Well to get started in Sylvius, I'm going to open the brain stem cross-sectional
atlas, and select all structures. And let's begin at the lumbar level of the
spinal chord. So I've just selected the lumbar chord.
And what I'd like to highlight for you is the dorsal column.
So here we have the dorsal column, and as I select it, the label that appears is the
gracile tract, no cuneate tract, right, because we're, we are in the lumbar cord
and we are below the level of the input from the arms, obviously.
So the only axons we find here in the dorsal column are those that are serving
mechanosensory signals from the lower body, the lower extremities primarily.
So if we now proceed to a section higher up in the spinal cord, you'll notice that
we are adding axons To the peripheral sides of this dorsal column region, so
there are some unlabeled axons that are out here laterally, so that's where we are
going to find the cuneate tract, alright, so the gracile tract medial for the lower
extremity, the cuneate tract lateral for the upper extremity.
So these are elements of our dorsal column medial lemniscal system.
Conveying mechanosensory signals from the spinal cord, all the way on up to,
the,[INAUDIBLE] medulla. Now before we get there, let's go back
down to the, lumbar cord, and let's consider the spinocerebellar pathways.
So there may be,[INAUDIBLE] from muscle spindles, and Golgi tendon organs and the
like. Our deep[UNKNOWN] receptors entering this
dorsal[UNKNOWN] entry zone and enter, and entering the dorsal column.
But as we ascend in the dorsal column system to the thoracic cord some of
these[UNKNOWN] are going to dive into the intermediate grey matter in a region that
is on the medial side of this gray matter and right about in this area we have what
we call Clarke's nucleus so the dorsal nucleus of Clarke.
This is the origin of the axon that then sweep out and run from thoracic cord into
the serebone. In the pathway that we call the dorsal
spinocerebellar tract. So there's the dorsal spinocerebellar
tract, now an important point that I want to emphasize, the dorsal spinocerebellar
tract is on the same side of the spinal cord as the entering[UNKNOWN] fiber.
So this allows to establish an ipsilateral Input to the cerebellum from the body
concerning proprioceptive signals. Now, we've been emphasizing that in the
cerebral hemisphere, the principle is contralateral representation.
But I warned you that when it comes to the cerebellum We have a different principle.
It's ipsilateral representation, and this is how it's established.
The an ipsilateral lateral dorsal spinal cerebellar tract that's relaying
information derived from the same side of the body, in this case the lower part of
the body. Now, what about the...
Proprioceptive signals from the cerebellum from the upper part of the body, well here
we are in the cervical cord, those signals are going to enter through the dorsal root
entry zone, and enter the dorsal column in the region of the cuneate fasciculus, and
from there the signals are going to ascend.
Let me just select the[UNKNOWN] tracts, so you can see what will happen there.
They will ascendinto the[UNKNOWN] medulla. So, here we are in the region of
the[UNKNOWN] medella, and we are now very close to where we would find our full
compliment of dorsal-column nuclei, including This far lateral external
cuneate nucleus. So here's our external cuneate nucleus,
and I'll highlight it here on the right side of this image to be consistent with
where we started. So this external cuneate nucleus that's in
view here received input from the ipsilateral apharens concerning
proprioceptions of the upper extremity. From here this external cuneate nucleus is
going to grow axons that will join up with those that are running right next to it
from the dorsal spinal cerebellar tract, and eventually we can follow that tract
all the way into the cerebellum, and at some point...
This tract begins to accumulate axons from the brain stem that are serving the other
purposes. And for that reason, we no longer call it
simply the dorsal spinal cerebellar tract, but rather, we now begin to call it the
inferior cerebellar peduncle. And that peduncle makes its way into the
cerebellum. Which is where we are now.
Okay, now let's return to the dorsal columns of the spinal cord, and consider
again what happens in the dorsal column-medial lemniscal pathway.
