Hey everyone, welcome to part two of the chapter 13 material where we are going over spinal cord and reflexes. So in part one we talked a lot about some spinal cord structural biology, meninges and things like that. We talked about the sensory ascending spinal cord tracks. Now we're going to talk about the descending motor tracts that carry motor commands from the cerebral cortex down to the skeletal muscles and elsewhere.
And then once we do that, we'll take a look at spinal cord reflexes, or what we call spinal reflexes. So here we have a situation where we have a new map. Now, these are descending pathways.
So what these are doing is we're taking motor commands to skeletal muscles. So these are somatic efferent pathways. Go check out the flow chart that we did at the first slide of this chapter in part one.
So these are somatic efferent portions. the peripheral nervous system so we have new vocabulary here we were looking at ascending pathways previously in the vocabulary there was first order second order and third order neurons here instead if you look up top here on the key we're looking at upper and lower motor neurons so if you look on this diagram the upper motor neurons are in red They all arise from the cerebral cortex. So they start here in the gray matter of the cerebrum.
They descend through the brain and into the brain stem. So this is the midbrain, and this is the medulla oblongata. So all of this is the brain. So from this point up is all brain.
So we have quite a lot going on in the brain stem here, so we have to pay attention to that. Down here is where the spinal cord is, and here is where we terminate at the level that your motor command is going to go out on the spinal nerve to the skeletal muscles. So what you notice here are the lower motor neurons.
These arise, their soma, their cell bodies, are in the ventral gray horn of the spinal cord, and their axons go out on the ventral root. These are motor neurons. These are the neurons that are going to skeletal muscle fibers, motor units, the whole bit that you learned in A&P 1. The upper motor neurons, these are the neurons that start in the cerebral cortex.
Their axons descend all the way through the brain, all the way down the spinal cord, and they synapse on that motor neuron soma in the ventral gray horn. So you only have two. You have the upper motor neuron.
And the lower motor neuron is what exits the central nervous system, becomes a peripheral nervous system neuron. So with these corticospinal pathways, we actually have three tracts here. If you look down here at the bottom, we have the anterior corticospinal tract. We have the lateral corticospinal tract. And we have the corticobulbar tract here up top.
Corticospinal tract, again, I mentioned before. You know a lot of times these are named for where they start and where they end up. Corticospinal, they start in the cerebral cortex and end up in the spinal cord going out through spinal nerves. Corticobulbar, what we are talking about with the bulbar portion here, is they're showing these areas here, here in the midbrain and here in the medulla.
where we have the motor nuclei of the cranial nerves. Okay, this isn't something we've talked about yet, either in lab or lecture specifically. But when we talk about spinal nerves down here, you know, dorsal root, ventral root, most of your body, the skeletal muscle and the sensory and all that, most of your body is innervated through these spinal nerves. So it starts at C1.
the first spinal nerve just outside your cranial cavity where the spinal cord emerges from the form in magnum, all the way down to the coccygeal nerve, and all points in between, those are spinal nerves. And that's where most of your body's innervated from, except there are 12 cranial nerves. So these are nerves that come directly off the brain.
And here we're seeing that some of these cranial nerves come off the brain in the medulla in the midbrain region. So they've just generically called these motor nuclei of cranial nerves, but there are lower motor neurons coming out directly off the brain. Just like down here coming off of the ventral gray horn of the spinal cord out the ventral root into a spinal nerve.
We have the same issue here, same situation with cranial nerves coming off the brain and going to skeletal muscles. So right now. My voice is coming to you because the muscles that run my voice box, and yours too, are innervated with motor neurons from cranial nerves directly from my brain, not from my spinal cord.
So when my primary motor cortex tells my voice box, let's start talking about the corticospinal pathway, those motor commands are coming off of these cranial nerves, not out of the spinal cord. So what we have here is a separate portion of the corticospinal pathways that is specifically the corticobulbar tract, where the upper motor neurons are terminating in the cranial nerve nuclei in the brainstem so that they can synapse to the lower motor neuron and come out. So I can line trace here.
One of these lines represents those corticobulbar neurons, upper motor neurons, that are terminating in the cranial nerve nuclei. So they're part of the somatic efferent pathway. It's just where they're synapsing, and that represents the corticobulbar tract.
All of the rest of this diagram are upper motor neurons that are entering the spinal cord and going to their spinal level and out their spinal nerve on the ventral root. All right, so that's all the rest of them. So these are the corticospinal tracts, right?
Cranial nerves, corticobulbar tracts. Down here, these are the corticospinal tracts. There are two of them, the anterior and the lateral.
