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Day 160: Know your spinal cord – The corticospinal tract

spinal cord cross section drawing

We’ve made it to day four of knowing your spinal cord. I’ve introduced a new category just for this, which makes these posts easier to find for future reference (yours or mine). For those of you just starting out, you may want to go from the first post on the medullary pyramids. For the rest of you, you’re probably here to learn about the corticospinal tract. This will be fun, so let’s get started.

Say you want to move. It’s easy and for the most part you can (and do) do it without thinking to much about it. In fact if someone throws something to you, chances are you will at least make the attempt to catch it without too much thought behind all the physics that goes into being able to do that. As we saw yesterday, the motor cortex is where deliberate movements start (reflexes happen elsewhere… hint, hint). Let’s look at the motor cortex one more time.

Primary motor cortex

The location of the motor cortex, highlighted in red

Now, we know that the motor cortex runs down to the pyramids, decussates, then travels down (we covered this in the medullary pyramids post). We also mentioned that only ~90% of the nerves actually decussate. In this case, we have two parts to the corticospinal tract, the lateral corticospinal tract and the anterior corticospinal tract. Let’s look at the location of these on a cross section of the spinal cord.

corticospinal tract

Locations of the lateral corticospinal tract and the anterior corticospinal tract

As you may be able to guess, the lateral corticospinal tract (located dorsally) is the major motor tract and controls limb muscles. This tract is what is the big controller of your movements. It’s organized like the motor cortex, in sections. Now, the image above I’ve created to highlight the fact that the spinal cord is a mirror image of itself. The left side controls the left side of the body and the right controls the right side. The neurons that run through this tract are part of the ~90% that decussate at the decussation of pyramids.

For the second portion of the tract, we have the anterior corticospinal tract. This varies in size and is inversely related to the size of the lateral tract. ut unlike the lateral tract it ends in the thoracic region (middle of the back). Why do we have this tract when the lateral corticospinal tract controls the limbs? Well, we need to be able to control the axial muscles too (trunk muscles, or your core muscles). This is the tract that is responsible for that control and also explains why it ends in the thoracic area, as it doesn’t need to run to the lower limbs.

Now for the weird. Unlike the lateral tract, the anterior tract does not decussate at the pyramids. Crazy, right? You may be wondering if that means the left side of the brain controls the left side of your trunk muscles since it doesn’t decussate that the pyramids, but this is not the case. The tract does in fact decussate just like all the other tracts, it just does so IN the spinal cord!

How does the spinal cord work like that? Well let’s look at it in more detail. First the anterior corticospinal tract. This tract decussates at the medullary pyramids, when the neuron reaches the level it innervates, it synapses on a neuron in the anterior horn (the grey matter). Then it exits the cord and innervates the muscle it controls. If you prefer a visual, below we have the entire pathway mapped out from motor cortex to muscle.

lateral corticospinal tract

Lateral corticospinal tract from motor cortex to muscle.

The anterior spinal tract has a similar pathway, it just decussates in the cord. In this case, when the fibers get to the target level of the cord, they decussate at the anterior white commissure then synapse on the anterior horn of the grey matter. Since we’re using a lot of anatomy terms, below we can see where the anterior white commissure and anterior horns are located.

cross section

Okay, so we know how it decussates and where it goes, let’s look at the graphic outlining the tract. Below we see how the tract travels, then decussates at the white commissure, then synapses on the anterior horn before exiting.

Ventral Corticospinal

They aren’t labeled, but the bottom shows how the tract decussates at the anterior white commissure, then synapses on the anterior horn of the grey matter before exiting.

Now that we’ve seen these two tracts separately, you may notice that both of them synapse on the anterior grey horn. Let’s take a look at just a spinal cord cross section showing both of the tracts at the same time.

corticospinal tracts (both)

Both of the corticospinal tracts and how they synapse on the anterior horn of the grey matter in the spinal cord

That pretty much sums up the corticospinal tract and how it travels from the brain to the muscles. We got our first taste of some of the inconsistencies that make the spinal cord weird! I absolutely love the quarks that make the spinal cord so unique. If you’re like me and think it’s fascinating that the anterior corticospinal tract you’re going to be in for a treat because that isn’t even the oddest part of the cord! I think next we will cover the medial lemniscus tract, it’s another major pathway of the cord. It isn’t as weird as the tract I’m thinking of, but we will get there.

Until next time, don’t stop learning!

 

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