The spinal cord is part of the central nervous system. It is composed of white and gray matter. As we can see here, the gray matter is this kind of H area.
Some people say it looks like a butterfly. And the gray matter is going to be composed of unmyelinated axons, specifically interneurons, somas, and dendrites. The white matter, which is going to be on the outside, is going to be composed of only myelinated axons. Now, as we can see, the spinal cord is going to be surrounded by layers of membranes that we call meninges.
There are three spinal meninge layers, and in between the layers are going to be spaces, and the spaces will be filled with fluid. The outermost layer, which is also the thickest layer, is duramator. This layer is very tough, and it's going to give the most protection to the spinal cord. Right underneath the duramator is going to be your subdural space.
About 50% of people have this, and those that don't, it's not a big deal, but this is another space that can be filled with... fluid, specifically cerebrospinal fluid. We know cerebral spinal fluid as CSF. Now, next is the middle layer. This is known as the arachnoid mater.
The arachnoid mater gets its name because it looks like spider webs. The word arachnoid means spider. Between the arachnoid mater and the innermost layer, It's going to be the subarachnoid space.
The subarachnoid space, and it is in this space that they will actually take spinal fluid and analyze it when individuals are sick. So you've heard of spinal taps? Well, they actually put the needle into this space and take some of the fluid to analyze it.
The innermost layer is the piamator. The piamator is going to cover the spinal cord. So it's a very thin layer and it's adhered to the spinal cord. Now what you'll notice is that this inner layer, the pia mater, is going to actually absorb and they help cure the spinal cord in place.
It's basically an extra layer or an extra way to secure the spinal cord between the middle of the vertebral column and the outside of the vertebral column. So here is a picture of a spinal cord. We can see on the outside we have the white matter. And it's kind of hard to see, but on the inside we're going to have your gray matter. This outer layer right here is your duramator.
And the dura mater and arachnoid mater are often fused together if that subdural space is missing, but the subdural space is also very tiny. And then the pia mater is going to be the one that's actually attached to the spinal cord. Now, what we're seeing here is an outside view of the spinal cord from the back of the body.
So this is not a cross-section like the previous picture. This is just a whole entire outer view. We have eight pairs of cervical nerves. This is the only area of the spinal cord where the cervical nerves do not correspond to the amount of vertebrae because we only have seven cervical vertebrae.
We have 12 pairs of thoracic, five pairs of lumbar, and five pairs of sacral nerves. These nerves are your spinal nerve. Now, in your spinal cord, you're going to have an area called the cervical enlargement. This area gets its name because it bulges out when all of the nerves are going to exit to go to your brachial and cervical plexus, which is basically a hot mess of nerves intertwining in a braided network.
So there's the brachial plexus I just circled. The brachial plexus is going to have many nerves coming off of it that are important. Median, radial, and ulnar nerves are some of the most important ones. These nerves are going to travel down your arm and help control many movements such as flexion, rotation, extension.
You also have your lumbar enlargement. Your lumbar enlargement actually...... starts and occurs before the lumbar section starts. You can see that it gets a little thicker where the lumbar enlargement is and the lumbar enlargement thickens because this is where the nerves for the lower limbs exit.
So your lumbar enlargement is going to give rise to the lumbar plexus. As you can see the lumbar plexus is a braided area of nerves also. At the end of the spinal cord We have our proper end and this is known as the conus medullaris.
The conus medullaris is where the spinal cord proper ends. It's basically a little pointed area and beneath all of that is just spinal nerves that have exited. The spinal nerves that exit, they kind of look like a horse's tail.
So they get the name cauda for tail, equina for horse. Now, an extension of the spinal cord is called the philum terminale, so the end of the spinal cord. And it's actually only made up of pia mater.
And the pia mater is actually going to attach the spinal cord to the coccyx. So it's another secure point. Another plexus that I mentioned earlier is the cervical plexus. You see it up here again.
It's a lot of nerves intertwining and forming a braided area. And then we also have the sacral plexus, which is coming out of the sacral region. What's interesting about the sacral plexus is it has the largest nerve in the body exiting, the sciatic nerve. Now back to the internal view of the spinal cord, we're going to learn a little bit about spinal cord anatomy and that's going to help you understand more about the function. So as we can see, this is the posterior portion of your spinal cord.
This is really important because this helps us understand movement of information in and out of the spinal cord. Now the back of the spinal cord is going to have an indention that we call the posterior median sulcus, PMS. Easy way to remember this is when girls are PMSing, they're often considered asses.