Look what happens here. of these upper motor neurons as they come down through the brain stem they uh they cross over they decussate in the medulla oblongata and there's a structure in the medulla oblongata called the medullary pyramid right there's a left and right medullary pyramids and these pyramids provide the decussation point for most of your upper motor neurons and in fact Write this value in right there. About 85% of the upper motor neurons decussate.
Those that do become the lateral corticospinal tract. That's what the lateral corticospinal tract is. is those 85% of those upper motor neurons that decussate in the medullary pyramids. The rest of these, I guess I'll just use plain old red here, the rest of these, about 15%, don't decussate in the pyramids. Those become the anterior corticospinal tract.
and they decussate at their spinal level. So these are the corticospinal pathways. These are how we innervate your skeletal muscles.
So there you go. All right, this diagram, if you look on your lecture outline, you'll see the... under the motor tracts, the descending tracts. So I have on there, you know, upper versus lower motor neurons, so you should understand that. I have then the corticospinal pathway where I have, you know, spots on there for cortical bulbar tracts and corticospinal tracts, medullary pyramids, lateral and anterior tracts.
Okay. Then what I have is something called the medial and lateral pathways. And what I want to try and illustrate for you is, look at that, I was hiding some things over there.
Actually, let me go over here first. Let me do, see the corticospinal pathways. Yeah, pathways.
And then we have the medial pathways and the lateral pathway. What I want to point out is here, we also have some vocabulary here where the corticospinal pathways for motor pathways, again, these are motor. pathways, all of these.
The corticospinal pathways are also called the direct pathways. That's another term for them. Whereas the medial and lateral pathways are called the indirect pathways.
Okay, so what are we talking here? If you consider, I can't really go back in this format to look at the slide just previously, but you have it in front of you on paper maybe, or you can hit pause and go back and look. Go look at these corticospinal pathways.
They come directly from the cerebral cortex down to the brainstem, and they meet up, you know, either in the spinal cord or in the cranial nerve nuclei, synapse with their lower motor neuron. The lower motor neuron goes out to the muscle. Boom, done.
That's a direct pathway. What do we mean by an indirect pathway? I only give you this diagram just to illustrate this topic.
I don't want you to try to memorize this diagram or really properly learn anything from it. Here's an illustration of a couple. of the indirect pathways.
Look down here, we have the rubrospinal tract and the reticulospinal tract. So there's just two of them on here. Look where we start. Generally, we start in the cerebral cortex with motor neuron function, male efferent motor output. Look what happens.
We synapse here and they're showing the lentiform nucleus here. These are the basal nuclei of the cerebrum. So you synapse here.
We've got some, you know, collateral axon action going on here. And look, you got some collateral action going on here. We synapse maybe in the red nucleus. We synapse maybe in the substantia nigra.
So these are nuclei in the brainstem. You pop over here. Maybe you got some collaterals here. You synapse there in reticular formation.
And then you pop down here to a lower motor neuron in the ventral gray horn, and you go out to a skeletal muscle. There ain't nothing direct about this. This is like a pinball being launched at the top and bopping its way down through the pinball machine. That's why they call these indirect. So what you have are you have an original motor command here, but there are lots of other structures in the brain that have...
a say in the regulation of that motor output. These are the indirect pathways. So we have four of them, and I've listed them over here. The first three, if you compare what's typed in here versus what's on your sheet, the first three are medial pathways. The rubrospinal tract is a lateral pathway.
Medial versus lateral mostly refers to the destination for those motor commands. So I'll show you the illustration here for a lateral pathway with the rubrospinal tract when we're done. But I'm just going to take these one at a time. Again, I'm not going to have you recognize a map for each of these, but I do want you to understand when we talk about the vestibulospinal tract what that's all about. Okay, so I'm just going to do these one at a time.
I'll zoom in up here a little bit. So for the vestibulospinal tract, now these originate in the inner ear. So here we're from the inner ear. Actually, let me fix that. From the inner ear.
We haven't done special senses yet, but we will. And when you talk about the ear, the ear is not just for hearing. That's one half of ear function.
The other half of ear function is balance and equilibrium. So the portion of your inner ear that has to do with balance and equilibrium is called the vestibular apparatus. So these are the vestibulospinal tracks. They come from the inner ear vestibular apparatus, all right, they come from the inner ear vestibular apparatus and what they do is they allow for reflexive motor responses to changes in equilibrium imbalance. Let me get done writing this.
all right so let's think about this um the best way i can explain this is if you've experienced where you're standing somewhere and the floor moves underneath you you're like hey wait a minute when does the floor move underneath me well this uh have you ever been on a subway on a train and a bus where you were standing up when it either hit the brakes took a turn slowed down sped up And all of a sudden you find yourself moving unexpectedly and you put your foot out or you put your hand out and grab something and stabilize yourself. Anything like that. If you're on a boat and suddenly hit a wave or something like that. Again, somebody guns the motor or whatever. So if your body all of a sudden ends up in motion.
