 What I'm drawing here is a sensory pathway. From fingertip to brain, from sensory receptor to sensory cortex. Through a peripheral nerve, the spinal cord, the brainstem, always ending up on the opposite side of the brain. Now if we take everything else away, we see that the pathway is divided into three parts. That's three separate neurons. The first, second and third order neurons. The first order neuron picks up the information and transfers it to the second order neuron, which begins somewhere in the spinal cord or the brainstem. The second order neuron conveys the information to the cell body of the third order neuron, which is usually in the thalamus. The third order neuron conveys the info to the sensory cortex. Now before we go any further, let's just cover some brief revision. Neurons consist of axons and cell bodies. Individually these are microscopic, but when bunched together, they appear white and gray. In the central nervous system, i.e. the brain and the spinal cord, we call discrete chunks of gray matter nuclei. Again, in the CNS, we call chunks of gray matter nuclei. In the peripheral nervous system, i.e. everything apart from the brain and the spinal cord, ganglia. Again, the discrete chunks of gray matter in the peripheral nervous system we call ganglia. The cell body of the first order neuron is packed into a ganglion called the dorsal root ganglion. There are heaps of these, one for every spinal nerve. Just a bunch of cell bodies from nerves that relate to the same spinal nerve. Okay, so now let's talk about some individual sensory pathways. Let's take the spinal thalamic pathway first. That's the brain, brainstem, spinal cord, and yeah, that's it. I'm going to cut away the spinal cord at the level that this nerve that we've been following enters. Let's say that's around T5 or the fifth thoracic spinal nerve. As we mentioned, the first order neuron has its cell body in the dorsal root ganglion. Its axon keeps going and drops its info on the cell body of the second order neuron which will be sitting in the dorsal horn of the spinal cord's gray matter. The dorsal horns there. Okay, from here, this is where the crossover happens for the spinal thalamic pathway. There are two options for where it will cross over to and which path it takes depends on the type of info that neuron is responsible for transmitting. So if that neuron is responsible for transmitting information about crude touch, then its axon will move anteriorly around here. If that neuron is responsible for transmitting information about pain and temperature, its axon will move laterally over here. And these chunks here are the anterior and lateral spinal thalamic tracts. Tract is just a word for a bunch of axons. If we zoom out to get our bearings, yeah, that makes sense, the anterior spinal thalamic tract here, lateral here. So information has already crossed over, let's say, into the anterior spinal thalamic tract. So we continue up toward the thalamus, which sits right on top of the midbrain and which contains the cell body of our third-order neuron. So the thalamus is essentially one big nucleus, all grey matter. And now we've found our cell body in the thalamus for our third-order neuron and its axon shoots to the sensory cortex to drop our information off. If we remember our sensory homunculus, we can work out where this axon will go. The hand and the fingers should be represented somewhere around here. Okay, and that's it for the spinal thalamic pathway. Let's move on to the dorsal column, medial lemniscus pathway. Same again. Our first-order neuron starts out here and has its cell body in the dorsal root ganglion. Let's cut the spinal cord again and move closer because instead of dropping its message here as in the spinal thalamic pathway, the axon keeps on going onward and upward up the spinal cord. Specifically, our axon moves in here to continue upward because all of this here between the dorsal grey horns is the dorsal column. Split down the middle, of course. Right and left. And this neuron will end up around here-ish because this half is split again into two separate parts. Two separate bundles. One called grass aisle, D for grass aisle, and one called cuneate. And if the nerve we were talking about was coming from the legs, they'd be in the grass aisle bundle because gracilis is in the leg. If the nerve's coming from the arms or the upper body, it'll be in the cuneate bundle. This actually makes a lot of sense because if we start adding white matter down here, down the legs, we're going to pack it on immediately, laterally. The leg neuron comes in here and moves up. And upper abdominal neuron comes in here and moves up. And as you can see, pack on medial to lateral. Somewhere along the way, we decided to arbitrarily divide these two bundles. Essentially this is a continuous piling on. But we've divided them in our minds into separate bundles. One that holds everything from below, the grass aisle bundle. And one that holds everything from T6 and above the cuneate bundle. As you recall, our axon was heading into the cuneate bundle. So let's continue on from there. On the dorsal aspect of the spinal cord, we continue moving up. Now first order neuron is still travelling up the cuneate bundle. Now in the medulla here, there's some nuclei that we should know about. There's two cuneate nuclei and two grass aisle nuclei. We want the cuneate obviously. And in there is the cell body of our second order neuron. So here we have C cuneate and G dorsal nucleus. And having reached our cell body in the cuneate nucleus, we're at the critical moments and we are going to swap sides. That's a bit confusing. So again, it's the job of the second order neuron. Axon of the second order neuron which does the crossing over. And now that we've crossed, we're going to move up in a ribbon of white matter called the medial lemniscus. I'll zoom out to get a better look at that. Okay, so here we have on both sides medial lemnisci. Ribbons of white matter. We're going to move up the contralateral medial lemniscus and the cell body of our third order neuron is again anathalamus and this is all revision now. The third order neuron heads from there out to the sensory cortex. Info travelling this pathway is either fine touch, proprioception which is our sense of where our various bodily bits and pieces are in space pressure and vibration. And that's it. I hope you feel like you understand these pathways just a little bit better now. Thank you as always for watching. I hope you liked this video and we'll see you next time.