foreign so we're going to start up now with our first lecture video over the peripheral nervous system so now we're on to unit 3 in AP1 and as you know by now unit 3 is going to cover the peripheral nervous system so to do that we're going to start with the spinal cord take a look at the structure supporting features kind of the basic overall idea from there we'll go into spinal cord Pathways so not just looking at the overall structure of the spinal cord physically or anatomically but looking at it more functionally instead so that will be kind of like our next step um all right so from there into spinal cord Anatomy the overall length of the spinal cord is about 18 inches which works out to about a half a meter um the width is about half an inch and there are several enlargements so um since this is theoretically the first time we've talked about any of this we need to very quickly talk about the different types of spinal vertebrae okay so when we're looking at either the vertebrae themselves or the spinal cord itself which are two very different things we refer to them by the same names okay so the top of the spinal cord near the neck is cervical your most of your back your upper back mid back is thoracic so that's the T L is lumbar and then s is sacral okay so we have cervical thoracic Lumbar and sacral vertebrae as well as regions of the spinal cord itself so there are several different enlargements or swellings of the spinal cord located between C4 and C1 T9 to T12 and L2 that's S5 and so what those represent are swellings that are due to uh presence of more neurons in those regions of the spinal cord okay so we know those are going to be somewhat important because they're essentially equipped with more processing power in those regions there is then going to be a structure called the conus medullaris or medullary cone right around the first or second lumbar vertebrae which we'll talk about more in a second as far as what that is uh structurally um not so much physically quite yet but structurally we'll just take a look at that one on this next slide um we then have what's called a terminal phylum okay so the terminal phylum is basically an extension of the Pia Mater so remember the spinal cord has the same meninges as the brain so dura mater arachnoid and Pia Mater the Pia Mater is going to extend itself from the lumbar spine and spinal cord down to the second second sacral vertebra second sacral vertebra that's hard to say okay so this is um we see the enlargements here so enlargement here the thoracic one lumbar terminal phylum is going to be the Pia which extends down from this a little bit further and then the cada equina which literally means horse's tail is this last part and so what this actually represents each one of these little structures here which looks sort of like I guess hairs on a horse's tail these represent what are referred to as spinal nerves and so spinal nerves are something that you're going to hear a lot about in this chapter okay spinal nerves are going to have entry points and exit points from the spinal cord and so of course entry points are going to be for sensory neurons going toward the brain and spinal cord and we're also going to have motor neurons exiting as well right from the spinal nerves into various other parts of the body to control muscles and glands okay so we have several different structures here to look at the enlargements the regions themselves the economicularis the terminal phylum The Equinox um just all various physical structures associated with the spinal cord okay so this is now the first time we're actually seeing a picture of the spinal cord so we need to kind of like take a look at some of the terminology on this picture what I really want to focus on in this picture for right now is the middle of it all right this part right here is the spinal cord and so whenever we see a cross section through a spinal cord the typical kind of configuration is to see this kind of shape in the middle of it and so this shape in the middle of it is gray matter and outside of that is white matter all right so gray matter inside represents cell bodies and dendrites white matter outside represents myelinated axons okay so this structure in the center is the spinal cord we see the Pia model surrounding it here we also have our other layers of dura mater is right here and then the arachnoid is kind of this gray layer right below that so blue is dromodor the really faint gray one is arachnoid and the layer actually surrounding the spinal cord is the Pia Mater all right so we have those structures now we also have in this picture um a few other things to look at for right now we're not going to go through all of these right now but we are going to talk about some of them all right so mainly on this slide we're interested in the meninges but I do want to go through and um just take a quick look at some what some of what these other terms are and what they mean because we're going to see all of these terms a lot in this chapter and actually a little bit in the next couple of chapters as well so we really want to make sure we understand it well here okay so we talked about the spinal cord got that knocked out um this is a vertebra right so everything around this in this beige color is bone and the body of the vertebra is this big thick part up top all right so