After anatomy lab today, I was fortunate to catch an explanation of rami communicans by one of our fantastic anatomy teachers. After having gone through so much of the human body’s gross anatomy, it’s a bit embarrassing that I still didn’t have a grasp of exactly how splanchnic nerves, the sympathetic chain, and white/gray rami communicans play together, but today’s explanation finally made it very clear.
We have 31 pairs of spinal nerves emerging from the spinal cord coming out at every vertebral level. A spinal nerve is comprised of ventral motor fibers and dorsal sensory fibers, thereby making it a mixed nerve. 14 of the aforementioned 31 pairs (from the thoracolumbar T1 – L2 region) have additional sympathetic fibers added to the mixed nerve from the spinal cord.
The sympathetic chains run on both sides of the spinal cord from the base of the skull to the coccyx (keep in mind that the spinal cord only runs from the base of the skull to around L1/L2). One can think of the spinal cord as a distribution center of the brain. Similarly, the sympathetic chains are distribution centers for the specific sympathetic region of the spinal cord (T1 – L2). This is how, for example, we get sympathetic innervation in our upper limbs to control things like sweating and vessel constriction even though the brachial plexus is above the sympathetic region (C5 – T1).
Rami communicans are the “bridges” between the spinal cord and sympathetic chain. All 31 pairs of spinal nerves receive gray (unmyelinated) rami communicans from the sympathetic chain, but only the spinal nerves in the thoracolumbar, sympathetic region (T1 – L2) give off white rami communicans (myelinated) to the sympathetic chain. This is precisely why we don’t have white rami communicans in the head/neck region as well as the pelvic region.
Splanchnic nerves (myelinated) are those which come from the spinal cord, pass through the sympathetic chain (they do NOT synapse there like the rami communicans), and proceed to synapse elsewhere. It should be noted that splanchnic nerves carry sympathetic fibers except for pelvic splanchnics which are parasympathetic.
As a side note, I’ve also found that in preserved cadavers, the sympathetic chain seems to be “stuck” along the sides of the spinal column while splanchnics tend to be the loose, nerve-like structures adjacent to them.
That’s the end of my lesson for today. I know it’s pretty dense and potentially confusing, so feel free to leave questions! 🙂