Understanding the Spinal Cord and Reflexes

All right, so this is the second lecture of this third unit, which is the spinal cord. So we did nervous tissue last time, and now we're going to look at the two organs of the central nervous system, which is spinal cord first, and then we'll look at the brain after. So we'll start off by looking at some just general anatomy and get a sort of a just sort of architectural understanding of how this spinal cord is set up and organized, and then we'll look at some physiological concepts primarily looking at reflexes and how those are passed through the spinal cord and sort of how those work. So as I said, spinal reflexes are what the primary thing that we'll be looking at as we go through our lecture and these of course are as you guys know rapid automatic responses triggered by some sort of specific stimuli and they're controlled by the spinal cord. The brain is not sort of getting involved at this time. Speed purposes, right? If you touch something that's hot or you're in a dangerous situation, you need a reflex to remove yourself as quickly as possible. You sort of want that reflex to happen very fast, and so the fact that it can just go to the spinal cord and back until traction is faster. The brain can come into rapid, fast, really quick, automatic responses. So we'll look at spinal reflexes and reflex arcs and how it works. Our cranial reflexes as well, off of cranial nerves, we'll talk about those next time we look at the brain. Alright, so anatomy of the spinal cord, you know, it's about 18 inches long, basically starts obviously at the end of the medulla oblongata, which is the last part of the brain stem, and then runs the length of the spinal cord. truly ends around L1 or L2, and then from that point it starts to taper a little bit and sort of take on a different form. But it's about a half inch wide, and what we call bilateral symmetry, which is basically giving a left and right. These grooves is posterior median silicus and anterior median fissure, which are grooves that divide it into half, essentially. Right, your brain is the same thing. You're going to have two separate distinct hemispheres. But there are certain locations where information can cross over, same with the brain, same with the spinal cord. One of the primary areas for that is called the corpus callosum. And the spinal cord, it's very close to the central sulcus, or central canal, see how that... So, the spinal cord also has enlargements, which are caused by an increase in gray matter in... regions and there's a cervical enlargement and a lumbar enlargement and basically that is because there are increased amount of nerves dedicated for the shoulders and the limbs and the same thing in the lumbar enlargement pelvis and the lower limbs and so you're actually getting enlargement of that distal end of the spinal cord as i said sort of changes below the lumbar enlargement and fill and turn it out which Sort of a thin thread of fibrous tissue, and ultimately it turns into what's called a cauda equina below that. See in this image, you can see the cervical enlargement, the lumbar enlargement, and then again you can see at the conus medullaris, it really starts to change and taper into what we call cauda equina, which is horse's tail, spreads out into fashion. sacral spine. And then again, if you look at a cross-section of the cord, again, you can see these divisions, right? You have a posterior median figure here, right? And then you have a posterior median surface here, and giving you sort of a look. You can also appreciate gray and white matter. We talked about gray matter and white matter before. Gray matter is primarily Neuron cell bodies with white matter primarily. There are 31 spinal cord segments based on where the vertebrae originate. The cervical nerves are named for the inferior vertebrae. All the other names are the inferior vertebrae. So you're learning that in lab right now when you look at the vertebral canal. And you see vertebrae. line up with one another. It creates what are called intervertebral foramen. These adjoining vertebrae come together, a lot of openings for these nerves to exist. All right, so the spinal cord is divided into two branches that we call roots. And if you look at the image, you can see there's a ventral root and a dorsal root. And basically, the dorsal root contains the axons of... sensory neurons, so information coming in, and then the ventral root has axons for motor neurons going back, and so it's sort of divided up, and then ultimately that extends out and forms what's called the spinal nerve. The dorsal and ventral roots join, form a spinal nerve, and ultimately they're able to carry both sensory and motor information. Close to the spinal cord, you see the split, and into this... sensory information coming in motor information so you can sort of see that right here here is the dorsal root right so information coming into the spinal cord goes this posterior route into the spinal cord and then motor information coming out will then hook up with an neuron and come and then again as you see it joins in the spinal nerve the sensory information comes in the dorsal root corresponding motor response then goes out That's what's called a reflex arc, and we'll look at reflex arcs. Also in this image, you can see the meninges. The meninges are a three-layered membranous system that is designed to isolate and protect the spinal cord, and is also actually continuous in the brain. So you have a spinal meninges and a cranial meninges, and they're basically made of the same materials, and they're designed really well. So here's a nice image, again, cross-section through the vertebrae and the spinal cord where you can see meninges. So the spinal, the meninges, as I said, is a specialized membrane system to isolate the spinal cord from the surroundings. Remember, the central nervous system, the brain, and the spinal cord, very delicate tissue that cannot survive in the, you know, general interstitial fluid. It would be far too toxic for... tissue and so it needs to be isolated. There are many different ways we isolate this nervous tissue. We can isolate biochemically, we can isolate it physically, and this is the way one of the ways we physically isolated this membranous wrapping system protects the spinal cord. It's continuous as we saw with the cranial meninges but also as a route for blood supply in the region. So you've probably heard of meningitis. Meningitis is just sort of infection or inflammation of this