in this segment of the lecture we're going to discuss direct motor pathways
and remember that motor behaviors involve all parts of the nervous system and so we're
going to start at the top and work our way down to the bottom starting with the brain
motor information exits the brain travels through the brainstem and spinal cord and
is associated with a set of nerves in your peripheral nervous system the spinal nerves
which we've talked about in the spinal cord segments and the cranial nerves which are
so special that they're going to get a whole lecture series all by themselves
so let's start at the very beginning at the very top in our premotor cortex so we talked
about the frontal lobe and how that's the area of your brain where you think and you
plan and you make decisions and so even before you're aware of wanting to get up and go to the
refrigerator it's all happening out here in your premotor cortex once you make that decision to
actually get up out of your chair and walk over to the refrigerator neurons in your precentral
gyrus which is your primary motor cortex are going to become activated we call these upper
motor neurons these neurons send out a very long axon and we're going to spend the next
several segments talking about where that axon travels once upper motor neuron in the precentral
gyrus is activated then that axon is going to travel through a great big fiber pathway within
the central nervous system called the internal capsule and you can see it here so here's the
internal capsule this is a coronal section and in a horizontal section I can't spell
never mind in this horizontal space section you can see the internal capsule here it's this big
fiber band the neurons that we're going to be talking about in the motor cortex that go
to the spinal cord to interact with neurons that are going to contribute to your spinal
nerves are all traveling in the posterior limb we'll talk about the posterior limb
of the so the posterior limb of the internal capsule is associated with motor behaviors
basically below the neck motor neurons that are going to innervate the targets in the brainstem
that innervate your cranial nerves so that the cranial nerves innervate structures in your head
and neck those primary or upper motor neurons still live up here but they travel in a special
place called the genu or the bend of the internal capsule so the genu is going to be motor behaviors
above the neck and what we didn't talk about in a great bit in a lot of detail in the sensory
areas is that the anterior limb is going to be carrying some sensory information but don't worry
about that we covered all we're going to cover with the sensory systems once the the neuron the
axons of those upper motor neurons actually leave the brain proper they become the crus cerebri and
you can see this on the ventral side of the brainstem and if you make a section through
this through the brainstem it looks like this this is the image that you're going to see if you
look at a neuronal axon an atlas of the nervous system we now call these fibers first they were
just kind of a random assortment of axons then they all came together in the internal capsule
now that they're outside the brain we call them the crus cerebri and i'm really sorry about this
i'm really sorry that this track changes names so many times as it leaves that goes from the
top of the brain all the way down to the spinal cord I wasn't there when they named it so I but
I apologize for my predecessors but in their defense they didn't really understand that these were the
exact same axons traveling from one place to the other so they basically just kind of named him for
where they could see them so this is the image that you will see in an atlas with this being
dorsal and this being ventral and this is the image that you kind of see if if you just cut
this and then looked at it straight because you can see the crus cerebri is up and here's the
crus cerebri again so I just flipped it so that you'd understand that it's the same structure
just from two different points of view in the next segment we'll take up this the crus cerebri and
how these fibers then travel through the pons and the medulla during in the rest of the brainstem