So here again in a view of the intermediate grey matter of the Sacral cord we find the location of these neurons. Now as I mentioned in broader view, what these neurons actually do is they exit the spinal cord through the ventral roots. They bypass our Paravertebral Sympathetic trunk. And rather they make their way to ganglia that are associated with the viscera. And that's where we find our synapse with our post-ganglionic neuron. And the post-ganglionic neuron then supplies, the visceral tissue, whatever that target organ happens to be. Now, before we move on and consider more central aspects of visceral motor integration, I want to suggest, make a couple of, of points of emphasis here. Now, I want you to understand that, there's always coordination of activity, in our sympathetic outflow and our parasympathetic outflow. Yes, they do represent, rather, extreme ends of a continuum of function, but that function is a continuum, and there is tone in the outflow of each division all the time. So, while sometimes it's helpful to emphasize, for example, that our sympathetic division is about fight or flight, or the mobilization of resources for action, I want you to understand that that's not the only circumstance in which there is outflow from our sympathetic division. And likewise. For our parasympathetic division sometimes we consider this to be a division that is restoring resources in times of resting and digesting. And it's perfectly fine to use those kinds of monikers as a way of understanding the principle actions of these divisions. But what I really want you to get is the idea that there is a continuum of action here where there's tone in each side of this visceral motor outflow. And, the, a couple of points that I want you to take away from this are that the functions are complementary, but they're coordinated. There's always some tonic activity in both divisions of the visceral motor outflow, and while it's possible to increase activation in, a wholesale fashion across multiple visceral organs. That is not necessarily how these functions are coordinated. Rather, there can be local coordination of sympathetic and parasympathetic outflow that can serve to coordinate the activity of just one organ system without necessarily engaging the entire visceral effector system, in an all-or-none fashion. Well one important context in which this ongoing coordination of parasympathetic and sympathetic activity is played out at the organ-specific level reflects the operation of reflex activity. So I want us to now begin to move into higher levels of processing in the central nervous system, but before we get there. I want you to appreciate the fact that there is a rich source of sensory information that is coming into the central nervous system, and this information is integrated at various levels. at one level of input, there is integration and an important nucleus of the brain stem that I'll talk about in a little bit more detail in just a moment called the nucleus of the solitary tract. There is also integration in the spinal cord for the viscera of the abdomen especially. And then there is outflow. From this network that's integrating these sensory signals to preganglionic neurons that then can engage in visceral motor response. So this kind of reflex activity is not unlike our segmental reflexes that organize the output of our alpha motor neurons. As we've talked about when we discuss the myotatic reflex, for example. So, there's sensory signals that are integrated at a very local level and that can influence the output of our preganglionic neurons, both parasympathetic and sympathetic. In this way, the activity in a visceral effector system. Can be regulated in a moment to moment basis with adjustments in the tone of our sympathetic or parasympathetic outflow, depending on the organ system in question. Well, I want to say a little bit more about, the sources of this visceral sensory input. So this visceral sensory input essentially comes, through two cranial nerves, and through collaterals of our anterolateral pathway. So, in this illustration, we see those two cranial nerves. They are the glossopharyngeal nerve, cranial nerve nine, and the vagus nerve, cranial nerve ten. So this sensory information comes via the neurons that are associated with the ganglia of these two cranial nerves. And their peripheral process, is very much like our somatic sensory elements that we've already discussed, mechano-sensory axons. no susceptive axons free nerve endings. But in the case of our visceral motor afferents, there are also specialized tissues in certain regions of the body that are sensitive to the chemistry of the blood that's flowing past the structures. Or the chemistry of the extra cellular fluids. And so there are chemoreceptors that are sensitive to changes in pH, in carbon dioxide, in oxygen tensions for example. So there are mechano-sensory afferents, there are nociceptive afferents and there are chemo-sensory afferents, all supplying information that is fed into the central nervous system. So as I mentioned, an important integrator of this incoming information, is this nucleus of the solitary tract. An we've already talked about, this nucleus when we talked about, our gustatory system. As you'll recall, this nucleus really has two principle divisions to it. There's a rostral division which is receiving this incoming special visceral sensory information about taste. And then there is a caudal division