C5 and C6 roots form the superior trunk. Damage to this trunk results in Erb-Duchenne palsy, characterized by the arm hanging by side, medially rotated and forearm pronated (think of a "Waiter’s tip").
This nerve originates from the superior trunk. It innervates the supraspinatus muscle and infraspinatus muscle (involved with lateral rotation of the arm).
The C7 root forms the middle trunk. This contributes to the lateral pectoral, musculocutaneous, median, radial, axillary, thoracodorsal and subscapular nerves.
C8 and T1 roots form the inferior trunk. Injury to the inferior trunk can have many manifestations, classically including hand weakness/paralysis due to impairment of the ulnar and median nerves.
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