meningeal system. Either a viral bacterial infection can occur and can be very dangerous right and the reason part of the reason for that is is that it's continuous with cranial meninges and so very easy for infection to work its way up to the brain which if we look at the spinal meninges and again with the cranial meninges you're both components you're going to see three layers you're going to do a mayor arachnoid mater a pia mater The durameter is the outermost layer, a pretty thick, tough, fibrous layer. You'll have the opportunity to see the layer in the front. to see this duramator when you do your brain dissection coming up later in the semester, and that's the first thing to pull off the brain is this thick, tough, fibrous duramator on the outside. Cranially, it actually fuses with the occipital bone and caudalate papers to form the costagia ligament. And so, essentially, this not only is providing a nice, strong outer protection, but it's also anchoring the nervous tissue inside those compounds. So the dura mater also has what's called an epidural space. Epi meaning above, dura meaning the dura mater. So this is the space between the dura mater and the wall of the vertebral in this location. If you look at the previous image, you can see adipose tissue is found in this, so loose connective in adipose tissue. And you may have heard of an epidural or an epidural injection before. Birth and maybe have yourself or know someone that has, right? This is how an anesthesia is done. The biologist delivers an anesthetic injection to a woman who's about to give birth, right? I want to sort of numb the area, and the injection site is delivered directly above the dura mater, injected or inserted in between the vertebrae and basically goes into that adipose tissue area, and that's where the next layer is what's called the arachnoid mater. So this is the middle meningeal layer. The arachnoid membrane has a simple squamous epithelia that covers that arachnoid meter. And a couple of components of it, you have a subdural space between the arachnoid meter and the dura meter, but you also have what's called a subarachnoid space, which is, of course, deeper between pia meter. And here, this is very important because this is where we have CSF. I talked about CSF being in the central canal of the spinal cord. and the ventricles of the brain, but also moves out into the subarachnoid space. So not only is it inside the nervous tissue, it's also surrounding the nervous tissue. In the subarachnoid space, CSF can be withdrawn, right? And you may have heard of a spinal tap or a lumbar puncture. That needle, again, similarly to how we talked about the anesthetic injector, it'll be inserted. This time it's going to go through the deromator, through the arachnoid mater, into the subarachnoid space, and then... You can pull the fluid, withdraw the fluid into a syringe and analyze it. The CSF is going to carry dissolved gases and nutrients and waste products and proteins and other types of things that may give you an insight into what is going on with someone's brain. If they're having issues or suspect Parkinson's disease or Alzheimer's or something, the physician might do a spinal tap to assess. So that's the CSF in that subarachnoid. Also in that subarachnoid space, again, you're going to see vessels, blood vessels, and unfortunately one thing that can happen, a type of stroke that can occur, is called a subarachnoid hemorrhage. We've heard of bleeding in that subarachnoid matic because fluid in that area already with the CSF, but with an additional current. The innermost layer is the piamator, and this is basically sort of a thin mesh of collagen and elastic fibers that's actually directly bound to the neural tissue beneath it, right? So when you do your dissection of the brain, you'll pull off the dura and the arachnoid, and you'll be able to see just sort of a glossy membranous covering, and that's the piamator covering that nervous system. So, again, blood vessels will be found in this area. through the subarachnoid space onto the surface of the spinal pia mater. And you can see in this image, you can sort of see that glossy covering that I was talking about, peeled away a little bit. You can see the anterior median fissure here. You can see the dorsal and ventral roots over here. The deramator is sort of pulled back a little bit there. You can see that. And you can also see a nice, so a lot to see there in that image of the spine. Now, as I said, the sexual anatomy of the spinal cord allows you to see the white matter and the gray matter. The gray matter is, again, containing primarily neuron cell bodies and glial cells, and we call these areas gray horns, right? And the white matter is organized as what we call columns. posterior, anterior, and lateral white columns. So if you look at this image, this gives you a really great understanding of how this system is organized in that you have the dorsal root over here, sensory information coming in, right? It's going to hook up with a motor neuron here and then go back out the ventral root, back out to the target. And so, again, that's what we call a reflex arc. But you can also see easily how... this gray matter and white matter are organized, right? The gray matter is organized into what we call horns, posterior, anterior, and lateral horns. And then the gray matter is organized into what we call collars. We have white column, white column, anterior, white commissure. And you can see those areas where information can cross over, right? Because here you have your posterior median cell case. Here's your anterior median commissure. We have this little space here, so if information does need to cross over to the side, that's where that's going to happen. So just make sure you have a good understanding of how the spinal cord is organized with the gray matter and the white matter, and have a really good understanding also of the dorsal pathway and the ventral pathway and the sensory information coming in. And there's another actual microgram. All right, so... This sort of summarizes a lot of things we talked about, so if you want to look at that to review, you can. But now we want to move on to reflexes, which again is sort of the primary physiological concept we want to take a look at and sort of see how these work. And reflexes are basically automatic coordinated responses within the spinal cord, and they will involve a sensory neuron, a motor neuron, And then sometimes it will also involve interneurons, as I talked about