Welcome back and we're in Sylvius now, so I'm going to open the Brainstem Cross
Sectional Atlas by clicking onto All structures.
And let's go ahead and have a look at the spinal cord first, and we'll consider the
distribution of anterolateral system fibers for the postcranial body.
So here, we're looking at a cross section through the lumbar spinal cord.
Our dorsal roots are entering in the upper part of this image, which is dorsal or
posterior. So here's our dorsal root entry zone, and
we see this really bright region here in the dorsal horn, that's a substantia
gelatinosa. So, out here in the marginal zone and in
the substantia gelatinosa that's where our C fibers are going to terminate.
Whereas more in these deeper parts of the dorsal horn, that's where the A delta
fibers will terminate. Well, from these parts of the gray matter
arise the axons of the projection neurons that grow the anterolateral system fibers.
So, a neuron here at the base of the dorsal horn, let's say this is the left
side of the spinal cord, will grow an axon that will cross in this region of the
white matter. This is what we call the anterior or the
ventral white commissure of the spinal cord.
The axons grow across the midline and then enter the anterior and lateral part of the
white matter here, just to the lateral margin of the ventral horn of the spinal
cord. So this is now what we call the
anterolateral system. So the axons in the anterolateral system
over here on the right side of the image, those axons were grown by neurons that sit
on the opposite side of the dorsal horn. And vice versa for these axons here.
Now, while I have this anterolateral system selected in Sylvia, so I'm simply
going to, now, section up through the spinal cord.
And you'll see that, as we collect up more and more axons, this pathway grows in
size. Now, we're in the cervical cord and we can
see it occupies quite a bit of the white matter in this anterior and lateral
region. As we enter the brainstem you'll now be
comfortable with the idea that additional structures are found that are not present
in the spinal cord. Some of them occupy the ventral part of
the brainstem, including structures like the inferior olivary nuclei out here.
So our anterolateral fibers end up in a more of an intermediate position, but
they're still far lateral. So, we are now in the caudal medulla, as
we ascend through the medulla. These axons sit more or less between the
trigeminal region and the inferior olive. And eventually other components that
occupy the basal or anterior region of the brainstem.
So here is our spinal trigeminal system. We'll get back to that in just a moment.
Here again is our inferior olivary nucleus, and right between the two, is our
anterolateral system. So let's continue to track the progression
of these axons in the lateral part of the tegmentum.
Now, the structures are just to the posterior side of all of these pontine
nuclei and pontocerebellar fibers. And finally, as we get into the midbrain
you'll see that these axons are getting into a more posterior position.
They're still pretty far out lateral, still in the tegmentum.
And where they end up is in a position to supply the ventral posterior complex of
the thalamus, which is right about there. So there is our ventral posterior lateral
nucleus of the thalamus receiving input from these ascending axons from
contralateral dorsal horn neurons. Alright, let's reset.
And consider the organization of the trigeminal counterpart of this
anterolateral pathway. So, this begins at the level of the
trigeminal nerve root. So here is the trigeminal nerve entering
the [unknown] part in the mid pons. You'll remember that the mechanosensory
function of this nerve terminates right here in the chief sensory or the principle
nucleus of the trigeminal complex. But the pain and temperature fibers, they
make a sharp downward bend at this level. And so, what we find is the development of
a compact tract of white matter that is in the lateral part of the tagmentum of the
brainstem. This is a tract made by the central
processes of our nociceptive afferent, so they're present in the trigeminal nerve.
So we have a tract just on the lateral edge of the nucleus and that nucleus is
the spinal trigeminal nucleus with the spinal nucleus of trigeminal complex.
So let's follow this tract on down through the caudal part of the pons and now into
the medulla. And as we progress into the medulla, we
continue to find this well-defined tract, and then, a nucleus that sits just medial
to that tract. Along the way, the axons in the tract are
diving out and terminating neurons that comprise this long column that we call the
spinal trigeminal nucleus. Now, one interesting feature of this
nucleus, is that the closer we get to the junction of the medulla and the spinal
cord, the more this nucleus begins to resemble the dorsal horn.
So in fact, we get to the caudal medulla and now we see a component of this spinal
trigeminal nucleus that looks like the substantia gelatinosa, in fact, we call
this the gelatinosa part of the spinal trigeminal nucleus.
And then, the more medial part of this nucleus begins to look like the base of
the dorsal horn. So, sometimes, we like to say that the
spinal trigeminal nucleus is really an extension of the dorsal horn of the spinal
cord. So maybe that's some help to you
conceptually to recognize that the organization of the second order
projection from the dorsal horn to the thalamus really is quite similar To the
projection from the spinal trigeminal nucleus to it's destination in the
thalamus. So now, we see the dorsal horn again, the
substantia gelatinosa, the base of the dorsal horn.
Okay, well let's now ascend and watch what happens to this trigeminal system.
So, here is our spinal trigeminal nucleus and we have different divisions of it
which are labeled differently is obvious. But now, we've got the most rostral part
of this nucleus in view here and we are about to again enter the level where the
trigeminal nerve hits. So now, we don't have the first order
afferents above this level, because those pain and temperature afferents are
descending. Rather, what we have in the tegmentum is
the second order axons. Now, I haven't indicated the precise
location of this so-called trigeminothalamic tract, sometimes, it's
called the ventral trigeminothalamic tract.
But it's essentially very near the location of the anterolateral system
axons, probably just near the medial edge of these anterolateral system axons right
around there. We know it's going to be in a position to
eventually enter the posterior part of the thalamus and provide input to the ventral
posterior medial nucleus, which is what we see here.
Okay one final point to mention before we conclude this tutorial on the anatomy of
the pain pathways. I want you to recognize the spatial
separation within the nervous system between the pathways for mechanosensation
and the pathways for pain and temperature. We talked about dissociated sensory loss
with spinal cord injury which is [unknown] in part upon the spatial segregation of
those pathways with an ipsilateral pathway in the spinal cord for mechanisensation
and a contralaterally derived pathway for pain and temperature.
In the brainstem, we have a different kind of spatial separation.
We have pain and temperature systems out here in the lateral tegmentum of the
spinal cord. I'm sorry, of the medulla.
But, near the midline of the medulla, we have a mechanosensory pathway, the medial
lemniscus. We don't yet have the facial component,
because we're in the medulla, below the level of the trigeminal nerve.
I say all of this to make the following point.
We will see patterns of strokes in patients that afflict just the medial part
of the medulla, sparing the lateral part, or vice versa.
We may see strobe patterns that affect the lateral part of the tegmentum, sparing the
medial part. And you could imagine what kind of
dissociations you might see in those patient.
One might imagine encountering a patient lets say this is a left side of the
medulla. Where one might find loss of pain and
temperature sensation on the right side of the post-cranial body, but the left side
of the face with no damage at all to our mechanosensory systems.
On the other hand, one might imagine a patient that has damage through the
mechanosensory pathway for the postcranial body, and, no injury to pain and
temperature sensibilities anywhere. Well, these kinds of stroke patterns make
great sense if you understand the distribution of pathways for the various
modalities of somatic sensation. So I hope that gives you some additional
motivation to take on this challenge and make.
Clear and make visible your understanding of this complex clinical neuroanatomy.
