Brachial Plexus Structures Flashcards

A patient presents with a 'winged scapula,' where the medial border of the scapula protrudes posteriorly. This is most likely caused by an injury to which nerve that originates from the roots of the brachial plexus?

Answer: Long thoracic nerve

The long thoracic nerve innervates the serratus anterior muscle. Paralysis of this muscle, which is responsible for protracting and stabilizing the scapula, results in 'winging' of the scapula, where the medial border moves away from the rib cage. [13, 18] The long thoracic nerve arises directly from the roots C5, C6, and C7.

Which of the following structures of the brachial plexus is formed by the union of the anterior divisions of the superior and middle trunks?

Answer: Lateral cord

The brachial plexus is organized into roots, trunks, divisions, and cords. The lateral cord is specifically formed by the convergence of the anterior divisions from the superior (C5, C6) and middle (C7) trunks. [6, 12, 24]

A patient suffers a fracture of the surgical neck of the humerus. Which terminal branch of the posterior cord is most at risk, potentially leading to weakness in abducting the arm and loss of sensation over the 'regimental badge' area?

Answer: Axillary nerve

The axillary nerve, a terminal branch of the posterior cord, wraps around the surgical neck of the humerus. [2, 9] An injury here can damage the nerve, affecting the deltoid muscle (a primary arm abductor) and the teres minor, and causing sensory loss over the lateral aspect of the shoulder, an area often called the 'regimental badge' area. [2, 22]

A newborn is diagnosed with Erb's palsy following a difficult delivery. This condition, characterized by a 'waiter's tip' posture, involves damage to which part of the brachial plexus?

Answer: The superior trunk (C5, C6)

Erb's palsy, or Erb-Duchenne palsy, is an injury to the upper parts of the brachial plexus, specifically the superior trunk, which is formed by the union of the C5 and C6 nerve roots. [4, 8, 15] This injury affects muscles innervated by these roots, leading to the characteristic adducted, internally rotated arm with a pronated forearm, known as the 'waiter's tip' hand. [1]

Which terminal nerve of the brachial plexus characteristically pierces the coracobrachialis muscle before continuing down the arm to innervate the anterior compartment muscles?

Answer: Musculocutaneous nerve

The musculocutaneous nerve, a terminal branch of the lateral cord, is unique in that it pierces the coracobrachialis muscle. It then continues between the biceps brachii and brachialis muscles, providing motor innervation to all three muscles of the anterior compartment of the arm. [5, 11, 14]

An injury resulting from a sudden and forceful hyper-abduction of the arm, such as when grabbing onto something to break a fall, is most likely to damage which structure of the brachial plexus, leading to a 'claw hand' deformity?

Answer: Inferior trunk

This mechanism of injury typically causes a traction injury to the lower parts of the brachial plexus. The inferior trunk, formed by the C8 and T1 nerve roots, is most vulnerable. Damage here affects the ulnar and median nerves distally, leading to paralysis of the small intrinsic muscles of the hand and resulting in a characteristic 'claw hand' deformity. This specific injury is known as Klumpke's palsy. [1, 10, 23]