before. Interneurons allow for more complex reflexes, but you may have sensory motor neurons, it would be a little more simpler. So, neuroreflexes, again, are rapid automatic responses to some sort of specific stimuli, and these are the basic building blocks of neural function. So oftentimes, you know, you go for a physical, a physician is going to tell you, do a reflex at your knee, you know the knee-jerk reflex, we're all familiar with that. When a newborn comes into the world, one of the things a pediatrician does is test their certain reflexes to make sure those are present, because again, it demonstrates normal, healthy neural function. Without those reflexes, there may be indicative of, so one neural reflex typically produces one motor response, and the wiring of this reflex pathway is again what we call a reflex arc. It begins at a receptor. And that effector, and typically it's sort of a negative feedback to oppose that original. So what we want to do is take a look at the five steps involved in the reflex arc. And so this is something you want to make sure you have a good understanding of in these. And so the first thing that happens, and this visual shows basically the same thing. First thing is you have arrival of the stimulus, therefore activating the receptor. So in this case, the person's hand. like it's been stimulated by the attack or a nail or something like that, right? Stimulates the nerve ending and then therefore activates the sensory neuron. Sensory neuron sends an impulse back to the spinal cord, of course via the dorsal root, right? Comes in through the gray, posterior gray horn. Information processing then occurs in the central nervous system, right? And so in information processing, that could be an interneuron that could then relay that information up to the brain for instance, but also could send an impulse to a motor neuron which would then send an impulse out the ventral root to the skeletal muscle. So step four is activation of that neuron and then of course the result will be five steps involved in this reflex arc. Activation of the stimulus, excuse me, arrival of the stimulus, activation of the sensory receptor, information processing in the central nervous system, activation of the motor neuron, and then finally response follows the hand away, right? So quick response causing the muscle to... and so reflexes can be classified by development, type of motor response, how complex they are, and then also by information process. So developmental reflexes, right? Some are innate, some things were born with, right? They're actually... formed before birth. Those are the type of reflexes I said, like a pediatrician will come to the hospital first day after a newborn. I test a lot of these reflexes. You're supposed to see certain things, but there are some that are quiet. Rapid automatic learn motor patterns that we learn like in, you know, playing a sport, for instance. If someone plays a sport, they're going to learn certain reflexes, or maybe someone who didn't play that sport may not have those reflexes, right? They're acquired, they're learned, but they do become rapid sort of automatic responses. There also are the type of motor response, of course, somatic reflexes, right, or reflexes of the skeletal muscular system, typically, right? So... like the knee-jerk reflexes, but there also are visceral reflexes. We talk a lot about autonomic reflexes, things that control other than the musculosystem, like in digestion or urination, for instance. We have the micturition reflex in the urinary system that makes us want to urinate, right? And so it's a stretch reflex that stimulates our brain and lets us know that it's time to, you know, release the urine from our bladder. And so... That's what we call a visceral reflex, right? So we'll talk more about those in A&P 2. For now, we're going to focus mostly on somatic reflexes. Reflexes can also vary in complexity. I talked about this previously. You can have monosynaptic reflexes, which are, you know, where you have one specific sensory neuron linking up with a motor neuron. Nothing else. Just one synapse between a sensory and a motor neuron. Or you can have more complex reflexes. that require multiple synapses. We call those polysynapses. So this is something that I want to talk about a little more here in a minute is monosynaptic and polysynaptic. And then finally the side of inflammation processing as I said there are cranial reflexes and those are going to do course occurring in the brain. Spinal reflexes are going to occur. Alright and so this gives you a nice overview of how reflexes can be classified by development. type of response, complexity. All right, so as I said, I want to spend a little bit more time talking about the different types of spinal reflexes that can range based on complexity. We have simple reflexes, and we're going to have more complex reflexes. Monosynaptic reflexes are going to be the most basic ones. Polysynaptic reflexes are going to be more complex. So monosynaptic reflexes, again, have the least delay between sensor input and motor output when you need something to happen very quickly, for instance, right? And so, for example, the stretch reflex, right, like the patella reflex, where you're having a physical and you're sitting up and the doctor strikes the knee, a tendon with a little mallet, and your knee kicks up, right? We've all experienced them. And this happens very quickly, 20 to 40 milliseconds. And that's a great example of a monosynaptic reflex, right? You have the person's leg sitting down. You're putting a stretch in the muscle. The stimulus is applied to the patellar tendon. That activates the receptor in the muscle spindle. That sends an impulse up to the spinal cord via the dorsal root, of course. So as you can see, there's just one sensory neuron, and there's just one motor neuron. So there's only one synapse, hence the term monosynaptic. That then hooks up with it. motor neurons, sends it out to the muscle, the effector results in contraction of the leg. So that's a great example, stretch reflex is a great example of a monosynaptic reflex. And so this image just shows where the receptor is actually in the muscle fiber itself, and then ultimately, so it's a great example of a monosynaptic, polysynaptic reflexes. are more complicated, right? They have typically inner neurons, remember we talked about inner neurons being a type of neuron that stays just within the central nervous system, to control more than one muscle group. Typically they're