Let's turn now from the post-cranial body to the face, the anterior two-thirds or so of the head, and ask, what's the pathway that serves mechano-sensation for the face? Well, that pathway is called the trigeminal lemniscus pathway, or sometimes we call it the pathway of the chief or principle nucleus of the trigeminal complex. And I'll explain why we call it the trigeminal lemniscus in just a moment. So again, it's a three neuron pathway. From skin to cortex. Okay. Note, that conservation, of the basic organization of the pathway. So the first order neuron, is a ganglion cell, only now it's in the ganglion of the trigeminal nerve. It enters the brain stem through the trigeminal nerve, which as you should recall, plunges right through the transverse fibers on the ventrolateral surface of the pons. At that level, we find our second order neuron, which resides in a nucleus of the trigeminal complex called the principle nucleus, or the chief sensory nucleus. The second order neuron grows an axon, that crosses the mid line, and ascends the remaining divisions of the brain stem, the upper pons and the mid brain, and enters the ventral posterior complex of the thalamus. This pathway we call the trigeminal lemniscus. And it essentially forms near the medial edge of the medial lemniscus. And I'll show you that in just a little bit. Okay, so the third order neuron then, is in a division of the somatic sensory thalamus that we call the ventral posterior medial nucleus, or VPM. So VPL was for the medial lemniscus. The VPM is the target of the trigeminal lemniscus. You should know where these neurons in the ventral posterior medial nucleus project. They project to the postcentral gyrus, but they project to the inferior one-third of that postcentral gyrus. And as they do, they provide a beautiful somatotopic map of the face across this inferior segment of the postcentral gyrus. So, the somatotopy that we see at the level of the cortex really reflects the way the VPL and the VPM map into that cortical to main. Okay, well let's turn our attention now from our system that makes us aware of our light touch sensations and our vibration sensations and our position sensations, to the kind of sensory feedback. Derived from the movements of our musculoskeletal units that serve to coordinate the ongoing control of movement. And what I'm referring to now is the role of this important part of the brain that we'll talk about in a couple of weeks, the cerebellum. The cerebellum, I like to think of as an organ of agility. It helps to facilitate agile movements of body and agile movements of thought. With respect to the body, the cerebellum gets inputs from the cortex about what we are trying to do. And the cerebellum gets feedback from our sensory systems about what we are actually doing. And if there's a problem, as there quite often is, then the cerebellum will generate an error signal that is sent back into our motor system for an adjustment so that some correction can be made as we, as we execute a voluntary movement. So, we think of this as real time, ongoing coordination of voluntary movement. Now, what I want to address is the means by which this cerebellum gets information about how we are actually moving. One important source will be our mechano-sensory systems. That supply the spinal cord. So there is a pathway that runs up the spinal cord connecting to the brain stem via one of the three peduncles, one of the three stalks of white matter attachment. And what I'm referring to specifically is the inferior cerebellar peduncle. So, let's look at this pathway. We consider this pathway part of our spinal cerebellar systems. So, these are spinal cerebellar pathways. So, the receptor here is going to be a proprioceptor. So its a muscle spindle afferent, a Golgi tendon organ, a joint receptor. One of those receptors that's stimulated when we move our musculo-skeletal units. The first order neuron is a dorsal root ganglion cell and its central axon enters the spinal cord and, as we'll see in a few weeks, gives rise to some local collaterals in the ventral horn. That are important for things like the myotactic reflex or the knee jerk reflex. But, what I want to focus on here is the central projection. So, if we're talking about an afferent from the lower body. Then the first order axon enters the dorsal column and ascends to the thoracic spinal cord. There, there is a synaptic connection on a nucleus of the intermediate gray matter of the thoracic spinal cord called Clarke's nucleus. It's also called the dorsal nucleus of Clarke, so either term is fine, Clarke's nucleus, dorsal nucleus of Clarke. Okay, I think I want to make an interjection here, and it's based on an experience with the first one we had through medial neuroscience, where some students were concerned about my declaration that the dorsal-nucleus Clarke is a nucleus of the thoracic spinal cord. And, yet we're looking at a slide that shows a cross section through the lumbar spinal cord with Clarke's nucleus. Now, I know that is a small point, but perhaps it's worth clarifying. Clarke's nucleus extends throughout the thoracic spinal cord, and then to the upper lumbar levels of the spinal cord. So, this figure actually is perhaps not ideal, but it's also not misleading. There is a caudal extension of Clarke's nucleus in the lumbar cord, so it's perfectly fine that we use the lumbar cord for illustration. But I do want you to associate Clarke's nucleus primarily with the thoracic cord. And let's just not be concerned with the inferior limit, whether it's L2, L3, or somewhere in that region. I'd be more pleased that you look for Clarke's nucleus in the thoracic cord and let's just leave it at that. And otherwise. Let's continue with this tutorial. From there, Clarke's nucleus gives rise to a second order projection that runs in the dorsal lateral white matter of the spinal cord. This is called the dorsal spinocerebellar tract. And this pathway runs right on into the cerebellum via the inferior cerebellar peduncle. So this is a pathway that serves the lower extremity. Now you may be wondering about the upper extremity. Well, the upper extremity is served by a pathway that runs in parallel. So again, first order neurons, or dorsal root ganglion cells, and the axons enter through the dorsal root entry zone, but rather than reaching Clarke's nucleus in the thoracic cord, these axons ascend in the dorsal column. And from there, they interact with a nucleus, that is external to the dorsal column nuclei we've already discussed. It's called the external cuneate nucleus, external cuneate nucleus. Not shown here in this illustration, but it's shown in the next illustration. And the external cuneate nucleus then is the second order neuron that projects to the cerebellum information about the upper extremity. Okay, so Clarke's nucleus is concerned with the lower extremity, external cuneate nucleus is concerned with the upper extremity. Now, here's an illustration from my friend and colleague here at Duke Doctor Nell Kent, where she has tried to put this pathway, this Spinocerebellar pathway, together with the dorsal column medial lemniscal system. And what we see, is that propioceptive information derived from things like muscle spindles, golgi tendon organs, enters the spinal cord via projections that run through the dorsal nucleus of Clarke. Through the lower extremity or the external cuneate nucleus for the upper extremity. And, then from there, the pathways enter the cerebellum via the inferior cerebellar peduncle. ICP for short. Now, as we move our bodies in space, we do have some concious awareness of those movements. In fact, the movements of our body in space are part of what consitutes our body schema or body image. So that implies that there must be a way for this information to also enter conscious experience. And there's some concern as to exactly how this happens in the human nervous system, so we don't have a definitive picture of this quite yet, but I think a provisional answer can be that it's these pathways, the dorsal spinocerebellar pathway and the pathway from the external cuneate nucleous. As they arise to enter the cerebellum, they give off collaterals and those collaterals seem to synapse upon neurons, that are either in our dorsal column nuclei, or very near them. And from there, the neurons around the region of the dorsal column nuclei grow axons that contribute to the medial lemniscal pathway. And this allows these proprioceptive signals derive from these final cerebellar relays to also enter our pathway for conscience awareness. Okay, well, we will say much more about this final cerebellar pathways when we talk about the cerebellum in a few weeks. Now what I'd like to do is transition to sylvius and show you what these pathways actually look like. As we ascend from the spinal cord all the way up through the brain stem.
