Every movement of your arm runs through one tangled nerve junction
Five nerve roots leave the spine at the base of the neck, merge, split and merge again before reaching the arm. This braid, the brachial plexus, wires almost every muscle and patch of skin from shoulder to fingertip. Anatomy students remember its stages with a mnemonic: Rich Tourists Drink Cold Beer.
The mnemonic stands for roots, trunks, divisions, cords and branches. The five roots come from spinal nerves C5 through C8 plus T1, occasionally joined by C4 or T2. C5 and C6 fuse into the upper trunk, C7 carries straight on as the middle trunk, and C8 and T1 form the lower trunk. Each trunk splits into front and back divisions, making six, which regroup into three cords named for where they lie beside the axillary artery. Those cords end in five terminal nerves: the musculocutaneous, axillary, radial, median and ulnar.
On the dissecting table, several of those nerves together form a recognisable M or W shape that helps locate the plexus. Smaller branches peel off earlier. The long thoracic nerve, drawn from C5, C6 and C7, drives the serratus anterior, the prime mover whenever you reach forward or push. Only two parts of the arm escape the plexus: the trapezius muscle, served by the spinal accessory nerve, and a patch of skin near the armpit.
Because the nerves are so sensitive to position, injuries are hard to prevent. The classic mechanism forces the shoulder down while the head is pulled away, as in motorcycle crashes or falls from trees, stretching or tearing the upper roots and paralysing muscles such as the deltoid and biceps. Lower injuries are rarer and can follow grabbing something to stop a fall, leaving the hand unable to make a full fist. In American football, players suffer stingers when a shoulder pad drives into the point where the plexus lies closest to the surface.
Birth can injure it too. When a baby's shoulder catches behind the pubic bone during delivery, the manoeuvres needed to free it can stretch the plexus, causing obstetric brachial plexus palsy. Reported incidence is 1.5 per 1000 births in the United States and 0.42 in the United Kingdom and Ireland. MRI at 1.5 tesla or above is the preferred way to image the plexus.
Source: Brachial plexus