Unexpectedly, that's picked up with the vestibular apparatus in your inner ear and it initiates a reflexive motor response because it has its own dedicated motor pathway out of that to muscles that it can tap into the regular descending motor tracks. and initiate a reflexive action of your skeletal musculoskeletal system to to address that motion and stabilize yourself so these are these are motor tracks but they're they're kind of indirectly connected into the regular spinal motor pathways so that when needed they can initiate reflexive motor actions That's what these are about. So here's what I'm going to do.
We're going to unveil these one at a time. So here I've written in for the tectospinal tracts. Alright, so the tectospinal tracts, the tectum is a portion of your brain stem.
It comes from an area called the midbrain. So from the colliculi of the midbrain, we're going to learn these structures in lab. The superior and inferior colliculi are brain structures in the midbrain and the tectum of the midbrain. And these allow reflexive motor movements of head and neck in response to visual or auditory stimuli. So you have specific small pieces of your brain stem.
The one set is tied into your eyeballs. When you see movement out of the corner of your eye, it is a reflex. for you to turn your eyes and then your head towards that movement. And that's driven by these colliculi.
If you hear something behind you, it is a reflex for you to turn your head to either orient your ears or your eyes to that sound. And this is, again, driven by these colliculi. So they have a tap-in. Just like the vestibulospinal group, they can tap into the normal motor pathways and co-opt them for a reflexive movement.
when they detect these stimuli. And these are the tectospinal tracts. So here we have the reticulospinal tracts.
These come from another portion of the brainstem called the reticular formation. And what we have here is a set of indirect pathways. Again, these just tap into other normal corticospinal pathways.
And these provide for subconscious regulation of posture and balance. And so your core postural muscles largely, when you're standing up and not, you know, usually don't think about, oh, I'm standing up. How do I stand up? I better stand up straight. 99.99% of your life, you're just standing up and not thinking about it.
Your posture and balance are being regulated through the reticulospinal tracts. All right, so that's another one of these indirect pathways. Let's take a look at the rubrospinal tract.
Oh, here we go. The red nucleus. The rubrospinal tracts, rubro in Latin means red.
So these are the red spinal tracts because they arise from a structure in the brainstem called the red nucleus. This is, it's red because it's highly vascularized and so on cadaverous sections it shows up quite dark and red. This particular structure in the brain works with the cerebellum to help with muscle coordination. And the red nucleus in particular helps coordinate distal upper limb movements.
So if we think about this, I'll do a self-portrait here. Here's me. I'm a happy guy and here here I am there's me okay you know here's my elbow right so I'll draw a little bump on my elbow there's my elbow there's my elbow over there okay now you got fingers five in each hand last I checked okay if you think about manual dexterity think about all of the fine motor movements distal to your elbow, everything you do out here with your hand and fingers. And controlling your hands and fingers is one of the biggest motor coordination learning events in your lifetime as an infant and toddler. So at some point you were sitting in a high chair and, you know, somebody put Cheerios in front of you and you sat there for, you know.
long time trying to figure out how to use two fingers to pick up Cheerios so upper motor or upper limb and Movements require a lot of motor coordination so the rubro spinal tracks one of the things that they do is they're a dedicated subsystem for helping refine These motor movements right here and so If you think about this, if you're standing here, you know, in the anatomical position, you know, this is one of the reasons why the rubrospinal tracts fall under what we call the lateral pathways. If you look on your sheet, the vestibulospinal, retectospinal, reticulospinal, they fall under medial pathways because the, by and large, the muscle groups that they influence core muscle groups whereas here with the ruba spinal tract the muscle groups that they are coordinating are lateral they're on the you know away from the midline and so that's where that term comes from with the lateral pathways so there you have it a basic introduction between what we did here and what we did on the previous slide to the descending motor pathways. Really, it's just the motor output to your skeletal muscles and just give you enough vocabulary to get you started.
So let's now shift gears and let's take a look at reflexes. Okay, everybody, let's talk reflexes. Okay, so this...
diagram. If you look at your lecture outline under reflexes, there's only a few things left for this chapter. And I have under there monosnaptic and polysnaptic. And then under monosnaptic, I have the stretch reflex.
So this right here is the stretch reflex. What does this mean? If one of your muscles gets unexpectedly stretched, now remember we talked about proprioception, your brain has a constant input of information about what your position of your body is in three dimensions, and it also has some predictive power to, you know, predict where your body will be in the immediate future. It knows what you mean to do. It knows if you're sitting in a chair.
