The brachial plexus is the bane of many med students’ existence during any sort of neuro block. So many nerves, so many connections, so many seemingly arbitrary names of different sections. It’s just a woven mess of misery. (especially when they start getting into the “where is the lesion” questions)
Thus I’ve decided to have a couple posts about the brachial plexus, hopefully demystifying it to some extent. This first doodle is about the brachial plexus and its anatomical relationship to some of the structures that show why anatomists who named the parts weren’t as crazy as they seem.
Important structures to remember because they explain why parts are named the way they are:
- Anterior and posterior scalene muscles
- Subclavian artery
- The arm (in its anatomical position)
Vertebrae: There are 7 cervical vertebrae and 12 thoracic vertebrae. To make things confusing the cervical spinal nerves exit ABOVE their named vertebrae (except for C8) while the thoracic, lumbar and sacral exit BELOW. This messes up the whole numbering system because there are SEVEN cervical vertebrae but there are EIGHT cervical spinal nerve roots. The brachial plexus generally includes the nerve roots C5-T1*
* I say generally because there’s are anatomical variations such as a “prefixed” plexus that goes from C4-C8 and a “postfixed” plexus that goes from C6-T2
Scalene Muscles: The brachial plexus is nestled between the scalenes in the neck. At this point the plexus is oriented up and down and therefore the trunks are superior (closest to your noggin), middle, and inferior.
Subclavian Artery/Anatomical Position: The artery is in front of the plexus at the level of the trunks and then the plexus starts to wrap around it (or at least seems to because we don’t keep our arms straight out to our sides in “anatomical position” at all times). The cords are named for their relationship to the artery. One is lateral (again, if the arm was held out to the side), one is posterior and one is medial (think closest to armpit).
Subdivisions of the Brachial Plexus
The parts are: Roots/Trunks/Divisions/Cords/Branches or, as I remember them being a classy east coast Canadian: Real/Truckers/Drink/Cold/Beer
Then you might think, “But how do I remember which of the terminal branches comes off where?” For that I think of the two “M” branches being on the M: Musculocutaneous, Median and (M)Ulnar and that the whole thing together can just be said as “MARMU” Pick the mnemonics you want, the brachial plexus is rife with them. I personally just like the sound of the word marmu.