in this segment we'll talk about the individual neurons that make up the three neuron chain
five fine touch reception so neuron number one of course as always lives in the DRG its peripheral
process is it going to be in the skin and it's going to project a central process into the
dorsal horn it doesn't synapse here that's the big thing to remember and it doesn't cross the
midline so it passes through here it may make it may give some information to some of the neurons
around here via what we call an axon collateral but that's not its main synapse it's gonna a send
on the same side in the dorsal column okay the dorsal columns are divided they have they have a
regional specificity so the fasciculus gracilis is here and the fasciculus cuneatus is here
gracilis is going to be concerned it's going to be carrying fibers from your lower extremity and
part of your abdomen okay the reason the way I remember gracilis is that there's a big muscle
in my thigh called the gracilis muscle and so I I always put gracilis in gracilis together the
fasciculus cuneatus is over here it's a little more lateral because if those axons that are going
coming from the arm are going to enter on their way up after the ones in your leg of injured okay
and the reason the way I remember cuneatus as being my upper extremity and my thorax is... cuenatus,
cuneatus kind of goes with cuticles which are you know around your fingernails so it's upper
extremity you'll figure out a way to remember it for yourself I'm sure the second neuron in
the chain is in the medulla oblongata and there is a nucleus there's a gracilis nucleus and a cuneatus
nucleus and that is where all of the axons from the fasciculus are gonna gracilis are going
to synapse here the axons from the fasciculus cuneatus are going to synapse in the cuneatus
nucleus so you have two divisions of that dorsal column gracilis for the leg cuneatus for the
hand or an arm and you have two nuclei the nucleus gracilis and the nucleus cuneatus where those
tracks are going to end this neuron is equivalent to that neuron in the dorsal Horn of your spinal
cord for gross touch so it's a gonna cross the midline so its axon crosses and then a sense we
call these fibers as they're crossing the arcuate fibers once this once the arcuate fibers have
crossed the midline we call this band of fibers that's a sending the medial lemniscus I can't even
spell so let me clear this and draw it a little more neatly there's a nucleus nucleus cuneatus
nucleus gracilis where they're specific tracks and neurons crossed the midline these are the arcuate
sarcuate fibers i have to remind you that a lot of these fiber tracts and things were named
before they knew what they were doing not not that the anatomist didn't know what they were doing but
they didn't know exactly where the fibers were coming from and where they were going to so they
named him arcuate because it's a curve it's an arc they then a send in the medial lemniscus
and this is the act that so the second neuron here is the one again that's going to talk to
the add to the neuron in the thalamus so fine touch and proprioception third neuron
in the chain just like gross touch is going to be in the thalamus and the thalamus the
neuron in the thalamus then it's projects to primary somatosensory cortex and again that is
your post central gyrus so in the next sections we'll go through and draw this pathway a couple
of more times just to solidify it and then at the end we'll kind of summarize what the
big differences are between gross touch and fine touch so that you get those straight in your mind
because they're important diagnostically and we'll talk about that a little bit at the end