It knows if you're walking down the street. So it has some predictive power about what should be happening to you now and in the very near future. All right. If one of your muscles gets unexpectedly stretched, it kicks in a reflex that causes that muscle to immediately contract. So imagine if this might have happened to you at some point.
You're walking through a doorway. Maybe you're wearing some loose clothing. and some of that loose clothing, say a sleeve or a piece of your shirt or jacket, catches on the door handle, and you're walking, and all of a sudden you're jerked backwards. If it catches you just right, you may reflexively pull your arm very strongly towards that, or some other reaction like that.
You may have run into something like this before. Now here, this diagram. The classic clinical diagnostic of the stretch reflex is the rubber hammer on the patellar ligament.
Look, the adaptive value of the stretch reflex has nothing to do with a little rubber hammer tapping on the patellar ligament. This is just a simple trick to trick the quadriceps group into initiating this reflex. Because by tapping on the patellar ligament, it pulls. very sharply on the upper you know distal portion of the rectus femoris and so when that happens and that sharp little rubber hammer tap pulls just right here it initiates the stretch reflex in the in the entire muscle group and so you kick your leg out all they're doing is they're doing a little clinical trick to see if this reflex arc is working okay has nothing really to do with its adaptive significance so what is this i want to you know use this because this is our simplest reflex arc that's why we always start with this one look at what we have we have red here represents the sensory neuron so there's a sensory structure here it says receptor and muscle near tendon we'll look at that here in a moment so there's actually a sensory structure in the muscle called the spindle and that's where that sensory neuron originates and when the spindle detects the stretch it sends the information in through the dorsal root right so there's the soma the cell body of this sensory neuron in the dorsal root ganglion you go into the dorsal root you're in the dorsal gray horn and you synapse now with a motor neuron Here's the cell body of a motor neuron in the ventral gray horn.
The motor neuron exits, so this generates an excitatory postsynaptic potential and initiates an action potential in the cell body of the motor neuron, and that causes the muscle to contract. So this is what we call a reflex arc. The sensation comes into the spinal cord and you have an immediate motor function goes out, motor signal goes out. And what we have here is a situation where there is only one synapse, one synapse between sensation and action. So we call this a monosynaptic.
reflex arc uh hey i gotta learn to spell here um arc so monosynaptic means this is the fastest um i just think you know action potentials are really fast especially in some myelinated neurons they go really fast 50 100 200 meters per second When you go through a synapse, you slow down because synapses go at the speed of chemistry. So you have to open some, you know, if this is a cholinergic synapse, for example, the action potential has to open voltage-gated calcium channels on the terminal. Calcium has to flow in by diffusion. It has to link up with synaptic vesicles, and then the vesicles have to exocytose. acetylcholine neurotransmitter into the synapse.
That acetylcholine has to diffuse across the synaptic cleft. It has to bind to some sodium channels. The sodium channels have to open. Sodium has to flow in, and then you have to initiate a new action potential in the postsynaptic cell. Synapses slow you down.
The more synapses you have in the way, the longer it takes for the response. So monosynaptic reflex arcs are the fastest. And this is our textbook example of a monosynaptic reflex arc. It's the stretch reflex.
So we want to be able to read these diagrams, right? We have the sensory neuron come in. We have a synapse.
You have the motor neuron go out. You have the impact, right? You have the effect.
The quadriceps femoris muscle contracts. Over here, they show us... the actual sensory structure called the muscle spindle.
So this is the muscle spindle. All right, so this is the actual sensory structure for the stretch reflex. you to pay attention here to this diagram up here so here we have a muscle attached to the femur right so we have the greater and lesser trochanters here so I think they're trying to illustrate here maybe the actual rectus femoris muscle and near the tendon not in the tendon but near the tendon you have this little structure a sensory structure called the muscle spindle see it actually has nerves coming to a peripheral nerve right And here they're showing a close-up of this muscle spindle.
Inside, so what is this muscle spindle? We have a sheath of connective tissue in a spindle shape. It's called a muscle spindle because it's spindle-shaped.
It's large in the middle and tapered on both ends. That's a spindle. And so this is what this looks like.
And this spindle structure is sheathed in this dense fibrous connective tissue that's pretty much the same as tendon. And then inside, we have tiny little skeletal muscle fibers. So here's the regular muscle fibers of the muscle, okay? And they call these the extrafusal muscle fibers. The tiny little muscle fibers inside the spindle are called the intrafusal.
muscle fibers. And these are the actual sensory structures. So these are modified muscle fibers.
They have the ability to contract, but that's not their function. Their function is to sit here and be wrapped by these sensory neurons. See all these little yellow lines?
Those are sensory neurons, sensory nerve fibers. And what happens is that if these little specialized skeletal muscle fibers, these intrafusal fibers, if they get stretched, then the sensory fibers are what, you know, kicks off the reflex arc, okay? So it's the sensory fibers here wrapped around these intrafusal muscle spindle fibers.
