Why the Brachial Plexus is like the London Underground

The brachial plexus is an interchange of nerve fibers that connects the cervical spinal cord to the major nerves in the arm.

This is analogous to the fuse box in your house, distributing electricity from the main electric cable to all of your individual appliances/outlets):

fuse box

Or a highway interchange which can route cars off an interstate to multiple local roads:



Brachial plexus injuries can be caused by trauma to the shoulder in accidents or during passage through the birth canal, and result in pain, numbness, weakness and muscle wasting in the affected arm.



This is a real human brachial plexus:

real anatomy plexus


However, in clinical practice, we are usually dealing with closed (traction) injuries to the plexus, so we are not too concerned about actual anatomy.

We are more interested in figuring out where the lesion lies within the plexus based on the patient’s clinical deficit.

We are therefore more likely to use diagrams of the plexus which distort anatomy, but emphasize branches and intersections, to localize these lesions:

post cord

For example, a patient with weakness of deltoid (axillary nerve) and triceps (radial nerve) but not biceps (musculocutaneous nerve) must have a lesion in the posterior cord (green in the figure above).


The iconic London Underground map, first designed by Harry Beck in 1931, is based on a similar principle.

Earlier maps were geographically correct, but as more lines and stations were added these maps became more cumbersome and confusing.

geographical underground map1

Beck realized that because the railways were underground, the physical locations of the stations were irrelevant to the traveller wanting to know how to get to one station from another, so he devised a simplified map showing only stations, straight line segments connecting them, and the river.  All lines ran only vertically, horizontally, or on 45 degree diagonals, ignoring the actual geography:


You can easily use this kind of map to find the best way to get from Park Royal to Oxford Circus, with the least number of stops and line changes.


Do you see the similarity?



Other examples of medical uses of Beck’s London Underground map: