Choice A. The extensor muscles of the wrist and digits are primarily innervated by branches of the radial nerve, which also provides sensory innervation to the posterior arm, forearm, and dorsolateral hand. The radial nerve is frequently injured in humeral mid-shaft fractures and use of improperly fitted crutches.
Choice B. The biceps reflex is mediated by C5 and C6 spinal nerves, with muscular innervation via the musculocutaneous nerve (lateral cord of the brachial plexus). Traumatic injuries to this system are uncommon but can occasionally be seen in high-velocity motor vehicle collisions.
Choice C. The ulnar nerve may be injured by fracture of the medial epicondyle of the humerus or, more distally, by deep lacerations of the anterior wrist. Symptoms include "claw hand" resulting from paralysis of the intrinsic muscles of the hand as well as sensory loss involving the medial hand.
Explanation:
Educational objective: Acute shoulder pain after forceful abduction and external rotation at the glenohumeral joint suggests an anterior shoulder dislocation, which may cause injury to the axillary nerve. The axillary nerve innervates the teres minor and the deltoid, and injury can result in weakened shoulder abduction and decreased sensation over the lateral shoulder.
Choice E. The long thoracic nerve innervates the serratus anterior muscle. Deep lacerations to the axillary region and axillary lymphadenectomy are common causes of long thoracic nerve injury. Damage causes scapula winging.