going to produce either an excitatory response or an inhibitory response to allow for a more complex response. So, withdrawal reflex, for instance, is a great example of a polysynaptic spinal reflex. to move your body away from some sort of stimulus, like if you touch something hot, sharp, or whatever, that is involving a polysynaptic reflex. So as you see in this example, the flexor reflex, you have a painful stimulus, of course, that activates with the sensory receptor, sends an impulse up to the spinal cord, the dorsal root, of course. hooks up with an inner neuron that may send information up to the brain or other locations in the body, but then also will hook up with a motor neuron to stimulate the flexor to contract and pull your arm. Right? But you may also see inhibition of the extensors to allow that muscle to contract. And we'll talk about that a little bit more in a minute. But as you can see, there are one, two, three, four synapses here. So, poly, right? More than one. So that's a great example of a polysynaptic. But again, the difference is this will take a little bit longer, right? Because there's synaptic delay. And so each time... depolarization and release of neurotransmitter occurs at these components you have to account for that time little just slightly longer than so as I said before this concept known as what we call reciprocal inhibition is when you get inhibition of an imposing muscle group to allow for this flexor reflex to work so the stretch reflex of the extensor muscle must be inhibited by inner neurons of the spinal cord. And so that's what we saw with tricep, for instance, being inhibited, allowing the bicep to pull the hand away to allow for, you know, prevent competition. All right, so reflex arts typically are what we call ipsy lateral, meaning they occur on the same side of the body as the stimulus, right? You touch something on your left hand that's hot, you're not going to pull your right hand away, typically the leg, right? And so stretch, tendon, withdrawal reflexes are all a great example of ipsilateral reflex arc. But there are occasions where we need what we call a contralateral reflex arc, where it actually crosses over to the other side of the body. And a cross extensor reflex typically occurs simultaneously, coordinated with that flexor reflex. And so a great example of that cross extensor reflex is like, for instance, when we... step on something that, you know, is sharp, right? I talked about this previously, but, you know, if you step on some sort of painful stimulus, obviously you're going to get a withdrawal reflex to pull your leg up from that stimulus, right? Just like we saw when you pulled your hand away from the hot stove, okay? But in this case, you also need the impulse to cross over to the other side of the body, right? So you don't fall over. So you get the extensors stimulated, the flexors inhibited, and that... allows you to maintain balance. And so as you can see, again, many synapses, if you look up at the spinal cord, one, two, three, four, five, six synapses here. So again, great example of a polysynaptic reflex that also crosses over the... So to wrap things up, I thought clinically we'd talk a little bit about what's called amyotrophic lateral sclerosis, or ALS. ALS involves these actual motor neurons that we've been talking about. And ALS is a lethal, incurable, neurodegenerative disease that basically degenerates these motor neurons, leads ultimately to muscle atrophy, and then typically the sort of fatal event is respiratory muscles aren't able to work, the impulse significantly that respiration's not. Most of the cases of ALS is... Okay, 90% of these cases are sporadic, meaning they have no genetic linkage. 10% are linked to inherited mutations, and of that 10%, 2% are caused by mutations to a gene SOD. Oftentimes, we want to study quite a bit to try to give us clues to perhaps understanding what's happening in the 90% of these sporadic cases. But still, at this point, there isn't a lot of progress made. So as you can see in this image, these are motor neurons, right? Motor neurons are going, somatic motor neurons are going out to skeletal muscle. In a normal, healthy condition, you have a nerve cell, right? It's sending an impulse down a myelinated axon, and it's going to move and contract. And when the muscle contracts, right, it stays very healthy. We talked about this with skeletal muscle. Use it or lose it, right? If you don't contract a muscle, it will atrophy, right? And so in ALS, the motor neurons are destroyed and damaged. The impulses don't get to the muscle, result is a wasted muscle. Among these sporadic cases, most widely held theories are issues with the glial cells, excitotoxicity, or perhaps exposure to some sort of metal or pesticide that could potentially cause these reasons. The other 10% again are specifically related to mutations. So one of those, again, is the superoxide dismutase, S-A-B-1 protein. This protein basically leads to destruction of these neurons that regulate these muscle contractions. There are some new treatments out there trying to look at perhaps blocking the production of SOD1, maybe silencing these individual genes, but again, whenever you sort of play with this genetic side of things, you have potential side effects which could cause even . SOD1 is a pretty well-studied . pathway of genesis. Alright, so death of motor neurons again in ALS leads to difficulty breathing and swallowing, weakness, muscle atrophy, paralysis can occur. Typically, you know, people are going to survive maybe just one to five years with this and again ultimately failure of the respiratory muscles or fatal event that occurs. The selectivity of the neurons is also sort of baffling and interesting. There's no cognitive impairment. Neurons of the bladder and the eye are typically spared, so it can be very challenging for meds. Lulazole is the only medication approved to treat ALS, and you have a very modest effect. On average, only improved survival by a few months. So typically non-pharmacologic therapies like... nutrition, respiratory function, occupational, psychological support, speech therapy, all these types of things are really designed just to help minimize the effect and try to improve the thought of life. All right, so that's it for the spinal cord. We will wrap it up with the next chapter, which is going to be... Looking at the brain, of course, and that'll be the last chapter for this unit, and then we'll move on. So look for the next brain lecture coming up.