this would be like ventral this would be dorsal in this particular picture Okay um so from there we also Let's see we can look at dorsal root ganglia so this ganglion represents a collection of cell bodies that we'll take a look at as well the dorsal root ganglion um is going to be a really important structure that we'll see exiting from each region of the spinal cord so these these spinal nerves this is a spinal nerve this is a spinal nerve as well there's going to be a pair of these exiting the spinal cord at each level of the spinal cord so T1 T2 T3 T4 each one has or most of them at least have these spinal nerves which represent again motor neurons and sensory neurons feeding into and coming out of the spinal cord we have something referred to as a denticulate ligament right here the denticular ligaments are kind of like um almost like bungee cords that suspend the spinal cord and keep it held in place they're not super rigid but they do keep the spinal cord kind of like centered more or less and and suspended within the CSF which is a very important uh feature we have some adipose tissue down here for cushioning right that's something we've talked about before um the ramay we'll talk about a little bit later on um autonomic ganglia this would be depending on the level of the spinal cord either sympathetic or parasympathetic so as we'll see the autonomic ganglia that our sympathetic are going to be primarily in the thoracic region and the lumbar and sacral regions are going to give rise to the parasympathetic division of the autonomic nervous system okay so in this picture now coming back to the text a little bit we have our dura mater of course the durable layer on the outside we have our epidural space outside of that we have our subdural space um deep to that and inside of that we have some areolar connective tissue and blood vessels all right so this is now all um in the subdural space right so we've got our connective tissue we've got blood vessels and then we move into the arachnoid space and eventually subarachtoid space just like we saw with the brain right same layers of meninges same spaces between them um really the same structure just a little bit different um emphasis I guess on the different layers um certainly not as thick as they are for the brain but still the same basic configuration all right so Pia motor is actually on the surface of the spinal cord and the Brain lots and lots of blood vessels here we're going to see a lot of nourishment from the Pia Mater uh we talked about our identicular ligaments right they're suspending the spinal cord within the CSF these actually are an extension of the Pia Mater um they give rise to the genticular ligaments all right so that those are all terms we talked about on that last picture so I don't think we need to spend too too much time on that one okay um so again our pictures here um a term that we have not talked about yet is a commissure so a gray commissure represents a crossing over of the spinal cord okay so we'll we're going to see that quite a bit um gray commissure uh is again the center part we have white commissure here we have our Central Canal which is the entire space within the vertebras remember this big space is the central Canal which is filled with CSF as well as these venticular ligaments and spinal nerves as well entering an exit the spinal cords and then there are the posterior or dorsal horns as well the horns are these structures okay so the gray matter within the spinal cord is going to look a little bit different depending on which level of the spinal cord we're looking at so in other words cervical versus thoracic versus lumbar the spinal cord kind of changes in appearance as it transitions from one layer of the spinal cord to another or one um one type of of region to another I should say um and then the ventral horns also um they're going to be the ones um actually these are ventral and these are dorsal over here dorsal side is by the vertebral actually I'm sorry this is the ventral side the ventral side is um yeah that's where the body of the vertebra is the spine of the vertebra is dorsal I was lost it for a second ventral side is where the body is dorsal side is where the spine is Central canals in the middle all right so that's kind of our overall look here so um these would be the ventral horns and the dorsal horns of the spinal cord sorry about that lateral horns uh if they're present are going to be to the side right so thoracic and Lumbar as it says we will also see some lateral horns here um cervical not so much sacral not so much but the two kind of Center regions of the spinal cord the two really big regions of the spinal cord we do see lateral horns as well um dorsal root ganglia again this is dorsal side here where the vertebral spine is dorsal root ganglia are going to feed into the spinal cord here her spinal Roots here um and then they kind of go right to the spinal cord ventral root is up top so here ventral roots enter root dorsal root dorsal root and so what we're going to eventually see here is that the ventral roots and the dorsal Roots have opposite jobs okay so specifically what they're going to be dealing with is sensory information coming in versus motor neuron or motor information coming out of the spinal cord okay so now we're getting into a little bit more of a diagrammatic view