BIOS252 Week 6 Lab 6 - Somatic Sensory and Motor Pathways Exercise Somatic Sensory and Motor Pathways To complete this worksheet, select: Module: Regulation Activity: Animations Title: Somatic Sensory and Motor Pathways Introduction 1. What are neural pathways ? The pathways along which information travels through the neurons of the brain. 2. What are the functions of somatic receptors and sensory pathways ? The function of somatic receptors is to allow for feelings of touch, pressure, vibration, stretching of skin, itching, tickling, warmth and cold, pain, muscle length and joint position and movement. The function of somatic sensory pathways is to relay information from the somatic sensory receptors to the cerebral cortex and then to the cerebellum. 3. What is the function of motor pathways ? The function of motor pathways is to initiate movement. Somatic Sensory Pathways 4. Three neuron types are needed for a somatic sensory pathway. Describe the role of each. a.
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