Day 189: Know your spinal cord – The Ramus
Today is day thirty-two in our know your spinal cord series. If you’re just joining in, as I usually do in the intro, we have a whole neuroanatomy category just for these posts so you don’t have to dig for them! Today we are covering a couple of structures that fall under the same broad category, ramus. What are they and what do they have to do with the spinal cord? Well that’s what we’re about to find out!
So yesterday and pretty much everything prior to today’s post would have you believe that the nerves exit (or enter) the spinal cord follow some determined path and innervate the part of the body they are supposed to run to. However, this isn’t quite the case, we’ve sort of black boxed the middle part of the pathway that connects the body to the spinal cord. This was somewhat intentional as we wouldn’t want to bombard you with information, so think of it as building up to this post.
So quick recap, yesterday we (re)introduced the spinal nerves. With thirty one pairs coming from the spinal cord, they run the length of the cord and technically beyond (the cauda equina). We know there are an afferent set and an efferent set, but the afferent set has the dorsal root ganglion. The two roots join together, exit the intervertebral foramen (shown above), then split into what is called the dorsal rami, ventral rami, and rami communicantes. Below you can see how they are arranged.
We give these their own name because these structures are separate and distinct from the spinal roots. As for what they look like, the ventral rami is larger than the dorsal rami. The rami are mixed nerves that carry both sensory and motor information, which is highlighted in the image below. They also branch off to form the grey and the white rami communicantes which make connections with the sympathetic ganglia. Ganglia are a group of neuron cell bodies and sympathetic ganglia are the ganglia of the sympathetic nervous system. As a refresher the sympathetic nervous system regulates many of the homeostatic mechanisms that keep us alive.
The ventral rami supply the antero-lateral parts of the trunk and the limbs, while the dorsal rami carry information that supplies muscles and sensation to the human back (shown in the featured image at the top). Interestingly, the dorsal ramus of each spinal nerve travels backward, with exception of the first cervical, the fourth and fifth sacral, and the coccygeal.
This leaves the ramus communicans, which is latin meaning “communicating branch.” There are two different ramus communicans, the white and grey. The white ramus communicans have a higher myelination of the nerve fibers that make it up. This gives it a white appearance (hence the name). The grey rami communicantes have more unmyelinated fibers than myelinated, which is why they are called grey. As mentioned before, the grey and white rami communicantes are responsible for conveying autonomic signals, specifically for the sympathetic nervous system.
That about wraps up our intro into rami. You’ll have to wait to see what tomorrow’s post will be on. Shhh it’s a secret. In any case I hope this helps introduce you to how the nerves actually travel from the spine to the rest of the body.
Until next time, don’t stop learning!