Speed purposes, right? If you touch something that's hot or you're in a dangerous situation, you need a reflex to remove yourself as quickly as possible. You sort of want that reflex to happen very fast, and so the fact that it can just go to the spinal cord and back until traction is faster. The brain can come into rapid, fast, really quick, automatic responses.

So we'll look at spinal reflexes and reflex arcs and how it works. Our cranial reflexes as well, off of cranial nerves, we'll talk about those next time we look at the brain. Alright, so anatomy of the spinal cord, you know, it's about 18 inches long, basically starts obviously at the end of the medulla oblongata, which is the last part of the brain stem, and then runs the length of the spinal cord. truly ends around L1 or L2, and then from that point it starts to taper a little bit and sort of take on a different form.

But it's about a half inch wide, and what we call bilateral symmetry, which is basically giving a left and right. These grooves is posterior median silicus and anterior median fissure, which are grooves that divide it into half, essentially. Right, your brain is the same thing.

You're going to have two separate distinct hemispheres. But there are certain locations where information can cross over, same with the brain, same with the spinal cord. One of the primary areas for that is called the corpus callosum.

And the spinal cord, it's very close to the central sulcus, or central canal, see how that... So, the spinal cord also has enlargements, which are caused by an increase in gray matter in... regions and there's a cervical enlargement and a lumbar enlargement and basically that is because there are increased amount of nerves dedicated for the shoulders and the limbs and the same thing in the lumbar enlargement pelvis and the lower limbs and so you're actually getting enlargement of that distal end of the spinal cord as i said sort of changes below the lumbar enlargement and fill and turn it out which Sort of a thin thread of fibrous tissue, and ultimately it turns into what's called a cauda equina below that.

See in this image, you can see the cervical enlargement, the lumbar enlargement, and then again you can see at the conus medullaris, it really starts to change and taper into what we call cauda equina, which is horse's tail, spreads out into fashion. sacral spine. And then again, if you look at a cross-section of the cord, again, you can see these divisions, right? You have a posterior median figure here, right?

And then you have a posterior median surface here, and giving you sort of a look. You can also appreciate gray and white matter. We talked about gray matter and white matter before.

Gray matter is primarily Neuron cell bodies with white matter primarily. There are 31 spinal cord segments based on where the vertebrae originate. The cervical nerves are named for the inferior vertebrae. All the other names are the inferior vertebrae. So you're learning that in lab right now when you look at the vertebral canal.

And you see vertebrae. line up with one another. It creates what are called intervertebral foramen. These adjoining vertebrae come together, a lot of openings for these nerves to exist.

All right, so the spinal cord is divided into two branches that we call roots. And if you look at the image, you can see there's a ventral root and a dorsal root. And basically, the dorsal root contains the axons of... sensory neurons, so information coming in, and then the ventral root has axons for motor neurons going back, and so it's sort of divided up, and then ultimately that extends out and forms what's called the spinal nerve.

The dorsal and ventral roots join, form a spinal nerve, and ultimately they're able to carry both sensory and motor information. Close to the spinal cord, you see the split, and into this... sensory information coming in motor information so you can sort of see that right here here is the dorsal root right so information coming into the spinal cord goes this posterior route into the spinal cord and then motor information coming out will then hook up with an neuron and come and then again as you see it joins in the spinal nerve the sensory information comes in the dorsal root corresponding motor response then goes out That's what's called a reflex arc, and we'll look at reflex arcs.

Also in this image, you can see the meninges. The meninges are a three-layered membranous system that is designed to isolate and protect the spinal cord, and is also actually continuous in the brain. So you have a spinal meninges and a cranial meninges, and they're basically made of the same materials, and they're designed really well. So here's a nice image, again, cross-section through the vertebrae and the spinal cord where you can see meninges.

So the spinal, the meninges, as I said, is a specialized membrane system to isolate the spinal cord from the surroundings. Remember, the central nervous system, the brain, and the spinal cord, very delicate tissue that cannot survive in the, you know, general interstitial fluid. It would be far too toxic for...

tissue and so it needs to be isolated. There are many different ways we isolate this nervous tissue. We can isolate biochemically, we can isolate it physically, and this is the way one of the ways we physically isolated this membranous wrapping system protects the spinal cord.

It's continuous as we saw with the cranial meninges but also as a route for blood supply in the region. So you've probably heard of meningitis. Meningitis is just sort of infection or inflammation of this meningeal system. Either a viral bacterial infection can occur and can be very dangerous right and the reason part of the reason for that is is that it's continuous with cranial meninges and so very easy for infection to work its way up to the brain which if we look at the spinal meninges and again with the cranial meninges you're both components you're going to see three layers you're going to do a mayor arachnoid mater a pia mater The durameter is the outermost layer, a pretty thick, tough, fibrous layer. You'll have the opportunity to see the layer in the front.

to see this duramator when you do your brain dissection coming up later in the semester, and that's the first thing to pull off the brain is this thick, tough, fibrous duramator on the outside. Cranially, it actually fuses with the occipital bone and caudalate papers to form the costagia ligament. And so, essentially, this not only is providing a nice, strong outer protection, but it's also anchoring the nervous tissue inside those compounds.

