Three Critical Vertical Pathways: Neuroanatomy Video Lab – Brain Dissections


[ Silence ]>>In order to do neurological
localization and figure out where in your
patient the difficulty is, in addition to the cranial
nerves, it’s important to know three tracts
or pathways. A tract is a collection of axons
coming from a similar place and going to a similar location. Now this tract has
junctions or connections or we call them synapses
in between, but the two sensory tracts come
from the sensory dorsal roots, here I’m showing
you the cauda equina because they’ve been
torn off up here and ascend either connecting
in the cord or in the medulla, relay in deep structures called
the thalamus and eventually come to perception in
the cerebral cortex. So one pathway, one sensory
pathway is the spinal, from the cord, to the thalamus,
the spinothalamic tract and that carries pain and
temperature and some touch. The second important pathway is
the pathway that’s called the dorsal column because some of
the information is travelling in these dorsal columns,
this area of white matter between the forceps here
on the dorsal aspect or the posterior columns
of the spinal cord, relaying up in the medulla
and again onto the thalamus and finally the cerebral cortex. That pathway is called the
dorsal column-medial lemniscus pathway and that carries
proprioception, vibration, joint position, knowing where
your body and muscles are in space as well as
fine and discrete touch. So we have two sensory,
somatic sensory pathways coming from the body and there are
comparable pathways just like it coming in from
the trigeminal nerve but we’re just going to talk about the spinal cord
contribution today. The spinal thalamic or anterolateral pathway
has three neurons. It is always the second neuron
that decussates or crosses to the opposite side to
ascend to the thalamus. The first neuron has its cell
body in the dorsal root ganglion and synapses almost
immediately in the spinal cord. The second neuron crosses to
the opposite side and ascends to the brain stem to the ventral
posterior lateral nucleus of the thalamus. The third neuron located in that
nucleus of the thalamus ascends through the internal
capsule to terminate in the postcentral gyrus, which
is primary somatosensory cortex. The second pathway for
sensation from the body, the dorsal column medial
lemniscus pathway has a very similar pattern and
trajectory except for one very important
difference, the first neuron the
one with cell body in the dorsal root ganglion
does not cross immediately but ascends on the same
side of the spinal cord to the posterior
part of the medulla. There it meets neuron number
2 and crosses to ascend to the thalamus on
the opposite side. The decussation then is in the
caudal medulla, it too ascends to the same thalamic nucleus,
ventral posterior lateral, and different neurons in that
nucleus then send their axons, these are the third neurons,
through the internal capsule to the post central gyrus,
primary somatosensory cortex. The third pathway is a
motor pathway that comes from the cortex and descends
through the spinal cord or down as far as the brainstem cranial
nerves to end in the cord or brainstem on motor neurons
that go out to muscles. Those muscles, striated muscles, voluntary willed movement
controlling my little finger or my big thumb or pinching all
that are willed, not reflexes, these are willed responses. This is the corticospinal
tract or the motor pathway. The important motor pathway
for voluntary control over your striated
muscles is called the corticospinal pathway. There are axons traveling
with this pathway coming from the face area of the
cortex precentral gyrus, also terminating
in the brain stem on cranial nerve motor nuclei. But this diagram shows you only
one going to the spinal cord, descending through the internal
capsule, through the brainstem and crossing, important crossing
or decussation at the junction between the medulla
and the spinal cord. At this point left brain
controls right body. This neuron, this motor control
unit is called the upper motor neuron because it comes
from upstairs in the cortex, it descends and terminates in
the cord therefore we call next or second neuron the
lower motor neuron. The lower motor neuron
is nothing more than the anterior horn cell, also called the ventral horn
cell or just plain motor neuron. This neuron sends its axon
through the ventral root to terminate as the motor
end plate on striated muscle. So three pathways we’re going
to look at corticospinal, motor, spinothalamic, pain
and temperature, dorsal column-medial lemniscus, fine touch vibration
and proprioception. The critical thing is
to be able to visualize where they are close
to cranial nerves and where they are
far apart from them at different levels
of the brainstem. So we’re going to come back
and have to look at our levels of the brainstem, we’re
going to look at medulla, we’re going to look at pons
and we’re going to look at the midbrain as we ascend. And we’re also going to have
to ring this information up to the cerebral cortex. I have a atrophied brain here
which makes the gyri stand out remarkably, because
there’s been so much loss of brain tissue that
the sulci are deep and these gyri are
narrow and remember to define the central sulcus you
find two parallel gyri that come from the inner hemispheric
fissure all the way the way down to the lateral fissure
so this is the central sulcus and keep in mind that behind
the central sulcus is the post central gyrus and this is
sensory cortex and in front of the central sulcus
is the precentral gyrus and that is what we
call motor cortex. So the willed voluntary movement
is going to start with neurons in the precentral gyrus
and the perception of somatic sensation is going
to end in the postcentral gyrus which is somatosensory cortex. [ Silence ]

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