Spinal roots of the brachial plexus nerves
Five main nerves of the arm each take a different combination of spinal nerve roots (the nerves that leave the spinal cord), from C5 to T1.
Remember
The radial nerve takes all five roots, C5 to T1, the median nerve typically takes C6 to T1, and the ulnar nerve takes only C8 and T1.
Mnemonic
MARMU
- M musculocutaneous
- A axillary
- R radial
- M median
- U ulnar
MARMU gives the order of the five main nerves of the arm.
The brachial plexus (the network of nerves in the neck and armpit that supplies the arm) is built from five spinal nerve roots: C5, C6, C7, C8 and T1. Its outputs are five main nerves: the musculocutaneous nerve (which supplies the muscles at the front of the arm), the axillary nerve (which supplies deltoid), the radial nerve, the median nerve and the ulnar nerve.
Each main nerve takes its own set of roots. The musculocutaneous nerve takes C5 to C7 and the axillary nerve takes C5 and C6. The radial nerve takes all five roots, C5 to T1. The median nerve typically takes C6, C7, C8 and T1, although some people also have a small C5 contribution. The ulnar nerve takes C8 and T1 only.
A story fixes the numbers: 3 musketeers, then assassinated, then 5 rats, 4 mice and 2 unicorns. The first letters give the nerves in order (musculocutaneous, axillary, radial, median, ulnar), and the numbers show how many roots feed the musculocutaneous, radial, median and ulnar nerves. On one hand, the thumb stands for C5, the index finger for C6, the middle finger for C7, the ring finger for C8 and the little finger for T1. Four mice, for the median nerve, means raising the index, middle, ring and little fingers, which gives C6, C7, C8 and T1.
Numbness of the little finger and half of the ring finger, with weak finger spreading, points to ulnar nerve injury, and the ulnar nerve is formed from C8 and T1.
Key facts
- Radial nerve
- takes C5, C6, C7, C8 and T1, the only one of the five main nerves of the arm usually taught as formed from every root.
- Median nerve
- typically takes C6, C7, C8 and T1, and some people also have a small C5 contribution.
- Ulnar nerve
- takes C8 and T1 only, shown by the ring and little fingers (two unicorns).
- Axillary nerve
- takes C5 and C6 and supplies deltoid.
- Musculocutaneous nerve
- takes C5 to C7 (3 musketeers).
- Finger key
- the thumb is C5, the index finger C6, the middle finger C7, the ring finger C8 and the little finger T1.
Exam traps
- Giving the median nerve all five roots as its usual pattern confuses it with the radial nerve, because the median nerve is usually taught as taking no contribution from C5.
- C5, C6 and C7 are the roots of the musculocutaneous nerve, so they are incomplete for the radial nerve, because the radial nerve also receives C8 and T1.
- The median nerve is the near miss for an injury of C8 and T1, because it also receives those roots, but it additionally takes C6 and C7, so it does not come from only those two roots.
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