Okay? So we sort of jumped ahead a bit to the
spinal cerebellar pathway. Let's get back to the dorsal column-medial
lemniscal pathway. So we have a tracts the cuneate tract and
the gracile tract that are going to ascend in the dorsal column region of the spinal
cord until they hit the[UNKNOWN] and what will happen is that a nucleus will be
encountered essentially on top of this column of white matter, there's a gracile
nucleus sitting on top of the gracile tract and a cuneate nucleus sitting on top
of the cuneate tract. And if we look a few millimeters above,
now we can see that the nucleus is engulfed by the, surrounding axons of the
tract. So this is where we would find our
second-order neurons of our dorsal column medial lemiscal system.
Ok. So we have tracked a nucleus, tracked a
nucleus. Now, these second-order neurons grow out
axons that are going to cross the midline and form A critical pathway of the brain
stem called the medial lemniscus. Now as these axons grow out, we call them
internal arcuate fibers. But once they cross the midline, they
begin to coalesce into a medial ribbon of white matter, and that's what medial
lemniscus means, medial ribbon. So now we can follow this medial lemniscus
all the way up the brain stem. Here we are through the medulla, and at
this level the medial lemniscus is, in the anterior and posterior axis of the brain
stem. But as we rise through the pons, the
inferior, or sorry, the anterior part of the meidal lemniscus begins to move out
laterally. It's as if this medial ribbon is beginning
to twist, and as it twists, while we rise superiorly through the pons and enter the
mid-brain... This pathway is now getting in a position
to hit the ventral posterior complex of the thalamus, that we'll find in the next
section in our atlas. So here's our ventral posterior lateral
nucleus of the thalamus. And it's in a position, ready to receive
the input from the medial lemiscus that we saw just in the previous section.
Ok. And from the ventral posterior lateral
nucleus of the thalamus grows the axons that enter the internal capsule and then
make their way to the post-central gyrus. Allowing for the third-order neuron then
to relay its signals to the level of cerebral processing in the cortex.
Ok. Now, let's complete our journey through
the brain stem for mechanosensory processing by thinking about the signals
derived from the face. And for that we need to get into the pons
at the level of the trigeminal nerve. So here are the nerve roots of the
trigeminal nerve, so the mechanosensory signals that the first order axons enter
the brain stem through the nerve root, where they encounter a nucleus.
So here is the chief sensory nucleus of the trigeminal complex, and from here
there is a outgrowth of a second order axon that sweeps across the mid-line and
ends up being very close to the medial edge and the medial lemniscus, so
somewhere around in here... Is where we expect to find the fibers of
the trigeminal lemniscus. So I'm just going to select the, the
medial lemniscus as a placeholder so that you that there are some[UNKNOWN]
concerning mechanosensation in the face near the medial edge of that pathway, and
then as we ascend through the brain stem on our way to the thalamus...
That medial edge of the medial lemniscus is now in a position to meet the medial
division of the ventral posterior complex. We call the ventral posterior medial
nucleus. So here's our home for our third order
neuron. This is where its cell body resides, and
it grows axons. That join the eternal capsule and project
to the inferior third of the post-central gyrus.
Alright, now you may be wondering how can I possibly learn the, all of this
information and keep this straight? Well, there is really one key strategy
that I would suggest, and that is to draw. I don't care how you draw, you can draw in
the dirt if you like. But I would encourage you to draw and to
make your drawings as large as possible. The bigger the better when it comes to
making visible your knowledge. And as you draw you can hold yourself
accountable to what you actually see from your own hand.
So what you produce is what you can then interrogate.
And here's how you might go about interrogating your drawings: Let's imagine
that you select the left medial lemiscus. Okay, so I'm placing my cursor here on the
left medial lemniscus. You might ask yourself questions like
this. Where are the cell bodies that grew these
axons? Or you might ask, where are the synapses
of these axons? Now I"m going to encourage you to hold
yourself accountable to the precision of your language.
It's my conviction, and my experience that precise language is a reflection of
precise thinking and that's what I want for you precise thinking.
Okay, in answer to the first question where are the cell bodies that grew these
axons the best answer would be in the contra lateral dorsal column nuclei.
