Remembering the Brachial Plexus: A Hand Mnemonic, Illustrated

Mnemonics help memorization via cognitive mechanisms that are incompletely understood; their use within medical culture is prolific and can be traced back hundreds to thousands of years.^1,2 Although most medical mnemonics utilize letter- or word-based creative modalities, (eg, acronyms, initialisms, rhyme, or play on words), other mnemonics are visual and rely on physical props, including the human body.³ Here, we appreciate a visual mnemonic that uses the human hand to represent the spinal nerve contributions to the brachial plexus, which has clinical relevance in anesthesiology primarily for understanding and placement of brachial plexus nerve blocks. I created an illustration of this mnemonic (Figure) to help facilitate its memorization and teaching, but also because, as an art enthusiast, medical mnemonics provide a unique source of inspiration, replete with bizarre imagery and unexpected use of language.⁴

Figure.: Illustration for the “3 Musketeers Assassinated 5 Rats, 5 Mice, and 2 Unicorns” Mnemonic The illustration shows how the fingers can be used to remember the spinal nerve contributions to the brachial plexus. The illustration shows to first designate each finger on one had (from thumb to little finger,) as spinal nerves the C5, C6, C7, C8, and T1, respectively. Use the thumb (C5), index (C6), and middle finger (C7) to form the “3 Musketeers,” encoding that C 5, 6, and 7 form the musculocutaneous nerve; Use the thumb (C5) and index finger (C6) to create the shape of a gun (used to “Assassinate”), representing the C5 and 6 contributions to the axillary nerve; outstretch all fingers to represent the C5, C6, C7, C8, and T1 contribute to the radial (“5 Rats”) and medial (“5 Mice”) nerves; lastly, outstretch the ring finger (C8) and little finger (T1) to represent their contribution to the ulnar nerve (“2 Unicorns”).

The genius of the “3 Musketeers Assassinated 5 Rats, 5 Mice, and 2 Unicorns” mnemonic is how it physically represents its subject matter. To witness this, first designate each finger on the hand from thumb to little finger as spinal nerves C5, C6, C7, C8, and T1, respectively. With this numbering, use the thumb (C5), index (C6), and middle finger (C7) to form the “3 Musketeers,” which signifies that C 5, 6, and 7 form the musculocutaneous nerve; next, use the thumb (C5) and index finger (C6) to create the shape of a gun (used to “Assassinate”), representing the C5 and 6 contributions to the axillary nerve; outstretch all fingers to represent the C5, C6, C7, C8, and T1 contribute to the radial (“5 Rats”) and medial (“5 Mice”) nerves; lastly, outstretch the ring finger (C8) and little finger (T1) to represent their contribution to the ulnar nerve (“2 Unicorns”), a physical act that, ironically, uses the ulnar nerve itself.

The mnemonic’s use is primarily educational; it aligns with “classic” anatomy taught at most medical schools. Brachial plexus anatomy can vary,⁵ however, and therefore confirmation of anatomy should be performed prior to any clinical procedure or intervention. Like many mnemonics, the original creator of the mnemonic is difficult to identify; a provisional search using the term “musketeers assassinated” or equivalent in PubMed and other literature databases yielded no significant historical results. Thus, it is unclear when exactly the mnemonic came into being. Indeed, much like nursery rhymes, mnemonics and other “pearls” of medicine are cultural treasures passed down from generation to generation, often by word of mouth — or hand.