of the spinal cord um all right so a lot of this we have seen already um but I do want to go through this again and just kind of take a look at all of our features um so let's start up here at the top like 12 o'clock position and we're just going to work our way around clockwise around the picture here okay so now thing to uh to notice first is this picture is opposite configuration to the one before remember the one before had ventral on the top and dorsal on the bottom this one has um yeah this is ventral and this is dorsal up top so it's it's opposite configuration so that's maybe why it's going to look a little bit different um okay so the posterior median sulcus just like we saw in the brain um sulcai or sulci are these little notches that we see in the spinal cord itself so this is just kind of like a reference point at the six o'clock position on this picture we also have an anterior median fissure all right so this fissure is like a larger version of a sulcus it's actually a bigger Notch it cuts further into the spinal cord than this one does all right so again posterior side dorsal side anterior side or ventral side is down here okay so from there we have our dorsal column all of this white matter is going to be something we'll talk about here shortly the column is the entire thing we're running up and down the spinal cord in one continuous column we have our dorsal horn also over here dorsal horn we have a lateral column and a lateral horn if present would be right here now this particular representation of the spinal cord doesn't actually have much of a lateral horn but if it did it would be right about here okay we have our Central Canal again for CSF we have ventral horns here we have our fissure we talked about we're going to get back to these in a second so we'll come back to those two regions okay so now let's take a look at what these roots are all right so remember if we go back a second so if we take this picture and flip it 180 degrees these structures here these structures are the same as this structure here and there would be also one on the other side right so when Dr Landis made this slide he just drew one spinal nerve on the on one side for for clarity purposes basically just remember there's one on the other side as well in real life okay all right so this um again anterior ventral side of the spinal cord these two horns down here represent gray bodies or gray matter sorry gray cell bodies and I can talk dendrites of the spinal cord from that we have a ventral root now this ventral root and the same thing over here with this one these ventral roots are going to be carrying information to motor neurons and ultimately to muscles and glands located throughout the body the ventral side of the spinal cord the anterior side of the spinal cord is the side responsible for controlling the effectors or communicating with the effectors I should say the door starts at the ventral side up here dorsal side up here again dorsal side up top and this picture again it's flipped this was dorsal on this on this picture this is dorsal on this picture right the dorsal side is going to be concerned more so with sensory information so this particular dorsal root and we'll talk about the dorsal root ganglion as well here in a second is feeding into the dorsal side of the spinal cord carrying sensory information that will then travel either just to the spinal cord itself or perhaps all the way up to the brain through one of those columns we talked about a second ago Okay so we have a motor neuron efferent pathway efferent pathway leaving the spinal cord going out to effectors throughout the body we then have an afferent nerve carrying information toward the spinal cord carrying sensory information located just outside the dorsal root somewhere out here actually in real life this structure would be just barely off to the side not actually in the dorser itself will be just kind of sitting off to the side adjacent to it these are all the cell bodies of the axons that will feed into the spinal cord so what really happens is this dorsal root this nerve carries information up here just off to the side that's where the cell bodies and dendrites are of the neurons that are then going to feed directly into the spinal cord so information from this from sensory information after an information pathway carries sensory information through the dorsal root off to the side to the dendrites and cell bodies of a new set of sensory neurons or Afrin neurons when that when these neurons cell bodies are stimulated that's going to cause the axons they possess to then carry the information the rest of the way into the spinal cord all right so since we're in ganglion into the spinal cord from there these are the axons from those cell bodies sitting right up there okay so from there I think we just have our two commissures to talk about commissures are essentially a place where something crosses over something else and we're going to see those again so I just want to very quickly mention them on this slide posterior white commissure anterior white commissure we're going to see more uh sorry as far as what those are four here shortly okay so hopefully I think I hit everything on this slide I'm just making sure I didn't miss anything I don't think I did okay all right so now now we're getting a little more into the actual