in this set of lectures we're going to discuss the spinal cord we'll talk about some of the basic
gross anatomy of the spinal cord its functions we'll talk about the white matter versus the
gray matter what their functions are and we'll talk about the peripheral nervous system so the
basic stuff about the spinal cord that is really important to know or that the spinal cord relays
sensory information from the periphery that means your surface to your brain and that is
both somatic sensory information and visceral information so your surface and your internal
organs the spinal cord contains motor neurons and those motor neurons then project out to your
muscles and that means your somatic muscles your skeletal muscles that move you around as well as
the visceral or smooth muscle of your organs so within the spinal cord there are direct or local
connections between motor and sensory information and this interchange of information is responsible
for your reflexes so in addition then to being a sensory relay it is also your spinal cord is
also a motor relay in that it contains the motor neurons that are going to receive information
from your brain and then carry that or relay that information to your muscles both somatic ie your
skeletal muscle and your hollow organs or your visceral so the basic setup of your spinal cord
is gray matter and white matter and as you recall your gray matter is where the neuronal cell bodies
live and their proximal dendrites and the white matter is where the myelinated fiber tracts are
and so on this image this is the gray matter and we call it in the spinal cord we call it our gray
butterfly and you can see it on this image of a real spinal cord that's been kind of magnified
because your spinal cord really is only about the size of your thumb it's not really that big which
is kind of amazing for what it does and here's the gray matter and then out in this area is
the white matter and that is white again because it's myelinated and so it contains quite a bit of
lipid and that looks white in a gross preparation so the spinal cord has five different regions and
two enlargements so the regions are cervical in this zone its thoracic and round in here lumbar
and then sacral and coccygeal just way down at the bottom you have two enlargements there's a
cervical enlargement and that innervates your arm and it's an enlargement because you have a lot of
more neurons there because you have a lot more muscles in your arm so you need more neurons to
innervate them you also have a lumbar enlargement which is down here and that innervates your leg or
your lower extremity so this side representations of each of those spinal cord levels this cervical
lots and lots of white matter not quite as much gray matter as you'll see as you progress down the
cord but a little bit of that cervical enlargement here because it's going to innervate your upper
extremity in the thoracic region where you know you're in the thoracic cord when you see this
little bump here this is called I'm just gonna abbreviate it for now and we'll get we'll talk
a lot more about this when we talk about on and on it autonomic s-- but this is your IML which
is short for intermediolateral cell column and that's a big mouthful but this
is where when we talk about the difference between somatic and visceral this is where those neurons
that innervate your hollow organs live in this little little bump here as you go down here's the
lumbar I'm sorry let's see this is thoracic lumbar and this is the sacral region I want
to take just a minute to explain the difference you're gonna notice if you look at this like oh
my goodness there's a whole bunch of gray matter and not very much white matter down here and then
there's a whole bunch of white matter and not very much gray matter up here it's kind of the analogy
I use and this may or may not work for you but think about it is a bus route so if the school
is your brain and the bus route represents kind of your spinal cord if everybody starts at the
brain also all of the nerve fibers that are going to innervate your entire spinal cord start at the
top and as they go down they exit so the spinal cord the fibers that are going to innervate your
arm leave then the spinal cord fibers that are going to innervate your thorax leave and so the
by the time you get to the sacrum you have very few fibers left right so all the kids get off the
bus on the bus route until there's hardly anybody left so you have fibers exiting at every level so
that by the time you get to the bottom you have less white matter it also works on the way up so
sensory information coming in starts to accumulate so you get more kids on the bus towards the at the
very beginning of the route which is way out in the boonies a few kids get on the bus a few fibers
join the spinal cord and then as you go up more and more and more fibers join or more and more
kids are on the bus until you get to the top and again everybody's on the bus right before you get
to school so that is kind of my analogy of why you have varying amounts of white matter up and
down the cord so lots of fibers at the top because everybody that's coming in is there and everybody
that is going to exit has begun and then as fibers leave you diminish as you go down and as fibers
come in you add as you go up so that accumulation of adding and subtracting is the net result as
fewer fibers in the bottom and more fibers as you get to the top so in the next segment we're
going to talk a little bit more about the spinal cord and the meninges that cover it