in this segment we'll talk about that final bit of the direct motor pathway and we call it the
corticospinal tract and it's called corticospinal because it starts in the cortex and it ends in the
spinal cord so the same motor neuron that started way up there in our motor cortex with our upper
motor neuron is finally across the midline now it's come down to the spinal cord we have the
lateral corticospinal tract which travels here and it innervates these are descending fibers and it
innervates the limbs the ventral corticospinal tract is here it is about half crossed and half
uncrossed and we'll talk more about that in the next slide or two and it's going to innervate
the trunk muscles and the trunk muscle at neurons live here limb motor neurons live
here remember when we talked about the spinal cord and we talked about the cervical and lumbar
enlargements you have all those extra neurons that innervate the muscles of your upper extremity and
your lower extremity so it makes sense that that lateral corticospinal tract travels here so it's
right next to its target neurons so again the corticospinal tracts lateral and ventral travel
here and they synapse then on what are called lower motor neurons also known as alpha motor
neurons and these are great big beautiful motor neurons that send their axons then out through
the ventral rootlets the ventral roots which then join with the dorsal to form the spinal nerve and
then these are all of the named nerves that we talked about when we talked about the plexus in
peripheral nerves to back in the spinal cord lecture and so one thing i forgot to mention
in the last slide that i want to catch you up on this side is again because the lateral is
lateral corticospinal tract is over here ventral here these are the neurons for the trunk these
are the neurons for the the the limbs this track the ventral is about half crossed and about half
uncrossed this is because your trunk moves as a whole I can't do half a sit-up I can twist but
my whole trunk moves together I can't move parts of my thorax or part of my abdomen independent
because it's one unit and so because you want it to move as a unit you want information from both
sides of your brain talking to both sides of your body so half is crossed and half isn't when it
comes to the lateral corticospinal tract that is totally crossed just like your sensory systems so
sensation from my right arm is being perceived on the left side of my brain when it comes to
moving my right arm those motor neurons are on the left side of my brain and they cross in that
pyramidal dist accusation in the at the bottom of the medulla and then will innervate the motor
neurons the alpha motor neurons in the spinal cord of my right right spinal cord and innervate
within my right arm so just to reiterate lateral corticospinal tract completely crossed
left brain works right arm ventral cortical spinal tract trunk both sides of my brain are talking to
both sides of my body okay so this is getting down to the last let me erase this part and
this is getting now down to the last part of this direct motor pathway in which we're talking about
our beautiful alpha motor neurons that live here sending their information out out to the actual
muscles and those then so it's corticospinal tract one neuron all the way up in motor cortex that has
sent it at its axon all the way down through the internal capsule through the brainstem partially
halfway down or maybe all the way down our spinal cord to synapse on this alpha motor neuron this
alpha motor neuron now takes that information and exits through the spinal nerves and then goes out
to innervate the muscles and now I want to remind you of how this actually works and that remember
we have our beautiful alpha motor neuron coming out and innervating muscle fibers and I want you
to remember a motor unit and that's the number of muscle fibers innervated by one alpha motor
neuron this is going to determine how fine the control is because a ratio of one neuron to you
know a hundred muscle fibers is going to give you really fine control one neuron to a thousand
motor fibers is going to give you kind of large gross gross motion big big broad movements not
fine control the way that the neuron talks to or more official and more scientific terms synapses
with the muscle fibers is called a neuromuscular Junction and the neuromuscular Junction is a very
special connection between the the neuron the motor neuron in the ventral horn and the muscle
fibers out in in your peripheral muscles this is a very special relationship muscles need neurons
you know that if there's a damage within a nerve or within the spinal cord that muscle will atrophy
if you don't if it's not innervated and neurons need muscles muscles make all kinds of growth
factors and things that then influence the health and well-being of this alpha motor neuron so it's
a mutually beneficial relationship between the neuron and the motor fibers in summary what I
really want you to remember is that efferent of course equals motor that I want you to know where
your upper motor neurons are I want you to be able to describe the path of that upper neuron axon all
the way through the internal capsule and brain stem and all the way down to the spinal cord and
then where do those upper motor neurons synapse what's their target what's the lower motor neuron
where do the motor neurons lower motor neurons live and what is the path that the axons of
those neurons take to get out to the muscle and then where do those lower motor neurons synapse
and what do we call it if you know all of these things you're going to be ready for the assessment