Pathways right um we saw this before on that previous slide this is the same same kind of picture of the spinal cord itself we see our dorsal side and our ventral side right we have dorsal horns ventral horns Central canal lateral horns if present and then our two commissure is located right inside of there okay so let's take a look at our motor neuron first a little bit easier so we're going to have so in this case there is a nerve exiting right here from this region of the spinal cord maybe this is a well this is a dorsal root here coming out of the spinal cord sorry ventral root here coming out of the spinal cord ventral root bottom side on this picture this is ventral ventral is carrying motor information to a larger nerve which is exiting and then from there this black line this big nerve is going to Branch repeatedly over and over and over and over into smaller and smaller and smaller bundles of nerves until eventually we're talking about individual axons that are controlling individual effectors okay so these or this pathway down here at the bottom all right this is a multi-polar neuron which we talked about back in our intro to the nervous system we're familiar with that we have cell bodies in the ventral horn so again ventral bigger ones here this is the ventral side the cell bodies and dendrites that belong to these axons that are leaving are located in this gray matter up here so imagine like many many many cell bodies dendrites in here they all exit the spinal cord at the same place at the ventral root and then this information is carrying on to the rest of the body to the effectors where they can be controlled on the other hand coming back the other direction right so now we have information coming into the spinal cord okay so we'll get back to that in a second again cell bodies ventral side axons from the ventral side carrying motor information now um so axon and ventral root and spinal nerve that's this part all right so remember cell bodies and axons here of the motor neurons cell bodies and dendrites here of the motor neurons cell bodies and dendrites axons leaving the spinal cord in this ventral root eventually moving on to the rest of the body okay eventually now this would not be an axon necessarily of the same cell inside the spinal cord it could be depending on on what it's controlling but it could also be one in sort of like a sequence I always talk about that as well later on um so cell bodies and dendrites axons leave eventually those axons either of this neuron itself or perhaps of a middleman sort of neuron is going to eventually terminate on a skeletal muscle where it can then influence its contraction so in that case we would be talking about a somatic uh situation because it's voluntary control right okay so that's kind of our our breakdown for the ventral side of the spinal cord the the belly side of the spinal cord if you will um entirely concerned with motor information on the dorsal side of the spinal cord now now we're talking about sensory information coming in so think about the spinal nerve I'm sorry up to this point right here as like a two-way street right we have motor going this way we have sensory coming this way like two two opposite directions at the at the um Fork on the road here this is where motor information is coming out of the spinal cord and this is where information is going into the spinal cord but at some point they do converge and form that spinal nerve from these two roots from the dorsal and ventral root dorsal and ventral Roots form the spinal nerve eventually okay so sensory information is coming in we're going to have uh Soma remember our cell bodies are in the ganglion which is usually just kind of off to the side over here so it's not in the way so neurons have to have cell bodies but in this case cell bodies are are kind of going to be in the way of the transmission so they're actually located just off to the side like a like a rest area on the side of a highway if we're going with if we're going with that analogy and sticking with it sensor information comes in and then we eventually these axons reach dendrites and cell bodies over here and then the axons of those dendrites and cell bodies come right back into the spinal cord and join up with the dorsal root okay so that's again gray matter Association of cell bodies and dendrites located just off to the side okay so we have our um uh Soma in the ganglion we have axons they come into the spinal nerve these are carrying information from sensory receptors out in the peripheral nervous system and again these could be picking up information about just about anything depending on what kind of sensory cells they actually are all right so hopefully um we're we're doing okay so far I know I misspoke a couple of times but I think I corrected it so um hopefully I didn't confuse anybody ventral side again horns are bigger dorsal side horns are smaller ventral side is for motor dorsal site is for sensory all right so we have our ventral root we have our dorsal root we have a dorsal root ganglia and then ultimately we have a spinal nerve and again please don't forget this is going to be repeated over here as well but having too much of this on one side gets to be a little bit overwhelming so we just have half of it shown with the assumption that we're going to remember that the same thing is