So the dura mater also has what's called an epidural space. Epi meaning above, dura meaning the dura mater. So this is the space between the dura mater and the wall of the vertebral in this location. If you look at the previous image, you can see adipose tissue is found in this, so loose connective in adipose tissue.

And you may have heard of an epidural or an epidural injection before. Birth and maybe have yourself or know someone that has, right? This is how an anesthesia is done. The biologist delivers an anesthetic injection to a woman who's about to give birth, right?

I want to sort of numb the area, and the injection site is delivered directly above the dura mater, injected or inserted in between the vertebrae and basically goes into that adipose tissue area, and that's where the next layer is what's called the arachnoid mater. So this is the middle meningeal layer. The arachnoid membrane has a simple squamous epithelia that covers that arachnoid meter.

And a couple of components of it, you have a subdural space between the arachnoid meter and the dura meter, but you also have what's called a subarachnoid space, which is, of course, deeper between pia meter. And here, this is very important because this is where we have CSF. I talked about CSF being in the central canal of the spinal cord. and the ventricles of the brain, but also moves out into the subarachnoid space.

So not only is it inside the nervous tissue, it's also surrounding the nervous tissue. In the subarachnoid space, CSF can be withdrawn, right? And you may have heard of a spinal tap or a lumbar puncture. That needle, again, similarly to how we talked about the anesthetic injector, it'll be inserted.

This time it's going to go through the deromator, through the arachnoid mater, into the subarachnoid space, and then... You can pull the fluid, withdraw the fluid into a syringe and analyze it. The CSF is going to carry dissolved gases and nutrients and waste products and proteins and other types of things that may give you an insight into what is going on with someone's brain. If they're having issues or suspect Parkinson's disease or Alzheimer's or something, the physician might do a spinal tap to assess. So that's the CSF in that subarachnoid.

Also in that subarachnoid space, again, you're going to see vessels, blood vessels, and unfortunately one thing that can happen, a type of stroke that can occur, is called a subarachnoid hemorrhage. We've heard of bleeding in that subarachnoid matic because fluid in that area already with the CSF, but with an additional current. The innermost layer is the piamator, and this is basically sort of a thin mesh of collagen and elastic fibers that's actually directly bound to the neural tissue beneath it, right?

So when you do your dissection of the brain, you'll pull off the dura and the arachnoid, and you'll be able to see just sort of a glossy membranous covering, and that's the piamator covering that nervous system. So, again, blood vessels will be found in this area. through the subarachnoid space onto the surface of the spinal pia mater.

And you can see in this image, you can sort of see that glossy covering that I was talking about, peeled away a little bit. You can see the anterior median fissure here. You can see the dorsal and ventral roots over here. The deramator is sort of pulled back a little bit there. You can see that.

And you can also see a nice, so a lot to see there in that image of the spine. Now, as I said, the sexual anatomy of the spinal cord allows you to see the white matter and the gray matter. The gray matter is, again, containing primarily neuron cell bodies and glial cells, and we call these areas gray horns, right? And the white matter is organized as what we call columns.

posterior, anterior, and lateral white columns. So if you look at this image, this gives you a really great understanding of how this system is organized in that you have the dorsal root over here, sensory information coming in, right? It's going to hook up with a motor neuron here and then go back out the ventral root, back out to the target. And so, again, that's what we call a reflex arc. But you can also see easily how...

this gray matter and white matter are organized, right? The gray matter is organized into what we call horns, posterior, anterior, and lateral horns. And then the gray matter is organized into what we call collars. We have white column, white column, anterior, white commissure. And you can see those areas where information can cross over, right?

Because here you have your posterior median cell case. Here's your anterior median commissure. We have this little space here, so if information does need to cross over to the side, that's where that's going to happen. So just make sure you have a good understanding of how the spinal cord is organized with the gray matter and the white matter, and have a really good understanding also of the dorsal pathway and the ventral pathway and the sensory information coming in.

And there's another actual microgram. All right, so... This sort of summarizes a lot of things we talked about, so if you want to look at that to review, you can.

But now we want to move on to reflexes, which again is sort of the primary physiological concept we want to take a look at and sort of see how these work. And reflexes are basically automatic coordinated responses within the spinal cord, and they will involve a sensory neuron, a motor neuron, And then sometimes it will also involve interneurons, as I talked about before. Interneurons allow for more complex reflexes, but you may have sensory motor neurons, it would be a little more simpler. So, neuroreflexes, again, are rapid automatic responses to some sort of specific stimuli, and these are the basic building blocks of neural function.

So oftentimes, you know, you go for a physical, a physician is going to tell you, do a reflex at your knee, you know the knee-jerk reflex, we're all familiar with that. When a newborn comes into the world, one of the things a pediatrician does is test their certain reflexes to make sure those are present, because again, it demonstrates normal, healthy neural function. Without those reflexes, there may be indicative of, so one neural reflex typically produces one motor response, and the wiring of this reflex pathway is again what we call a reflex arc.

