NEUROANATOMY OF THE UPPER EXTREMITY
The Nerve Root Supply of the Upper Extremity
In the cervical spine, nerve roots are numbered by the cervical level above which they exit: The C1 root exits above the C1 vertebra, the C2 root above the C2 vertebra (between C1 and C2), the C7 root above the C7 vertebra (between C6 and C7). The root exiting between C7 and T1 is the C8 root (there is no C8 vertebra). This is different from the thoracic, lumbar, and sacral levels, where roots are numbered by the vertebral level below which they exit: the T1 root exits below T1 (between T1 and T2), the L1 root exits below L1 (between L1 and L2), the S1 root exits below S1 (between S1 and S2) (see Fig. 15–1).
The lateral upper arm is supplied by C5, the lateral forearm and lateral hand (including the thumb) is supplied by C6, the middle of the hand (including the middle finger) is supplied by C7, the medial hand (including the ring finger and the fifth finger) and medial forearm are supplied by C8, the medial upper arm is supplied by T1, and the axilla is supplied by T2; the index finger may be supplied by C6 or C7. (Fig. 16–1). On the dorsum of the arm/hand this same pattern is maintained. To remember this, trace around your own arm naming the dermatomes: On the arm with the palm facing upward, trace around the arm from lateral upper arm (C5) to lateral forearm (C6), around the hand from the thumb (C6) to the index and middle fingers (C7) to the ring and fifth fingers (C8), around to the medial forearm (C8) to the medial upper arm (T1) to the axilla (T2).
Schematic showing dermatomes of the upper extremity. The index finger may be supplied by C6 or C7; see also Fig. 15-2. Reproduced with permission from Waxman S: Clinical Neuroanatomy, 27th ed. New York: McGraw-Hill Education; 2013.
The nerve roots supplying the upper extremity (C5-T1) join to form the brachial plexus. The terminal branches of the brachial plexus are the nerves to the upper arm, forearm, and hand: the axillary, musculocutaneous, radial, ulnar, and median nerves. Nerves that arise from the plexus proximal to the terminal branches supply the shoulder muscles (long thoracic, suprascapular, subscapular nerves), pectoral muscles (medial and lateral pectoral nerves), and sensation to the medial upper arm and forearm (medial brachial cutaneous and medial antebrachial cutaneous nerves).
The best way to learn the brachial plexus is to draw it over and over again (Fig. 16–2). A mnemonic theme in drawing it is the theme of threes. Figure 16-2 is a schematic of the left brachial plexus, ...