Deka sating they're crossing at the pyramids so what we need to know here that it's happening what's happening here at the module of where the pyramids are these fibers are a very very good chunk of them about how much again is important to remember about 80% 80% of these corticospinal fibers are actually going to cross and go to the contralateral side so a good good chunk of these guys are gonna cross the remaining part the remaining you know

15 to 20 percent are not going to cross and they're gonna stay itsy lateral okay so about good maybe 15 20 % here is gonna stay tipsy lateral and they're gonna go into the ventral part of the spinal cord now from here why is this important okay we said how much is actually going down through these decussate of fibers about 80% so about 80% of these are actually crossing how much is not crossing about 15 to 20 percent okay so about 15 to 20 percent is not crossing and again where is this occurring where is

this actually occurring this crossing it's occurring at the pyramids so this is called the pyramidal decussation all right so so far we have covered a pretty good amount work you we're almost there guys as we come down these fibers start entering into their respective columns of the spinal cord now what happens here from here let's let's follow the lateral first as we followed down these fibers that cross they went to the contralateral side they go into the lateral white columns so you know over here if you're just a little bit of spinal cord anatomy just

a really quick bit on it these are your lateral white columns or your lateral funiculi this is your dorsal white column or your dorsal funiculi and these are your anterior white columns or your ventral Finnick you line this is the posterior grey horn this is the anterior or ventral gray horn and then here is your central canal and you have the anterior white commissure there right so just a little bit here look what happens these fibers that are coming down into the lateral white column what do you think these fibers are called this is called

your lateral corticospinal tract what these guys are going to do is is they're going to come down into the lateral white column and they're going to sign apps on the cell bodies of the motor neurons located within the ventral or the anterior gray horn so look at this they're going to come over here and they're going to sign apps when they sign apps they're gonna sign apps on these motor neurons let's actually show them all right here here's an alpha motor neuron and you can also have other ones which we'll talk about very briefly called

gamma motor neurons now from here let's actually draw the gamma let's do the gamma and this nice purple here so there's a gamma motor there's a gamma motor okay so this is our gamma motor neuron and this brown one is the alpha motor neuron now from here these alpha motor neurons and gamma motor neurons are going to go where they're going to go to the skeletal muscles so they'll actually go out through the ventral root and into the ramus and a part of us right and again what could be moving with it this is the

gamma motor neuron all right so now these alpha motor neurons and gamma motor neurons are going to go out to the skeletal muscles and we said for our example that I want to flex the biceps right I want to be able to flex the bicep saw reflects the actual forearm by contracting the biceps brachii the brachialis all those different muscles that are associated with that right so let's pretend here I draw a skeletal muscle right now you know skeletal muscle we already said that it's made up of different types of fibers we have extra fuses

so what kind of fibers would we have so we say we have we have extra fusil which are the parts that contract so this is the contractile one the one that actually I shouldn't just say contractile it's the one that shortens and linkedin's the muscle that's a better way of explaining it so one that shortens and lengthens the muscle the other ones are called your intra fusil fibers and these are the ones that are actually the muscle spindles these are the ones that are associated with that proprioception so let's take this alpha motor neuron and

it's going to go to this muscle here and it's going to innervate the extra fuse of muscle fibers so what is this muscle here let's just say that this is some type of flexor at the elbow okay whether it be the biceps brachii brachialis brachioradialis doesn't matter we just want to flex at the elbow so we're going to have the alpha motor neurons innervate the extra fuse of muscle fibers to cause the muscle to contract and shorten now what about these gamma motor neurons you know inside of the muscle you have these different types of

things you have here nuclear bag fibers and nuclear chain fibers now what these guys do is is they pick up the different types of stretch you know nuclear bag fibers pick up the onset of stretch nuclear chain fibers pick up the progression and the stretch so what they do is they're important for being able to help our muscles to constantly have a nice contraction so what we want to do is these gamma motor neurons are going to come over here to these muscle spindles and constantly cause them to become very taut and that totnes helps

us to keep a nice tone of the muscles so it's going to come over here and stimulate the actual muscle spindles and cause the muscle spindles to be nice and taut and because these alpha and gamma motor nerds are working in tandem we call this alpha coat we call this alpha and gamma Co activation all right so again we're gonna have the alpha motor neuron stimulate the extra few cells to contract the muscle shorten gamma motor neurons to cause the muscle spindles to become nice and tight to maintain nice good contractions okay that is for

the lateral corticospinal now lateral corticospinal is very specific though so we have to say something about this when the latter corticospinal the lateral corticospinal is more associated with distal limb flah limb musculature so more of the distal limb musculature so more of the actual arm movements the finger movements so it's for very fine movements very fine and precise movements okay very fine and precise movements the other one though that says actually that we're going to talk about next is going to be called the the actual anterior or the ventral so the anterior cortical spinal tract

this one is more associated with the axial musculature more so do association with in axial musculature so it's for more of gross movements or large motor movements okay so lateral corticospinal distal limb limb musculature and the anterior is more for the axial musculature okay so this one we said right here was the anterior or the ventral cortical spinal tract right so here we'll put right here this guy's going to keep going down though so let's have them come down we'll have more room here if we come down here through the anterior white column and then

it's gonna come all the way down here and look it comes right here same thing with this guy follow him down he's gonna come down through the interior white column and then come to about right here here's where it gets tricky it's never always that simple right these guys have to cross we have to have pretty much most of the fibers from one part of the cerebral cortex have them going to the opposite limb so now this this tract right here what was this track to you're called guys this was called the anterior or ventral

corticospinal tract and we said that this was going more for the axial musculature when they go down into the the actual anterior or ventral funiculus all right this column that white column guess what it does it crosses over to the opposite side so it crosses over here to the opposite anterior or ventral gray horn then if it crosses over to the opposite anterior eventual gray horn guess what it's going to stimulate it's going to stimulate the alpha motor neurons and it's gonna stimulate the gamma motor neurons and these are going to be more specifically associated

with what type of movements axial muscular movements so from here these fibers are gonna go out these fibers are going to go out and where are they going to go to they're gonna go to the axial musculature for more of the gross large muscle movements okay and same thing here they're gonna go to the let's just draw a muscle here they're gonna go to muscles associated with what type of movement the axial musculature okay all right so again gamma just to reiterate because repetition is always good gamma is gonna go to the muscle spindles and

then alpha is gonna go to the extra fuse on muscle fibers all right Nazir so we pretty much went over for the most part all of the actual corticospinal tracts I hope it made sense I really do hope that you guys enjoyed if you guys learned a lot please hit that like button comment down in the comment section please subscribe also guys if you have the time please go check out our Facebook or Instagram even our patreon account alright engineers as always until next time [Music] you [Music]