Know your spinal cord – The spinothalamic tract
Day six already! Today is day six of knowing your spinal cord and we’re talking about the spinothalamic tract today. If you’re interested in the other posts, the first covers the medullary pyramids and I even have a category just for these posts. Of all the tracts of the spinal cord, this is probably my favorite becuase it is just so weird! You’ll see what I mean, so let’s get to it.
Yesterday we covered the medial lemniscus tract this was our first look at an afferent (ascending) pathway. That pathway if you recall was responsible for fine touch and proprioception, but as humans we have so many other senses! This tract is responsible for pain, temperature, and crude touch.
For those who don’t know what crude touch is, crude touch tells you that something is touching you without being able to localize where that touch is coming from. Sure it seems odd, but this is a real thing and works in parallel with fine touch. Fine touch is how we localize where something is touching us and we know that these are two seperate things because these tracts can become damaged and people will be able to identify that they are being touched without being able to localize where that touch is coming from.
The spinothalamic tract is divided into two further tracts. These are the lateral and anterior spinothalamic tracts. The lateral pathway transmits both temperature and pain information. The anterior tract transmits crude touch and pressure information. Let’s look at where they are located within the spinal cord.
Like the medial lemniscus tract, the spinothalamic tract is comprised of three neurons. Nerves coming from the skin go into the pseudounipolar neurons located within the dorsal root ganglion, which we covered in our last post. These neurons go to the dorsal horn (or the posterior) of the spinal cord at that segmental level. These neurons are of two types which we will define for you here:
- A delta fibers are large diameter axons that transmit (as fast as 6 milliseconds!) fast immediate pain. This type of pain is rapidly localized (if you’ve ever stepped on a Lego piece, you will certainly know the location of the injury!), and travels in the lateral spinothalamic tract.
- C fibers will make several connections in the dorsal horn before ascending. They transmit slow, aching pain that may result from inflammation (think about the pain you may have if you have arthritis for example). The slow type of pain is poorly localized, which explains a lot about why people sometimes have trouble explaining where they hurt.
Now as you may have guessed from the name of the tract, it travels to the thalamus (located in the brain). HOWEVER, and that is a big however, there is quite a bit of interesting things the neuron does before it gets to the brain. We’ve pointed out that most neurons decussate at the medulla and we even got into some of the neurons that do not decussate there. This tract is one of those tracts that do not decussate at the medulla. Unlike the previous tract which decussates right after it entered the spinal cord, this one does something completely different.
In this case the neuron enters the dorsal) horn at the segment, but THEN it ascends or descends one to two levels of the spinal cord. This happens in something called the tract of Lissauer (or Lissauer’s tract) and is named after the guy who found it. That is located at (you may have guessed this) lissauer’s fasciculus, which we can see labeled below.
After it gets to a completely different level from where it entered, it then synapses with the second neuron in one of two areas located within the grey matter, the substantia gelatinosa or the nucleus proprius, which you can see the location of both below. The image above shows the pathway as well, but only a single area.
Now the substantia gelatinosa is named because it is a gelatinous mass of neuroglia and nerve cells and it extend the entire length of the spinal cord all the way to the medulla! So once our nerve has synapsed here, the story is very similar to the medial lemniscus tract, it decussates (crosses the midline of the spinal cord) at the anterior white commissure. Then it finally travels to the thalamus. I get that this was a lot, so let’s recap a little to make this more succinct.
Our nerve enters at the dorsal root ganglion (like all ascending tracts do). Then it travels through the tract of Lissauer, where it can ascend or descend one to two spinal cord levels. Here it synapses on our second order neuron (the second neuron in our chain of neurons to the brain). Next, it decussates through the anterior white commissure. Finally, it travels to the thalamus via the spinothalamic fasciculus. It synapses there on our third order neuron, then finally arrives at the somatosensory cortex! Wow, that was a lot!
Well, this took quite a bit longer than I had hoped, so I’m not sure what I’m going to talk about tomorrow yet, but I will figure it out. I really hope that I’ve made it clear why I’m just so excited about this tract!
Why does it travel one to two levels before it decussates? I have no idea! That’s what makes it so fascinating! Like i said, it’s my favorite tract and maybe after today it’s your favorite tract too.
Until next time, don’t stop learning!