Know your spinal cord – Medullary Pyramids

I’m excited that today we are starting the know your spinal cord series that I’ve been working on. Today we are going to take it from the top, no really. We’re starting at the top of the cord and we will work our way down. So without further delay, let’s look at the curious case of the medullary pyramids!

Most of us know this, but the left half of the brain controls the right half of the body and the right half of the brain controls the left side of the body. That is to say, if you are right handed, you’re using the left side of your brain! Now for basic neuroanatomy, that is where most classes draw the line. However, have you ever wondered what the heck the left half of the brain is doing controlling the right side of the body?

It turns out, it may just be due to your spinal cord! Well, evolution, but we really aren’t sure WHY the left brain controls the right body, that is just our best guess for the moment. We do however know how it happens. Believe it or not, there is a reason why the brain controls the body the way it does. That magic is called the decussation of pyramids!

First, let’s define decussation. Decussation is just a fancy term that means something crosses over the midline of the body. If you had a pen and you drew a line from your left shoulder to the right shoulder, we could say this line decussates at the sternal notch (right between the clavicles).

In this case, we have neurons that start on one side of the body and make a cross to the opposite side. In our brain, this happens at the medullary pyramids and in particular at the decussation of the medullary pyramids. The decussation of pyramids is also where most people define the end of the brainstem and the start of the spinal cord. This structure is located in the medulla oblongata (or just medulla) and the medulla is located at the end of the brainstem. For those of you who need a refresher on your neuroanatomy, the brainstem and the location of the medulla can be seen below.

Way at the bottom of the brain stem, we have the medulla!

Now that we know where we are in the brainstem (hopefully). Let’s talk about what the neurons do here. A (very) simplified model of how the brain connects to the spinal cord would look something like this. You want to move, so your motor cortex (See below) sends a signal via neurons to the spine. This is done with one very long neuron that innervates (connects) the motor cortex to a certain area of the spinal cord. Yep it’s pretty much one really long neuron. Don’t worry if that seems odd, it was odd to me at first too, but for our simplified view, let’s leave it there and trust me that is the least weird bit about the spinal cord.

The pink lines show how the motor cortex innervates the spinal cord

However, as we discussed the right side of the body is controlled by the left side of the brain, why is this? Well the honest answer is that it isn’t a straight pathway from the motor cortex to the spinal cord. Neurons first exit the motor cortex, travel down the brainstem and to the decussation of pyramids. Here is where the decussation part comes in (and remember it is a fancy term for crosses the midline of the body). Once the neurons reach the pyramids, about 90% of the fibers decussate and travel down the spinal cord.

You may have noticed something odd. The first is that there are 10% of neurons unaccounted for here. They do their own weird thing and form a totally different spinal tract. We will get to it eventually, I promise. There is something else too, notice that while the left side of the brain controls the right side of the body, this isn’t the case for the spinal cord. Once the neurons decussate that’s it, which means the left side of the spinal cord controls the left side of the body and the right side controls the right side of the body. These neurons (the ones that go from the motor cortex to the spinal cord) make up the lateral corticospinal tract. Which is just one of several of the tracts of the spinal cord. This particular tract controls your movement, which is why it starts at the motor cortex.

So there it is, that one weird trick your brain uses to control the opposite side of the body (the fancy term for that is contralateral side). Next we will talk about the organization of the spinal cord, which is a LOT like the organization of the brain. After that we can discuss the different tracts that make up the spinal cord. I’m excited about this becuase we will see what happens to the 10% of the neurons that don’t decussate at the medullary pyramids and if you’re a big weirdo like me, it’s super exciting and just plain weird.