There's some fun stuff here. I mean, it's a some fairly interesting physiology here. Because these muscle fibers that are part of the muscle, the extrafusal fibers, they're innervated by motor neurons.
So these are alpha motor neurons. So alpha motor neurons are regular motor neurons. So they're showing this motor end plates here. And so since this is one alpha motor neuron, you're seeing a group of fibers that are all being innervated by the same motor neuron. all right so that's a motor unit we learned that vocabulary in amp1 but we also have gamma motor neurons so these are type 1 myelinated these are type 2 myelinated and these motor neurons are actually innervating the intrafusal muscle fibers because When the brain tells the whole muscle to contract, it tells the intrafusal fibers to contract also.
These will stay at the same contraction state as each other because they're receiving the same motor commands. And that's the way to make sure that the spindle is always in proportion to the larger muscle. It's kind of cool, right?
So it's the sensory nerve fibers wrapped around these that detect that stretch. And when they do... they send, they initiate the reflex arc. Now this isn't the whole story though. I mean it's the whole story as far as a stretch reflex arc.
That's it. Boom. You're done.
But I wanted to make sure you understood that what you're looking at here is not completely anatomically correct. These Muscle spindles are in every muscle near the tendon. And they're not just there to initiate a stretch reflex.
They actually have a day job. I mean, they have a 24-7 job in that muscle because these are proprioceptors. They're part of proprioception. So not only are they detecting unexpected stretch, they're just detecting...
on a moment-by-moment basis the position of the whole muscle. That's their actual job. And so they're constantly sending action potentials in. And I want to have us understand that when these, we're going to follow this sensory neuron into the dorsal gray horn, and this sensory neuron is carrying with it proprioception information.
So what's going to happen is you're going to have an axon collateral. And let me make my pen bigger. There's going to be an axon collateral. So here comes this red sensory neuron. You know, sure, it's going to be synapsing here with a motor neuron for potentially initiating a stretch reflex.
But you're also going to have Let me get my pen going here. You're also going to have a collateral here. And that collateral is going to enter the posterior column pathway.
Because remember, the posterior column pathway is our proprioception pathway. And in fact, if this is coming from the leg, I'm guessing it's below T7. And so this is going to be entering the fasciculus gracilis, and it's going to go up to the brain. And up to brain on the posterior column pathway. You're also going to be having a collateral, an axon collateral, coming off of here, right?
Because there's another proprioception pathway, and that's the spinocerebellar pathway. And so this is going to come off here, and that's going to go to the brain too. Only this is going to go up to brain on the spinocerebellar pathway.
So the brain is going to get informed of that motion and that activity. Now it might also this. afferent sensory neuron might also partake in the reflex arc but more often than not is partaking and just sending information up to the brain based on hey what position is the knee in and you're like well how does it know whether to initiate the reflex arc or send information in the brain or both i mean how does it know that's where you get into some of the interesting physiology with these different sensory fibers, primary and secondary.
We're not going to do this here, but Suffice it to say, there are different kinds of sensory fibers, and it's kind of the comparison between the primaries and the secondaries that tells you whether or not you're just simply reporting on position or you're initiating a reflex arc. Okay, so it's, like I said, there's some interesting physiology there, but here we're just trying to learn some reflex. vocabulary and concept here.
So this is our our simplest stretch reflex. It's monosynaptic. It's the fastest. We have afferent sensory neurons, afferent motor neurons.
We have a cause, you have an effect there. So let's take a look at some more complicated reflexes. Okay here is the Golgi tendon reflex. It's listed on your sheet as simply the tendon reflex. You'll see it listed either way.
So here it is. Here's the Golgi tendon reflex. Now, what do we have here? Let me lower this down.
There we go. So we have our sensory neuron, and this diagram is in green, right? And we have a specialized sensory structure called the Golgi tendon organ here. And so that's actually in the tendon.
Remember the muscle spindle was muscle fibers in the muscle near the tendon. This receptor is actually in the tendon itself. It goes into the dorsal gray horn.
Here it's actually showing the anatomically correct version of the collateral axon going up through an ascending tract to the brain. And this is going through the posterior column pathway. Again, probably the fasciculus gracilis because it's coming from the leg. Because the Golgi tendon organs, again, they have a day job. These are proprioceptors.
But they can also initiate a reflexive contraction. Or actually, in this case, not a contraction. Because the Golgi tendon reflex, what does it do?
This prevents dangerous... Oops, I ran out of... Whiteboard here, hold on. Prevents dangerous levels of contraction.