happening right over here all right okay so from there let's take a look at some reflexes right we tend to think about sensory information going into the brain being processed and if it's a significant significant enough perhaps a motor impulse being generated in response to that information all right so we have our kind of our classic pathway of sensory neurons in the body interneurons in the brain motor neurons exiting the spinal cord and traveling out to the periphery right but that's not always the case sometimes the brain isn't involved in a pathway sometimes it's just a spinal cord so there's a tendency I think for people to think of the spinal cord as just like um the middleman between the brain and all the sensory neurons out in the body and the spinal cord is just a pathway right it's not just a pathway sometimes the spinal cord is involved directly in this pathway of sensory to interneuron to motor neuron all right and so in that case we have what's called a reflex which I'm sure we're all at least generally familiar with um sometimes sensor information goes from the periphery to the spinal cord the spinal cord then makes a decision about what to do with it and sends information back out via the motor pathway okay so we're actually bypassing the brain in that situation and so what you might want to think about is a situation where that might need to happen and a lot of times that type of situation is one where we need to save a little bit of time rather than sending the information all the way up to the brain and then all the way back down again which doesn't take that long in the grand scheme of things but it does take time it might actually be beneficial to just um skip the brain step and go straight into and out of the spinal cord for our reflux to take place and that's what's going to happen all right we actually have sometimes there are actually there are situations where we have Sensory neurons that meet directly with motor neurons in the spinal cord so sensory comes in axon synapses with the dendrites and cell bodies of a motor neuron which then fires and has an axon which is going to communicate with muscles for example okay so in that case we actually are skipping the interneuron step we go straight from sensory to motor neuron with no interneuron in between and what's more is this is all happening in the spinal cord so it's a much quicker Pathway to go from sensory in the body out in the body in the periphery to the beginnings of a motor neuron of the spinal cord and write that out again with no internet in between that's going to be like more or less the fastest type of reflex we're going to see sometimes there are interneurons in the spinal cord that are involved in processing just like they would be in the brain they're really very similar they're not consciously controlled obviously but in terms of involuntary control that's very very quick this actually this setup works really well to um to kind of like bridge the gap between sensory and motor neurons okay so when these occur they are going to be in the ventral horn and they are going to form what's called a reflex arc all right so there's again two types there is a reflex arc that involves just a sensory and a motor neuron and there are reflex arcs which include a sensory neuron interneuron and a motor neuron all right so that's going to be a little bit different but it's the same end result it's it's an involuntary um response to a stimulus that involves skeletal muscle all right so normally we think of skeletal muscle as being consciously controlled voluntarily controlled it's not always the case they actually can be overwritten with invalid involuntary control with these reflex arcs that we were talking about okay so this is uh it's essentially um this is the overall pathway of information flow in the body okay so we have motor neurons we have spinal neurons we have the brain stem primary motor cortex which we'll talk about in a second the thalamus which we we mentioned back in the brain anatomy section remember the thalamus is kind of like the call center sensory information sensory information coming in from the body from the periphery eventually goes to the thalamus and then the thalamus determines where the information is sent next so whether it's visual information or auditory information or olfactory information or tactile whatever it is the thalamus will then direct that to the appropriate part of the brain where it can be processed okay from the thalamus then there are different options basal nuclei we talked about a bit back in the brain anatomy section as well as well as the cerebellum those can then feed into the association cortex so this is now getting into conscious control and conscious thought up here so now we're in the planning step which is going to involve conscious Control Association cortex is sort of like um uh if you see a fire burning outside you got a campfire going um you see the campfire and you automatically associate it with being hot and kind of indirectly as well associated with possibly damage occurring pain Burns those kinds of things occur you don't think about that necessarily but when you see a fire you associate that with some possible degree of danger right that is the association cortex the initiation of the motor act again this is conscious control now over um skeletal muscle okay