This is to the white matter of the spinal cord. So white matter divided into funiculi. There will be three on each side.
And we'll find tracts going through here. What are tracts? Collections of axons.
So we'll have some ascending and descending tracts. So taking information, sensory input 2. Higher areas of the central nervous system, descending brain motor output, down and eventually out to our effector. Primarily, we're going to find myelinated fibers since it is white matter, but there will be some unmyelinated fibers running through these tracks also. So we look here, we can see their labels.
Posterior. Lateral anterior. So funiculi is plural. Funiculus is singular. So there's a lateral funiculus over here also.
And we'll see some symmetry between those two sides. Because we've got two sides of the body, right? So here's some examples of some ascending tracts.
So we've got one over here and one over here, right, for each side of the body. We'll see symmetry with gray matter also, where we're getting. Setting out information, receiving information from. You do not need to know any of these names, okay? I can't get rid of the table, so it's just there, right?
So our columns, our funiculi, posterior, lateral, anterior. And that's really it for spinal cord white matter. Okay, so next then, while we don't need to learn any specifically named tracts, ascending or descending, we are going to look at a general pathway.
What is the pathway that input is going to take as it goes up to and into our central nervous system? So we'll generally see three neurons in our ascending pathway. So taking that sensory input from a receptor in the peripheral nervous system up into the central nervous system to be analyzed and interpreted.
We'll have a first, second, and third order neuron in that pathway. First order neuron. Conducting those impulses from those receptors, so maybe a receptor in the skin, a first-order neuron is receiving that information from that receptor. We're going to take that stimulus to then conduct that input. So first order neuron picking up that stimulants in the peripheral nervous system.
Carry it then to our second order neuron. I'm going to jump to a picture here and I'll come back to this slide. So here, the toe, we're walking on some carpet. So that receptor in the skin is stimulated.
And the information, the impulse carried by our first-order neuron through that posterior root into our cord, and we're going to come on up and eventually synapse with our second-order neuron. So, well, our first-order neuron is part of the peripheral nervous system. Second-order neuron is an interneuron that we find in the central nervous system.
Let's see, cell body of our second-order neuron. It's often in that posterior horn of the cord. Occasionally we'll find the cell body of a second order neuron in the brain stem instead of in the posterior horn.
We just looked at that, right? Posterior horn contains interneurons that receive sensory input. Bam.
Second order neuron. Then our third order neuron, still an interneuron because it's still in the central nervous system, is in that structure we call the main entrance, the cerebral cortex, where pretty much all sensory input goes through first. What structure was that? The thalamus. So we're kind of bringing those things we've been talking about all together here.
So if we go back to our picture, maybe I'll find it. So we started down here with our first-order neuron, and they're labeled. Again, you do not need to know any of those fancy named tracks, okay? So first order neuron brings in information from the receptor through our posterior root, enters our cord, and ascends. Wee!
Looks like here our second order neuron's in the medulla, which is part of the brain stem. So in this example, brain stem, but other pathways will synapse in a posterior horn. Here, boom, second order neuron crosses over. What's a fancy name for that? Degasation, yeah, degasates, crosses over.
We're at synapses with the third-order neuron in our thalamus. Sort, edit, then relay to the appropriate place in our somatosensory cortex. So if we went back and looked at that map, the somatosensory cortex, this is likely where information is sent for foot. toe area. Let's see, what's the other one?
Fingers, same thing. Receptor, first order through that posterior root. Ascends, synapses in our medulla again. That second order neuron crosses over.
Synapse in the thalamus again, but now we're going to sensory area devoted to the hand. Boom, boom, boom. mm-hmm.
Then we integrate it. What does it mean? Determine a response if necessary, and we're going to send it out along a descending pathway to our effector.
So motor pathways will pretty much always have an upper and lower motor neuron. Occasionally we'll see an interneuron, and here are examples that they have interneurons in between those two structures. So, upper motor neurons. There are cell bodies.
Where do you think they live? There's got to be some green matter if I can out-cell bodies. Where does motor output come from? We're going out to skeletal muscles.
That's coming up. We're not there yet though. We're starting up a little bit higher.
Where does the information originally come from? Cerebral cortex. We've got to start way at the top for this information.
And we are again looking at going out to skeletal muscle, so somatic descending pathways in this example. So upper motor neuron cell bodies are in that cerebral cortex. Coming on down, descending to our spinal cord.
So I'll come back to this if you're still writing. Let's look at our picture here real quick. So cerebral cortex, cell bodies. Coming on down then, projection fibers through that white matter, a little basal nuclei that's also going to influence our pathway.
Keeps on descending, those motor tracks keep descending down through our brain stem. We saw that, right, with midbrain, pons, medulla. Looks like in the medulla we are crossing over.
We're still upper. Do-do-do-do-do-do-do-do. In.
Here we are synapsing with an interneuron. What's up interneuron? It is then going to synapse with our lower motor neuron. And our lower motor neuron cell body lives where?
That's where the anterior horn is coming in. Yep. Anterior horn, a little broader, right? Remember those roots? Posterior root has that bulge at ganglion.
Anterior roots don't. Help us figure out which side, if necessary. So cell bodies of those lower motor neurons live in that anterior horn. So that somatic motor neuron we talked about, cell body's in, anterior heart goes out to our effector.
It's the lower motor neuron. That somatic motor neuron is the lower motor neuron of pathway. And, of course, this is way oversimplifying how this works.
Because before, these can even fire. The basal nuclei has to tell the thalamus it's okay to send input to even let those fire to influence movement. Of course, crossing over, synapsing, lower out to our effector.
And then, of course, we've got to get input from the cerebellum, too, about what motor program, help to pick the proper motor program and influence that. So, yeah, way simplifying it, but still pulling a lot of those parts together that we've been talking about. Bam!
Any questions? Oh, so fast, absolutely, yeah, milliseconds. I mean, for me to keep talking, how many times does that information have to come down there? A lot. It's crazy and awesome!
I'm going to stop that.