A patient in the emergency room has been stabbed in the back of the neck and complains that he is unable to lift his shoulder. Which of the following nerves has likely been damaged?
**Core Concept**
The patient's symptoms suggest a lesion affecting the motor innervation of the trapezius muscle, which is responsible for lifting the shoulder. The trapezius muscle is innervated by the accessory nerve (cranial nerve XI), specifically its spinal root.
**Why the Correct Answer is Right**
The accessory nerve has two roots: a cranial root and a spinal root. The spinal root arises from the cervical spinal cord (C1-C5) and innervates the sternocleidomastoid and trapezius muscles. Damage to this nerve can result in weakness or paralysis of these muscles, leading to difficulty lifting the shoulder. The trapezius muscle is responsible for elevating the scapula, which is essential for abduction of the arm above 90 degrees.
**Why Each Wrong Option is Incorrect**
**Option A:** The suprascapular nerve, although involved in scapular rotation and abduction, does not innervate the trapezius muscle. Damage to this nerve would primarily affect the supraspinatus and infraspinatus muscles.
**Option B:** The long thoracic nerve innervates the serratus anterior muscle, which is responsible for forward elevation of the scapula, not lifting the shoulder.
**Option C:** The axillary nerve innervates the deltoid and teres minor muscles, primarily responsible for shoulder abduction and external rotation.
**Clinical Pearl / High-Yield Fact**
The accessory nerve is unique in that it has a dual origin, with both cranial and spinal roots. This distinction is essential for diagnosing and treating patients with suspected accessory nerve damage.
**Correct Answer: C. The axillary nerve innervates the deltoid and teres minor muscles, primarily responsible for shoulder abduction and external rotation.