so the red step the planning step is conscious the green step and the blue steps are more involuntary or unconscious control think of it that way okay so sensory comes in it can then go into either of these steps up here um and then the execution step coming back down again we have the brain stem so the medulla and so forth we have our spinal neurons we have our motor neurons these are all involved in the execution of some type of effector response okay so the programs ones are involuntary the execution is involuntary but the planning in large part is voluntary okay so that's kind of like the overall picture of what a reflex arc can contain but not all reflex arcs do contain the same structures like we said sometimes it's just limited down here sometimes sensory information just feeds right into the spinal cord and right back out again okay so imagine sensory coming in here to a spinal neuron right so the spinal nerve is feeding into the spinal cord at some particular place sensor information comes in it's processed and then it comes right back out again to the motor neurons and you get a response Okay so there are different types of reflex arcs there are monosynaptic and polysynaptic reflexes a monosynaptic reflex is where you have a sensory sensory neuron synapsing directly with a motor neuron like I just described sensory comes in synapses with motor and a response happens that is a single monosynaptic reflex arc a polysynaptic reflex arc involves a sensory neuron at least one interneuron and then motor neuron so we're adding in now the interneuron or the association neuron between the other two that would be a polysynaptic reflex arc in this case um so we've talked about those as well motor neurons so those are all of our basic parts receptors afferent neurons synapse efferent neurons and then a response and then between that there are varying levels of other control steps okay so let's take a look at our first reflex arc here this is a monosynaptic reflex arc um it's involved in What's called the muscle stretch reflex okay MSR or muscle stretch reflex um another one is called the flexor withdrawal reflex okay so I'll explain these here in a second but these are just different names for different reflex arcs that we're going to talk about we have a different one called a crossed extensor reflex and then we have another one called a Golgi tendon reflex or deep tendon reflex GTR okay um deep tender reflex or Golgi tendon reflex all right so I think actually I have I do okay I'm just going to give you a quick rundown on these for now and then we're going to go back in each one specifically on the next couple slides all right okay um the stretch reflex um is a way for essentially your body to protect your muscles from strain or damage or tearing or however you want to say it okay so if you stretch a muscle um past a certain point your body is going to sort of kick in and say you know what I think that's a bit too much I think if we stretch any more than we already are there's a chance that something might be damaged so once you get to a certain level of stretch of a muscle so imagine you're stretching out your hamstrings you're sitting on the floor you're doing a hamstring stretch you're you're reaching forward towards your toes there's a point where if you reach really really really hard like really intensely your muscles actually won't let you go past a certain point voluntarily because of this muscle stretch reflex if you go any further you actually risk muscle injury so your muscles will only allow you to stretch up to a certain point but not Beyond voluntarily now obviously people tear muscles all the time people strain muscles all the time same thing but when that occurs it's not because of uh conscious sort of effort typically it's due to a fast rapid jerking twisting um type of situation where the muscle stretch reflex doesn't have a chance to kick in and actually protect the muscle from injuring itself okay so when someone's running and they over stretch and tear one of the muscles in the leg um the muscle stretch reflex wasn't it didn't happen in time it wasn't fast enough to protect that muscle from itself because the muscle contraction was was too rapid or in the wrong way or something wasn't stretched properly or whatever the damage actually does occur but it's not because of conscious effort you can stretch as hard as you want um but this reflex will not let you voluntarily stretch past a point where you're going to incur muscle injury okay these next two come together okay the flexor withdrawal reflex and the crossed extensor reflex are going to be two that occur kind of sort of in the same situation think about both of these as being um like jerking your hand away from a hot stove kind of a thing so we'll get back to that one the Golgi tender reflex is a mechanism that's there to help protect your tendons from overstretching okay so remember tendons are going to connect muscles to bones um they are made of dense regular connective tissue white fibrous tissue but these tendons actually have sensor receptors built in so that when the muscle attached to them contracts the the sensory cells the what are called the I guess well the Golgi structures these are not Golgi apparatus like in the cells that produce proteins or help reduce proteins these are different bodies these are called