So it kind of looks like a butt crack right there. The front of the spinal cord is called the anterior median fissure. You'll notice that the anterior go all the way to the gray matter. Space for it that allows both sides to connect. Then we have the white matter.
We name our white matter columns. So think about all the houses on St. Charles Avenue in New Orleans. They're very large and they have columns in front. What color are the columns mostly?
If you said white, you're correct. So the white matter of the spinal cord is known as columns and it's named for its location. So the posterior columns are in the back, lateral columns on the side, and anterior columns are on the front. But we can't call the gray matter columns. They're known as horns.
We name the gray matter for its location, except for there's going to be one exception. So we have our posterior gray horn there in the back. You'll notice that in the posterior gray horn, we're going to have dendrites present. Sorry, not dendrites, axons present from neurons that come into the spinal cord. gray horns.
In the anterior gray horns we're going to see cell bodies of neurons exiting the spinal cord. So here's the cell body and the neurons are going to exit the spinal cord. But we also have one other horn.
We also have the lateral gray horns. Now what's unique about the lateral grey horns is they're only located in the thoracic and the lumbar region of the cord. So we call it the thoracolumbar region. So if you are looking at a cervical region or a sacral region, you will not have these lateral gray horns. As you can see from the lateral gray horns, there is somas, or cell bodies, of neurons exiting the spinal cord.
In the middle of the gray matter, we have the central canal. This is also where cerebral spinal fluid, CSF, circulates. Surrounding the central canal, we have the gray commissure.
This is where information can cross from one side of the spinal cord to the other side of the spinal cord and vice versa. We also have the anterior white commissure, also known as just the white commissure. It is only going to be located in the front of the spinal cord right before the anterior median fissure meets the gray matter. So we have an area that the white matter can see from left to right to left. Now let's talk a little bit about how information travels.
Sensory information is always going to travel into the back of the spinal cord. So right here, where we have these axons, this is known as the dorsal roots, are also known as the sensory roots. This tells us that sensory information is always traveling into the back of the spinal cord. So as we watch that blue arrow, the sensory information is going... into the back of the spinal cord.
Now sensory neurons that are for somatic senses are always going to be unipolar. So if you look here, what we've seen is a soma with one projection. Part of it goes to the dendrites, the other part goes to the axon. Now whenever we get lots of somas bundled together, we get a swelling.
This swelling of ganglia. Ganglias are named specifically for where they're located, so this ganglia is on the sensory roots. Now, we also have roots in the front.
We call these ants, also known as motor roots. The motor roots are going to get out the front. front of the spinal cord. So sensory information always goes in the back, motor information always comes out the front. Now there are two types of motor neurons that can occur.
Are the motor neurons can be autonomic motor neurons. Somatic motor neurons are going to what part of, or what type of nerve? If you said skeletal, skeletal which is voluntary.
Notice that these motor neurons, the cell body is based on grey horns. But we can also have autonomic motor neurons. Autonomic motor neurons can also be in the anterior gray horns, but they're both motor neurons and they're both going to exit the front of the spinal cord. Now the last thing is the spinal nerve. If we look here, the spinal nerve and the sensory roots merge together.
So what we can see here is in this spinal nerve we have both information going into the spinal cord and motor information coming out. These merge together before the spinal nerve. Now, it's often dangerous to say all in biology, but in this case, it's true. All spinal nerves are mixed because they all contain both sensory and motor roots. Now, sensory roots go in the back of the spinal cord, motor roots come out the front, but when they meet, it forms your spinal nerve and all spinal nerves are mixed.
Now, the last thing is that in between the sensory neurons coming in and the motor neurons exiting, you can have an interneuron. If you have an interneuron, that's going to be where the integration occurs. Now integration always occurs in the gray matter, and usually we do have an interneuron, but you don't have to.
If we have an interneuron present, what happens is it crosses with the interneuron, so we have one synapse, the motor neuron, and we have synapse. Anytime you have more than one synapse in the integration, center, it is called polysynaptic. Again, you do not have to have an interneuron in the integration center for the information to be analyzed. So this is just a real cross-section of a spinal cord.
Notice it's the thoracic. So since it's the thoracic, we will have the lateral gray horns, which are... In the lateral grayhorns, you will often have autonomic cell bodies that are motor neurons exiting. So what's going to happen, this is the back of it. We're going to have sensory coming in and then motor coming out.
Where they meet together is going to form your nerve. And remember, nerve is always mixed.