Know your spinal cord – The rubrospinal tract
This is day nine of know your spinal cord. Now for my usual bits, we have a whole category that I created just for these posts. They are in chronological order with the newest posts first, so I would recommend you start from the beginning post or dive in where you’re interested. That said, we’re going to tackle one of the smaller, but still important tracts, the rubrospinal tract!
Yesterday we were talking grey matter and we had two tracts that were mentioned that I had not covered yet, this is one of them. The rubrospinal tract (in humans anyway) is fairly small compared to the other tracts and is involved in motor control and in particular it’s thought to have a hand in mediating voluntary movement. But wait, you say! We covered a tract that mediated voluntary movement, the corticospinal tract is the one responsible for voluntary movement. You would be correct, but like a lot of things in the body we have some redundancy (possibly!).
Technically speaking, there are several motor control tracts that run in the spinal cord. In humans this tract is small and fairly rudimentary (compared to the other more complex tracts of course). However, we’ve found through experimentation that in other primates this tract can actually assume almost all the duties of the corticospinal tract when the corticospinal tract is lesioned.
Interestingly this tract does not run the full length of the cord, it terminates primarily in the cervical spinal cord. This suggests that it functions in upper limb but not in lower limb control. The tract is primarily responsible for flexion and handles both large movements and fine motor control. Not to bad for a smaller tract, right? However, we still consider it a vestigial tract for the most part (at least in humans).
Since the name doesn’t quite give it away, let’s talk about the path of this tract. The rubrospinal tract starts in the midbrain (above). The midbrain is part of the brainstem (further highlighted above), and is associated with vision, hearing, motor control, sleep and wakefulness, arousal (the fancy way we say alertness), and temperature regulation.
Technically speaking, it originates in the part of the midbrain called the magnocellular red nucleus. This part of the red nucleus (shown broadly above) receives most of its information from the motor cortex and cerebellum. While the red nucleus itself is involved in motor coordination, mostly in other primates and not so much humans. Since this is a spinal cord topic and not a brain topic, that’s about all the detail you will get about the red nucleus.
From there, it crosses to the other side of the midbrain, then it descends in the lateral part of the brainstem tegmentum, which we’ve shown as a slice through the brainstem in the image above. Next, it descends through the spinal cord traveling through the lateral funiculus, which is adjacent to the lateral corticospinal tract.
In the midbrain, it originates in the magnocellular red nucleus, crosses to the other side of the midbrain, and descends in the lateral part of the brainstem tegmentum. In the spinal cord, it travels through the lateral funiculus of the spinal cord, coursing adjacent to the lateral corticospinal tract. Below is an image showing the location of the rubrospinal tract (yellow). You can also see the size difference between the lateral corticospinal tract and the rubrospinal tract.
Now we’ve said that in humans it is pretty much a vestigial tract. However, there is some research that shows it is particularly important in newborns. Here’s the important bit though, we’ve thought organs were vestigial in the past only to find out they served a purpose. Like with a lot of things about the brain and spinal cord, we don’t quite know exactly what this tract does, but we have a good idea. It may turn out to be a very important tract, but for now it’s a lot of speculation outside the narrow purpose that we know it has.
Next up we will tackle another tract we have yet to cover, the spinocerebellar tract. Which is admittedly a particularly complex tract. After that, I’m not sure what we will cover, but we’ll figure it out.
Until next time, don’t stop learning!