Golgi bodies that are sensory cells that detect the amount of stretch or the amount of force being generated on a tendon so that it's protected from tearing okay so um let's talk about another example let's do bicep curls let's say you know you're at the gym and you're you're doing bicep curls maybe you curled 25 pounds with a dumbbell I don't know um as long as you're being fairly conscientious about how you're doing your curls um you're not going to have any problem at all with damage maybe you decide you want to double your weight normally you do 25 but for some reason you want to see if you can curl 50 pounds that's going to take pretty much all the effort at least for me all the effort I have in my best biceps muscles I can't curl 50 pounds probably anymore um but if you're doing those curls consciously with control and doing it conscientiously you're basically zero risk of tearing um biceps tendons right or biceps muscle for that matter because the MSR is protecting your biceps muscles um we'll talk about what those actually are not right now we don't have to worry about it right now but eventually we'll talk about uh brachialis and biceps brachii and those kinds of things we'll get back to those in our last unit but for right now we're just going to talk about biceps like you know level of understanding that a dude with Jim has biceps biceps big muscle in your arm right we're not going to worry about what they specifically are yet but if you're doing those biceps curls voluntarily using good technique um your muscles are not going to contract so hard that they tear and your muscles are also not going to contract so hard that they tear your tendons all right because if they start to contract so hard that the tendon is in danger this Golgi tendon reflex is going to cut power to that muscle and prevent injury from occurring to that tendon if that makes sense I know it's sort of like not maybe the most um scientific or um fancy sounding terms but uh if the muscle's Contracting too hard the Golgi tandem reflex is going to cut back on that power so that the tendon does not actually tear now of course this is not something we're thinking about this is something that is built in and done automatically for us uh it's there to help protect these tissues in our body okay um now what happens if you go to the gym and you want to do bicep curls and you're really excited about it so you go to the gym you get ready you run right over to the dumbbell rack and you pick up your 50 pound dumbbells and you just try to jerk the dumbbells up as fast as you can you're not you're not warmed up you haven't stretched you're doing way too much weight really fast trying to jerk it up to show how strong you are um that is when you're going to hurt yourself right so if you if you try to lift too quickly then this Golgi tendon reflex and the monosynaptic stretch reflex don't have time to protect the tissues like they're supposed to which is why it's important to do things in like a smooth controlled way especially if you're doing a lot of weight because if you do too much too fast these reflexes don't have enough time to do what they're supposed to do and you're at a greater risk for injury alright so they talk about good form and good technique when you're lifting that's why that's exactly you know one of the reasons why injuries occur foreign okay so let's take a look here at our um spinal reflexes a little bit and then we're going to wrap this section up and start up with section two of this lecture okay so the first player here in a spinal reflex that we're going to talk about are the Alpha motor neurons or otherwise known as the lower motor neurons all right Alpha motor neurons lower motor neurons kind of the same idea okay so these are the motor neurons that are going to help control our skeletal muscles in our body these are motor neurons that are going to allow us to move voluntarily and contract voluntarily right so they're really important cell bodies are going to be in the ventral horn All right so again ventral horn is on the bottom here dorsal horn is on the top I didn't mix it up this time these are ventral the big ones these are dorsal ones up here okay the cell bodies of these Alpha motor neurons are located within these ventral horns and they can be relatively large because they are going to eventually synapse with anywhere between 12 to 1500 muscle fibers each neuron controls up to 1500 muscle fibers all right so we have a new term on the slide that we need to talk about motor unit so a motor unit is defined as a single motor neuron and all of the muscle fibers that it innervates or attaches to or controls that kind of a thing right so a single motor neuron and all of the muscle cells that it controls is referred to as a motor unit okay so another thing to think about here that we I don't think have defined explicitly yet in class is a muscle fiber is synonymous with muscle cell muscle fibers and muscle cells are the same thing okay so I'm going to use those two terms interchangeably because I really want you to get used to hearing both of them and thinking of the same thing muscle cells equal muscle fibers now within the muscle fiber there are lots and lots and lots of other small or very important things that we're not worried about yet we're just worried about okay we know we have muscles they're