and that's the part that I want to focus on here. It's this caudal division that's receiving this input from these cranial nerves and from collaterals of our anterolateral system. So this nucleus of the solitary tract, then, is really a key center that integrates incoming visceral sensory signals. So just to give you a view in a brain stem cross section. This nucleus is really quite distinct. It's found in the upper part of the medulla. And we recognize it. As something that looks sort of like a bull's eye. there is a dark milinated spot right in the middle of the nucleus, so that spot, that is actually the axons of these afferents that are entering this nucleus and its dark because these afferents are well milinated. And the gray matter of the nucleus of the solitary tract surrounds this tract. So, hence, you can understand why this is called the solitary tract. Because it's sort of sitting there by itself in isolation. And it's isolated then by the gray matter of the nucleus that surrounds the solitary tract. [INAUDIBLE]. Well, that's where you would find, this nucleus. The upper part of the medulla. The dorsal aspect of the tegmentum. Now, I want to make just one other comment about visceral sensory afferents. And it's reflecting some fairly recent discoveries about. the way that visceral pain is processed in the central nervous system. There is now a, a well understood pathway that runs through the medial part of the dorsal columns of the spinal cord that conveys visceral pain information. Now this is In addition to the pathways that run through our anterolateral system. So I don't want you to hear me saying that this conflicts with what I just said about the anterolateral afferent supplying the nucleus of the solitary tract. Rather this is an additional pathway that was discovered in the late 80s and the early part of the 90s. And what we think is happening here is a pathway that connects especially the lower GI system. from the first order afferent, which would be a nociceptive element in the appropriate spinal root, to a set of second order neurons that are found near the central canal of the spinal cord. And these neurons send an axon, not to the anterolateral system as we might've anticipated. But rather up along the medial edge of the dorsal columns. And there they synapse with neurons in the dorsal column nuclei. That grow an axon that appears to enter the medial lemniscal pathway and make synapses on neurons in the ventral posterior complex that project not up to the postcentral gyrus, but rather into the insular cortex where representation of. Visceral sensory information, seems to be elaborated at the level of the cerebral cortex. Well, this pathway is worth mentioning because it highlights an important clinical phenomenon that I should mention, and that is referred pain. Now, referred pain is often considered a phenomenon involving a crosstalk between dorsal root ganglion neurons that supply, let's say, skin tissue and dorsal root ganglion neurons that might supply visceral tissue. So, here I'll just indicate that this is maybe the lumen of the gut and there is some kind of a visceral sensory axon innervating that tissue. Well, the central processes of, of these neurons might very well converge upon the same cell in the dorsal horn of the spinal cord. So here is the, spinal cord. Let's just say this is the dorsal horn and somewhere in here is a second-order neuron of the anterolateral pathway. So one might imagine that the pain associated with this pathological process in the viscera might actually get referred to the more peripheral structure. And the fact that these patterns are so predictable, suggest that they actually do align with the organization of the body map, in the dorsal horn of the spinal cord according to the segments of the spinal nerves. So, illustrated here, for example, are visceral pain patterns associated with the esophagus, with the heart, and with the left ureter. So, this dorsal horn cross-talk hypothesis provides a reasonable framework for explaining these referred pain patterns. Well it has occurred to me certainly, and others, that this newly discovered dorsal column visceral pain pathway provides yet an additional possible explanation of referred pain. consider the gracile nucleus, for example. It's integrating inputs from the lower part of the body from mechano-sensory afferents that are more peripheral in their distribution. In the cutaneous surfaces, in the deeper tissues of the limbs, for example. But in experimental studies using known human primates, for example, we know that the very same gracile nucleus neurons may also respond to distension of the bowel. So they seem to be receiving input from visceral nociceptors, in addition to their mechan-osensory afferents concerning the lower extremity. So this might be yet another means by which some kind of crosstalk. might occur at the level of innervation of a distinct gray matter structure. So, just keep in mind referred pain doesn't necessarily have to happen via interactions in the dorsal horn to spinal cord, it actually might happen at multiple levels of our somatic sensory pathways. At least I think this gracile nucleus is a good candidate where some visceral pain might be referred to a more peripheral target. Well, let's get back to thinking more broadly then, about the central integration of visceral sensory signals, as the output of the visceral motor system is organized and ultimately coordinated.