If you've ever paid attention to athletic activity, things like this, people that do extensive weight training, sometimes you hear about folks detaching muscles. And what that means is They pulled, sorry, they pulled so hard on this muscle that it tore the tendon off the bone. That's not generally a good thing, right? That's a tremendous or catastrophic amount of pull.
Or, if it didn't pull the tendon off the bone, it ripped the muscle right in the belly of the muscle. So you have a torn muscle or a detached muscle. So we actually have...
reflexive response the Golgi tendon reflex that if this tendon organ detects contraction strength in the tendon that's too strong for the for the muscle too strong for the tendon it'll initiate a reflex that will shut down the muscle so if the muscles working so hard that it's dangerous There's a reflex to shut it off. It's an emergency off switch. So I'll write that in. It's an emergency off switch. So how do we do this?
Well, let's take a look at the reflex art. So we have the signal coming in on the afferent sensory neuron. There's the uh, unipolar neuron cell body. So, you know, if you can, you can kind of imagine, here's the dorsal root coming in, and there's the dorsal root ganglion, okay, coming into the spinal cord.
And then that sensory neuron enters the spinal cord, there's a collateral axon that goes into the posterior column pathway up to the brain, and there's a collateral axon here. Now, we have what's called an interneuron. That's new.
We have an interneuron here. So if you look at what happens, this sensory neuron excites or stimulates this neuron. And this neuron actually inhibits the motor neuron.
So here's the motor neuron going out to the muscle that's working at a dangerous level. And with an inhibitory interneuron, you shut off or inhibit the motor neuron. and that shuts off or inhibits the muscle. So here we have an interneuron. So you have one synapse here and one synapse here.
We have a polysynaptic reflex arc. So polysynaptic, okay. This means it's going to be a little bit slower, but you tend to see polysynaptic reflex arcs where you need to inhibit things.
Because this sensory neuron coming in can only generate EPSPs, excitatory postsynaptic potentials. And that's going to excite this neuron. But this, since this is an inhibitory interneuron, it has the ability to generate IPSPs.
And it can inhibit the downstream neuron here. So here's what I want you to be able to do. I want you to be able to look at a reflex arc.
I'm going to zoom in here. I want you, for any time there's a synapse here, let me get my pen out, get the right nib size. Any time there's a synapse, right, here's one.
I want you to be able to determine what kind of postsynaptic potential is happening there. Sensory neurons always generate an EPSP. When they enter the gray matter of the spinal cord, they're going to excite the next neuron.
This interneuron here, this inhibitory interneuron, it now... If it's an inhibitory interneuron, it's going to generate an IPSP. At the synapse, you're generating an inhibitory potential.
And what that does is it inhibits the next neuron in line. In this case, where we want to shut down the activity of a muscle fiber or a motor unit, we need to inhibit it. And so inhibitory interneurons generate IPSP. potentials and so this causes the muscle to relax reflexively and to to release the tension on the tendon this is the golgi tendon organ and this is the golgi tendon reflex now we have the same idea here we said look in the muscle spindle and now the golgi tendon organ this sensory structure has a primary job, a day job, I joked. It's supposed to send proprioception information up to the brain, and it does that through the posterior column pathway, and it's also going to go up to the spinocerebellar pathway, just like the muscle spindle on the previous slide.
But you're like, well, how does it know the difference between normal stimulus and dangerous stimulus? And again, you know, we're not going to get into the specifics here, but yeah. You have more than one type of sensory nerve fiber.
And so they're comparing their data. And when you reach a certain threshold for the stretch and tension in there, then that's what kicks in that reflex. So it's not an everyday thing where you trigger the reflex arc.
You might go most of a lifetime without. triggering that reflex arc if you don't do strenuous physical activity. But it's there hiding in the background if you need it.
So there's a bit of complicated physiology about the comparative sensory input between the multiple nerve fibers. That's really what causes this. Here they're just basically showing a simplified diagram for how it works. But I like this because it gives us our first look at a polysynaptic reflex art, and it's a relatively simple wiring diagram to follow. And it introduces us to the idea of an inhibitory neuron.
It's an interneuron because it's between the afferent and the efferent. It's something else. So we call it an interneuron. And this one is inhibitory, and it means it's generating IPSPs on its target cells. All right, here's the withdrawal reflex.
So classic textbook diagram. You reach onto a hot stove and grab a cast iron skillet and there's nothing indicating that that sucker's hot. And you touch it with your hands and then you find out it's hot.