It begins at a receptor. And that effector, and typically it's sort of a negative feedback to oppose that original. So what we want to do is take a look at the five steps involved in the reflex arc. And so this is something you want to make sure you have a good understanding of in these.

And so the first thing that happens, and this visual shows basically the same thing. First thing is you have arrival of the stimulus, therefore activating the receptor. So in this case, the person's hand.

like it's been stimulated by the attack or a nail or something like that, right? Stimulates the nerve ending and then therefore activates the sensory neuron. Sensory neuron sends an impulse back to the spinal cord, of course via the dorsal root, right? Comes in through the gray, posterior gray horn.

Information processing then occurs in the central nervous system, right? And so in information processing, that could be an interneuron that could then relay that information up to the brain for instance, but also could send an impulse to a motor neuron which would then send an impulse out the ventral root to the skeletal muscle. So step four is activation of that neuron and then of course the result will be five steps involved in this reflex arc.

Activation of the stimulus, excuse me, arrival of the stimulus, activation of the sensory receptor, information processing in the central nervous system, activation of the motor neuron, and then finally response follows the hand away, right? So quick response causing the muscle to... and so reflexes can be classified by development, type of motor response, how complex they are, and then also by information process. So developmental reflexes, right?

Some are innate, some things were born with, right? They're actually... formed before birth.

Those are the type of reflexes I said, like a pediatrician will come to the hospital first day after a newborn. I test a lot of these reflexes. You're supposed to see certain things, but there are some that are quiet.

Rapid automatic learn motor patterns that we learn like in, you know, playing a sport, for instance. If someone plays a sport, they're going to learn certain reflexes, or maybe someone who didn't play that sport may not have those reflexes, right? They're acquired, they're learned, but they do become rapid sort of automatic responses. There also are the type of motor response, of course, somatic reflexes, right, or reflexes of the skeletal muscular system, typically, right? So...

like the knee-jerk reflexes, but there also are visceral reflexes. We talk a lot about autonomic reflexes, things that control other than the musculosystem, like in digestion or urination, for instance. We have the micturition reflex in the urinary system that makes us want to urinate, right?

And so it's a stretch reflex that stimulates our brain and lets us know that it's time to, you know, release the urine from our bladder. And so... That's what we call a visceral reflex, right? So we'll talk more about those in A&P 2. For now, we're going to focus mostly on somatic reflexes.

Reflexes can also vary in complexity. I talked about this previously. You can have monosynaptic reflexes, which are, you know, where you have one specific sensory neuron linking up with a motor neuron. Nothing else. Just one synapse between a sensory and a motor neuron.

Or you can have more complex reflexes. that require multiple synapses. We call those polysynapses. So this is something that I want to talk about a little more here in a minute is monosynaptic and polysynaptic.

And then finally the side of inflammation processing as I said there are cranial reflexes and those are going to do course occurring in the brain. Spinal reflexes are going to occur. Alright and so this gives you a nice overview of how reflexes can be classified by development.

type of response, complexity. All right, so as I said, I want to spend a little bit more time talking about the different types of spinal reflexes that can range based on complexity. We have simple reflexes, and we're going to have more complex reflexes. Monosynaptic reflexes are going to be the most basic ones.

Polysynaptic reflexes are going to be more complex. So monosynaptic reflexes, again, have the least delay between sensor input and motor output when you need something to happen very quickly, for instance, right? And so, for example, the stretch reflex, right, like the patella reflex, where you're having a physical and you're sitting up and the doctor strikes the knee, a tendon with a little mallet, and your knee kicks up, right? We've all experienced them. And this happens very quickly, 20 to 40 milliseconds.

And that's a great example of a monosynaptic reflex, right? You have the person's leg sitting down. You're putting a stretch in the muscle. The stimulus is applied to the patellar tendon.

That activates the receptor in the muscle spindle. That sends an impulse up to the spinal cord via the dorsal root, of course. So as you can see, there's just one sensory neuron, and there's just one motor neuron. So there's only one synapse, hence the term monosynaptic.

That then hooks up with it. motor neurons, sends it out to the muscle, the effector results in contraction of the leg. So that's a great example, stretch reflex is a great example of a monosynaptic reflex. And so this image just shows where the receptor is actually in the muscle fiber itself, and then ultimately, so it's a great example of a monosynaptic, polysynaptic reflexes.

are more complicated, right? They have typically inner neurons, remember we talked about inner neurons being a type of neuron that stays just within the central nervous system, to control more than one muscle group. Typically they're going to produce either an excitatory response or an inhibitory response to allow for a more complex response. So, withdrawal reflex, for instance, is a great example of a polysynaptic spinal reflex.

to move your body away from some sort of stimulus, like if you touch something hot, sharp, or whatever, that is involving a polysynaptic reflex. So as you see in this example, the flexor reflex, you have a painful stimulus, of course, that activates with the sensory receptor, sends an impulse up to the spinal cord, the dorsal root, of course. hooks up with an inner neuron that may send information up to the brain or other locations in the body, but then also will hook up with a motor neuron to stimulate the flexor to contract and pull your arm.