made of fibers they're controlled by neurons that's basically the level that we're at to this point okay so like we see in this picture the actual configuration of motor neuron organization in the spinal cord is actually mapped okay so we know that um extensors are on the ventral ventral side flexors are more on the dorsal part of the ventral side trunk and extremities are kind of in the middle so trunk muscles here extremities out here even into the lateral horn if there is one um that's where those are going to be controlled okay so don't let this Arrow up here control you this or confuse you this is sensory information coming in here but we're just referring to the the top part of the ventral side of the spinal cord so the dorsal region of the ventral side of the spinal cord the top part up here so trunk is more toward the middle extremities are more lateral flexors are more dorsal extensors are more ventral and again all of that is within the ventral side of the spinal cord okay so if that doesn't make sense make sure you ask I want to make sure we understand that very well because if you don't it's going to be very confusing because it seemingly goes against a lot of what we talked about so far in this in this lecture this ventral side is concerned entirely with motor function but different regions of this side are concerned with controlling different regions of the body's skeletal muscles okay trunk flexors extremities and extensors um okay extensors I don't think we've talked about those terms either or flexors as well an extensor is a muscle that increases the angle between two bones that are joined together okay so if you think about let's think about arms again it's probably the easiest one to kind of visualize you can look at it while I'm talking um your triceps back part of your upper arm back part of your humerus are your triceps muscles those are extensors because when they contract the angle between your radius and ulna which is your forearm and you're humerus which is the upper part of your arm that angle gets bigger and about to about 180 degrees if you extend your your forearm completely all the way out um your humerus and radius are at 100 degree angle sorry 180 degree angle as compared to your humerus now your flexors are going to decrease the angle between two bones so when you contract those biceps muscles and your forearm moves closer toward you now you're decreasing the angle not down to zero because at least I can't track my arm all the way so they're you know next to each other that closely but but close yeah maybe there's a I don't know 10 degree angle between the um humorous and um radius and all or something like that so flexors decrease the angle extensors increase the angle between two adjacent bones um so another type of motor neuron are the G motor neurons and they are going to be the ones that are involved with sensing and um maintaining the proper amount of tension in muscle spindles all right so the muscle spindles are the cells in the muscles that are there actually to detect stretch of those muscles we talked about that before that muscle stretch reflex right there are these Golgi bodies or muscles primarily are just made up of of protein fibers that are contractile and therefore cause you to move but interspersed among those different muscle fibers are these muscle spindles that contain the G motor neurons spindle sense the tension and send that information off to the central nervous system where it can be processed and then if needed um we can either back off the amount of contraction that's occurring or allow for more contraction to occur if if we're not at the full kind of tension level yet um so that's kind of the idea here we've got a skeletal muscle we have individual nerve fibers within that and then associated with those are these muscle spindles that detect stretch okay so gamma motor neurons G motor neurons detect stretch they also are sometimes referred to as intrafusal fibers because they're different from the actual contractile fibers of the muscle some of the muscles made up of actually most of the muscles made up of muscle fibers that contract but it does contain a small percentage of these introfusal fibers controlled by g-neurons gamma neurons which ensure that the muscle is not going to contract at a level that's going to be dangerous to it or that might cause some sort of injury kind of the same thing over here with the Golgi tendon organ again now this isn't a tendon right this is our tendon this is the muscle up here but if this muscle contracts too much it could actually put this tendon in Jeopardy so as this is Contracting there's a limit to how hard it can track how hard it can contract so that this tendon is protected and unless you try to lift something really really heavy really really fast and jerk and twist and lift in every possible wrong way you can think of otherwise this tendon is going to be pretty well protected but if you do something improperly then this GT um GTR or gold retainer reflex doesn't have enough time to kick in and protect the tendon and you actually are or could be at risk of tearing a tendon which is not going to be a pleasant situation okay so let's cut this section off here it's going to be a bit long I think so we're going to cut this one off and we'll start up in just a moment with our second lecture video