Alright, so here we have painful stimuli coming in on your afferent sensory neuron. There's the cell body and again we can just imagine. Here's that.
dorsal root and there's the dorsal root ganglion and into the dorsal gray horn where we then synapse. Okay, so the idea here is we want to stimulate our flexors to pull the hand away. Okay, now uh you know those have you ever uh have you ever watched a cat? who is so confused about whether to chase its tail or not or it gets so scared but curious it doesn't know whether to run or stay and it just sits there and freezes and you can see this in any anything where the the number of choices you have is overwhelming and so you don't choose any of them and you're frozen in place right well These types of reflexes can be the same way because if you put your hand on that pot, that is a nasty painful stimulus and it could be really hot. And what happens then is the potential is that the muscles of your hand, wrist, forearm, and upper arm could all clench at the same time because it's so painful and shocking.
So if you are trying to use flexors to pull the hand away, but the extensors are also contracting, then the hand's going to stay right there. You're not going anywhere, and you're going to tattoo the shape of a cast iron skillet handle onto the palm of your hand. So what we have to do is, if we're trying to stimulate flexors, we need to inhibit the extensors actively as part of the reflex arc. You want to make sure that the extensors don't get in the way. So that makes for a bit of a complicated wiring diagram.
So let's follow this one piece at a time. So we want to stimulate the flexors, the biceps, for example. So what do we have? We have the sensory neuron comes in and synapses with an interneuron. So again, I said, well...
I want you to be able to illustrate for me, when you see it, where an EPSP is being generated and where an IPSP is being generated. Here, this is an EPSP. We said sensory neurons always generate excitatory potentials. They are always going to stimulate the next neuron they find in the dorsal gray horn.
this right here is an excitatory interneuron. Alright, it's an interneuron because down here we have our motor neurons. So these are the motor neurons.
So what do we have with our excitatory interneuron? It has one branch going up to the brain sending that sensory input up. through one of the ascending pathways.
So it's going to be one of the spinothalamic pathways that carries pain, right? So we got one branch going up that way on the spinothalamic pathway. We have one branch coming down to the motor neuron. And if I change this up just a little bit, this is the motor neuron that's going to the flexor we want stimulated.
And this is an excitatory interneuron. So this is zoom in here get my my pen going here Right here, this is an EPSP, an excitatory postsynaptic potential. You are exciting the motor neuron from the flexor, and you are going to cause that flexor to be stimulated and to pull your hand away. Okay. Now the third collateral for the axon here for our excitatory inner neuron is coming here.
And so let's zoom in on this. And it's an excitatory interneuron. The only thing it can do is generate an EPSP.
That's what it does. It's an excitatory neuron. Now we are exciting this neuron.
Now this black dotted line neuron, this is an inhibitory interneuron. Okay, this inhibitory interneuron, curiously enough, is sending information up to the brain to an ascending pathway. But what we are more concerned about here is the idea now is we're going to inhibit the extensors.
We're going to make sure they don't get in the way of our withdrawal response. And since this is an inhibitory interneuron, it's going to be generating an IPSP here. and it's inhibiting the motor neurons going to the extensor muscles.
So this is a kind of a complicated polysynaptic reflex arc because it has both excitatory and inhibitory interneurons, and part of it is stimulating flexors, and also at the very same time, inhibiting extensors. So there's a few things here. Let's put some vocabulary up. So this is a polysynaptic reflex arc. So it's a polysynaptic arc.
And this kind of situation here, we call this reciprocal inhibition. Reciprocal inhibition. So you're inhibiting the agonistic muscle group, you know, the muscles that are in opposition to the ones you're trying to stimulate.
In this case, you're stimulating the biceps, you're inhibiting the triceps. So this is a withdrawal reflex. It's a very common one and it works pretty well.
I've got one more for you though and what this next one is is a complication of the withdrawal reflex. What I'm going to show you next is just a different implementation of the withdrawal reflex but instead of involving the arms it involves the legs and that makes a difference. So let's take a look. All right let's take a look at this.
So You come down the stairs, I guess this guy's in his boxers, you know, you come out in the middle of the night to go to the bathroom and you hit the bottom of the steps and you step on a tack. Oh, yeah, we've been there, right? So there's that painful stimulus.
Well, what you've got here with this right leg is the withdrawal reflex. I'm going to label that. There's the withdrawal reflex.
Okay, if you do the withdrawal reflex with your hand on your arm, no big deal. You withdraw that hand or arm and, you know, then you've fixed the situation. You are no longer associated with the painful stimulus.
You do the withdrawal reflex with your leg, you fall down. Unless you do the cross. extensor reflex. We're seeing two different reflexes in action right here. One, you would draw the foot from the tack, but with your legs you have to plant the other leg to keep from falling over.
You can't just would draw off the tack, you will fall over. So, What you're going to see here on the withdrawal reflex side, everything here on the right leg side, is a mirror image of what you just saw with the hot pan on the previous slide. You're going to have the exact same wiring layout. Let's take a look.
