Day #171: Know your spinal cord – The cauda equina
Here we are at day fourteen of knowing your spinal cord. By now you’re all experts on the spinal cord and I’m not even sure what I’m doing here. For those just joining us, we have a neuroanatomy category with all the posts so you don’t have to dig for them. If you want to start at the beginning, that would be the medullary pyramids. For the rest of you this is the end, not the end of the posts because we have a lot more to cover, but the end of the spinal cord. Let’s talk the cauda equina!
By now you may have noticed your spinal cord doesn’t run the full length of the spine. Well once upon a time, it did. In humans your spinal cord stops growing when you’re just a child (around four years old). You on the other hand don’t stop growing until you’re in your 20’s so the spinal cord so what does this mean for the cord?
Due to normal anatomical variations, the final location of the end of the spinal cord may occur anywhere from T12 (twelfth thoracic vertebra) to L3. When you’re born the spinal cord runs basically the full length of the vertebral column and spinal nerves exit at the same level where they originate on the cord. However, as you grow the spinal cord travels up the vertebral column, so the nerves (especially at the lower level of the cord) need to grow into the vertebral column to remain attached. This results in a bundle-like structure of nerve fibers that extends caudally from the end of the spinal cord, gradually declining in number further down as individual pairs leave the spinal column.
This is why the sacral nerves originate at the upper lumbar region. It also explains why the spinal cord segments do not correspond to vertebral segments in the adult (which is a point of frustration for me in particular because it gets confusing). Let’s look at what this looks like. Below we can see the cauda equina. It starts below (caudally) the conus medullaris and occupies the area in the spinal cord called the lumbar cistern, a space filled with cerebrospinal fluid.
This makes it look like it takes up more space than the spinal cord. It doesn’t, it’s just nerves. We could see this in a cadaver photo, which maybe we will, but I prefer to show what this looks like using MRI. Below is an MRI of the cauda equina, you can see where the conus medullaris ends and the cauda equina begins, which I’ve labeled to make it easier to find. More importantly, this gives you a better idea of how the cauda equina sits in the lumbar cistern.
Note that the spinal cord and cauda equina show up darker than the surrounding spinal fluid (which I also labeled to make it easier to see what this image is showing). The cauda equina compared to the cord is fairly small compared to the spinal cord. So the nerve fibers are very small. Since this is a very delicate structure I prefer to show images of the cauda equina instead of trying to describe it, for completeness, let’s look at one more image of it. This is a cadaver dissection drawing showing the cauda equina and several of the structures around it. You can see a “casing” around it, this casing is the meninges that encapsulate the spinal cord and provide protection.
To finish this post, let’s talk about how many nerves make up the cauda equina. It has approximately 10 fiber pairs at its base and consist of three to five lumbar fiber pairs, five sacral fiber pairs, and one coccygeal nerve. Functionally, the innervates the lower limbs and the pelvic organs, which consist of the bladder, the rectum, and the genitals. Now, if there is one take away from all of this I would recommend that you simply remember that the cauda equina forms because you grow even after your spinal cord stops growing. To me that is the most interesting thing about the cauda equina.
Until next time, don’t stop learning!