So I want you to be able to characterize with words, with pictures in various
media, what these pathways are and how they're organized.
And I want you to be able to recognize them when we actually look at histological
sections through the brain, brain stem, spinal cord.
Okay, well, I'll just remind you of the broad organization of our somatic sensory
pathways. We have two pairs of pathways.
One pair serves the postcranial body, one pair serves the anterior cranium namely,
the face. So, for the postcranial body below the
head and including the posterior portion of the head.
There's the dorsal-column medial lemniscal pathway for mechanosensation.
And what we're going to talk about in this session is the anterolateral system, this
is the pain and temperature pathway for the postcranial body.
For the face, the mechanosensory pathway runs through the chief or principal
sensory nucleus at the trigeminal complex. The pain and temperature pathway runs
through an inferior division of the trigeminal complex called the spinal
trigeminal nucleus. So, I'm going to want you to get familiar
with those distinctions, both in the brainstem and in the spinal cord.
Okay, we briefly introduced our central pathways for pain.
When I talked more generally about our pain systems and we recognize that there
are two qualities of pain, two categories of pain, a first pain and a second pain.
That first pain was sharp, it was somatotopically localized and it was fast.
Well, that first pain is associated with activation of our A delta fibers.
And those A delta fibers provide input to the dorsal horn of the spinal cord.
And from the dorsal horn of the spinal cord, we have neurons that grow axons all
the way from the level of the spinal cord to the level of the thalamus.
Specifically now, for the postcranial body, the ventral posterior complex of the
thalamus. The ventral posterior lateral nucleus to
be most specifically. So, because of the direct connection
between spinal cord and thalamus, this pathway is called the spinothalamic tract.
And once these signals are received by the thalamus, there's a synaptic connection.
And a third order neuron then relays information up to the somatic sensory
cortex. So this is relatively rapid.
This is the transmission of this fast somatotopically precise information about
what body part was injured. And the somatotopic precision about this
is attributable to the fact that this information ultimately gains access to
this exquisite mapping of the contralateral body that we find in our
primary somatic sensory cortex in the postcentral gyrus.
Well, that's what we call first pain or this sharp somatotopically localized, but
thankfully usually transient sense of sharp shooting pain.
The pain that typically follows is more of a dull and aching nature, and the fact
that it feels so differently and often persists for a longer period of time
suggests that, perhaps these are signals that are percolating through a more
widespread set of brain structures. And indeed, that's the case.
Our second pain pathways then have access to a much broader array of brain stem
structures mostly associated with the reticular formation of the brain stem.
But also, a variety of other structures including this important structure, the
periaqueductal gray that is one place where top-down feedback signals might
limit the transmission of nociceptive signals in the spinal cord.
Well, the second pain pathways are distributed all the way up into the
forebrain, structures like the amygdala and the hypothalamus can receive
information about the presence of this painful stimulus.
Once this information gains access to structures like the amygdala and
hypothalamus, then it's in a position to have more widespread impact on the
physiology of the body and in our cognitive state.
In addition to projections that are entering the brainstem core and the medial
part of the forebrain, there are projections that are reaching medial parts
of the thalamus. Those parts that have projections up to
the insular cortex and to the anterior part of the cingulate cortex.
This insular cortex is quite a fascinating part of the brain.
We'll talk briefly about it when we talk more about the visceral motor system.
It seems to come to represent parts of our body, namely our viscera, that are often
in mind when we address questions like how are we feeling.
Well, those feelings tend not to come so much from the skin surfaces.
They tend to come more from our guts, our inside.
And we think that insular region is part of the brain that's integrating these
somatic sensations from visceral sources, as well as more peripheral sources, and
contributing to our emotional sense of well-being that often gets modulated by
pain. Well, all of this complexity on the right
side of the figure is really meant to illustrate the impact of second pain on
our cognitive status and our processing of pain within the context of our emotional
life and our thoughts about the future. Well, thankfully, the anatomy on the
right-hand side is much less clear from a point to point perspective.
So I won't attempt to talk any more about that detail in this session.
Rather, what I want to do is I want to take us through our first pain pathways.
First, for the postcranial body, and then, for the anterior part of the cranium, for
the face. So let's look first at our spinothalamic
tract. So, the first order neuron is a dorsal
root ganglion cell. And we've already considered in a previous
session, the distribution of the free nerve ending in the tissues.
So we're not going to look at that here, rather, we're going to focus on the
central termination of this fiber in the dorsal horn of the spinal cord.
So that's really the first and most important point to make and that is that
the first order afferent of our pain system terminates in the dorsal horn.
So that's where we find our first synapse. Not up in the caudal medulla, but right at
the level of entrance of that first order axon.
Actually not illustrated here, that first order axon may bifurcate and travel up and
down perhaps as much as a segment or two in the spinal cord.
But not much further soon after that distribution across couple of spinal
segments, there will be a synapse. And so, the second order neuron is going
to be within a segment or two of the dorsal root that attaches to the spinal
cord and supplies that axon its entrance into the central nervous system.