I'm going to zoom in here. We have sensory neuron comes in, synapses with an excitatory interneuron. that sends a signal to the brain, also excites the flexor, look, flexor stimulated, and it also excites an inhibitory interneuron.
The inhibitory interneuron sends a signal to the brain, and it also excites, or I'm sorry, inhibits the extensor motor neurons, extensor inhibited. This is exactly what we saw. with the hot frying pan. But the cross extensor portion of it, these interneurons both decussate.
They send collaterals across the gray commissures to the opposite side of the spinal cord. And in this case, the excitatory interneuron. Now, in order to plant the off leg on the other side, you have to stimulate the extensors. You were inhibiting the extensors on the withdrawal side. Here you have to stimulate them to plant the leg.
So whatever action is occurring on the withdrawal leg, the opposite action is occurring on the plant leg. So the excitatory interneuron, B-cussates, and a collateral here stimulates the extensor motor neurons. The inhibitory interneuron, B-cussates. And it inhibits the flexor motor neurons on the opposite side. So basically, you have the opposite here of the withdrawal reflex.
And it causes the one leg to come up and one leg to physically plant, just maintain balance. So it's a bit of a complicated diagram, but it's really just two sides of the same coin. So what do we got here? Here's an EPSP.
Here's an EPSP because you're exciting. It's an excitatory interneuron. I'll label these excitatory interneuron. So all it can do is generate EPSPs, right?
And even over here. EPSP. So that tells you which neurons are being excited.
Now over here we have an inhibitory inner neuron. So down here we have here's your IPSP and over here Here's an IPSP there. So we have here, let me shrink this up just a little bit, and remember, so here's a sensory neuron and these are motor neurons. So I, you know, I think you should be able to label every neuron in the diagram. Ones that are excitatory interneurons, ones that are inhibitory interneurons.
And I want you to be able to label what potentials are being generated. Okay, that's what I want you to be able to do. Oops, this is not working so well.
There we go. So what do we got? We have the withdrawal reflex.
We also have the cross extensor reflex. These are polysnapter reflexes, obviously. There's another kind of embedded idea here. And this is called, and you'll see this in the reading, this is called contralateral innervation.
so this is a mirror image well that's that's probably not the best way to to think of it I won't use the word mirror image because that implies exactly the same and it's not okay it's not exactly the same this is creating the opposite reaction, let's say, on the contralateral side. So you can't have these two reflexes in opposition without contralateral innervation. And it's where you have these inner neurons decussating. And where the excitatory inner neuron is exciting the flexor on this side, it's exciting the extensor on the other side, creating the opposite reaction on the contralateral side. So this is contralateral innervation.
So you've got a lot going on in this diagram. But if you understand the wiring diagrams, sensory, motor, inner neurons, excitatory, inhibitory, EPSPs, IPSPs, you can work your way through these diagrams. There's another version of this diagram associated with the Saladin textbook and you know the the artistry is better but I actually think the way they try to do it it's a little more difficult to read but this is showing the withdrawal reflex with the cross extensor reflex. illustrating the idea of contralateral innovation. So if you wanted to look at this diagram and read the associated text in the book, it's going to give you the same thing that we just did over here.
I think those first, you know, in some cases the simpler... I'm sorry this is really bugging out of me. This doesn't want to go over...
there we go. I think the simpler artist they all read diagrams work better for understanding the relationships right they don't have to be so lifelike this one i like because it gives you the full key and it's uh you know it's laid out nice so there you go uh this last uh slide that i want to show you just kind of wraps up some general concepts let's take a look so here we are at the end of the chapter This diagram, I don't know, I put it in here, I guess it's just because I like it. I like this diagram.
It's not anything specific I want you to remember necessarily. It just kind of visually wraps up some of the things we've been talking about. See, it shows the sensory neurons coming into the dorsal root here, entering the dorsal gray horn.
You know, sometimes they're synapsing and going up. to the brain on ascending tracks sometimes you get information coming down on descending tracks and you're going to have synapses and interneurons involved and then you can have motor output going out to the to the muscles so the when they when we show reflex arcs you know it kind of shows the simplest relationships where we're going to go next is into the brain And really, the beauty of a complex nervous system is in all the interconnections inside that allow for information processing. And so it's the interneurons. And that's really where the game is at. So we're going to take a look at some more complicated stuff, really.
But again, this is not a neuroscience course. This is just a basic introduction to nervous system anatomy and function. So we won't get that complex, but we're going to take a look at some interesting stuff. And we're going to see what happens to the brain and from the brain.
We're going to see what happens in the brain. But this gives us a little bit of an idea of what's going on in the spinal cord. We've got stuff going up, we've got stuff coming back.
We've got stuff decussating back and forth. and are right here in the gray matter coming in going out some fun stuff So I'll see you in Chapter 14.