Right? But you may also see inhibition of the extensors to allow that muscle to contract. And we'll talk about that a little bit more in a minute.

But as you can see, there are one, two, three, four synapses here. So, poly, right? More than one. So that's a great example of a polysynaptic. But again, the difference is this will take a little bit longer, right?

Because there's synaptic delay. And so each time... depolarization and release of neurotransmitter occurs at these components you have to account for that time little just slightly longer than so as I said before this concept known as what we call reciprocal inhibition is when you get inhibition of an imposing muscle group to allow for this flexor reflex to work so the stretch reflex of the extensor muscle must be inhibited by inner neurons of the spinal cord. And so that's what we saw with tricep, for instance, being inhibited, allowing the bicep to pull the hand away to allow for, you know, prevent competition.

All right, so reflex arts typically are what we call ipsy lateral, meaning they occur on the same side of the body as the stimulus, right? You touch something on your left hand that's hot, you're not going to pull your right hand away, typically the leg, right? And so stretch, tendon, withdrawal reflexes are all a great example of ipsilateral reflex arc.

But there are occasions where we need what we call a contralateral reflex arc, where it actually crosses over to the other side of the body. And a cross extensor reflex typically occurs simultaneously, coordinated with that flexor reflex. And so a great example of that cross extensor reflex is like, for instance, when we...

step on something that, you know, is sharp, right? I talked about this previously, but, you know, if you step on some sort of painful stimulus, obviously you're going to get a withdrawal reflex to pull your leg up from that stimulus, right? Just like we saw when you pulled your hand away from the hot stove, okay?

But in this case, you also need the impulse to cross over to the other side of the body, right? So you don't fall over. So you get the extensors stimulated, the flexors inhibited, and that... allows you to maintain balance. And so as you can see, again, many synapses, if you look up at the spinal cord, one, two, three, four, five, six synapses here.

So again, great example of a polysynaptic reflex that also crosses over the... So to wrap things up, I thought clinically we'd talk a little bit about what's called amyotrophic lateral sclerosis, or ALS. ALS involves these actual motor neurons that we've been talking about. And ALS is a lethal, incurable, neurodegenerative disease that basically degenerates these motor neurons, leads ultimately to muscle atrophy, and then typically the sort of fatal event is respiratory muscles aren't able to work, the impulse significantly that respiration's not.

Most of the cases of ALS is... Okay, 90% of these cases are sporadic, meaning they have no genetic linkage. 10% are linked to inherited mutations, and of that 10%, 2% are caused by mutations to a gene SOD. Oftentimes, we want to study quite a bit to try to give us clues to perhaps understanding what's happening in the 90% of these sporadic cases.

But still, at this point, there isn't a lot of progress made. So as you can see in this image, these are motor neurons, right? Motor neurons are going, somatic motor neurons are going out to skeletal muscle. In a normal, healthy condition, you have a nerve cell, right?

It's sending an impulse down a myelinated axon, and it's going to move and contract. And when the muscle contracts, right, it stays very healthy. We talked about this with skeletal muscle.

Use it or lose it, right? If you don't contract a muscle, it will atrophy, right? And so in ALS, the motor neurons are destroyed and damaged. The impulses don't get to the muscle, result is a wasted muscle.

Among these sporadic cases, most widely held theories are issues with the glial cells, excitotoxicity, or perhaps exposure to some sort of metal or pesticide that could potentially cause these reasons. The other 10% again are specifically related to mutations. So one of those, again, is the superoxide dismutase, S-A-B-1 protein.

This protein basically leads to destruction of these neurons that regulate these muscle contractions. There are some new treatments out there trying to look at perhaps blocking the production of SOD1, maybe silencing these individual genes, but again, whenever you sort of play with this genetic side of things, you have potential side effects which could cause even . SOD1 is a pretty well-studied .

pathway of genesis. Alright, so death of motor neurons again in ALS leads to difficulty breathing and swallowing, weakness, muscle atrophy, paralysis can occur. Typically, you know, people are going to survive maybe just one to five years with this and again ultimately failure of the respiratory muscles or fatal event that occurs. The selectivity of the neurons is also sort of baffling and interesting.

There's no cognitive impairment. Neurons of the bladder and the eye are typically spared, so it can be very challenging for meds. Lulazole is the only medication approved to treat ALS, and you have a very modest effect.

On average, only improved survival by a few months. So typically non-pharmacologic therapies like... nutrition, respiratory function, occupational, psychological support, speech therapy, all these types of things are really designed just to help minimize the effect and try to improve the thought of life. All right, so that's it for the spinal cord.

We will wrap it up with the next chapter, which is going to be... Looking at the brain, of course, and that'll be the last chapter for this unit, and then we'll move on. So look for the next brain lecture coming up.

Transcript for:Understanding the Spinal Cord and Reflexes

Transcript for:
Understanding the Spinal Cord and Reflexes