Which nerve primarily innervates the trapezius, with proprioceptive input from C3-C4?

• A. Dorsal scapular nerve (C5)
• B. Spinal accessory nerve (CN XI) with proprioceptive input from C3-C4.
• C. Axillary nerve (C5-C6)
• D. Long thoracic nerve (C5-C7)

Answer: (B)

The main idea is how the trapezius is controlled both in movement and in sensing its position. The trapezius is driven primarily by the spinal accessory nerve (cranial nerve XI), which provides the motor output needed for elevating, retracting, and rotating the scapula. Proprioceptive input—the sensory feedback about muscle length and tension—comes from the C3-C4 spinal nerves and travels with the same nerve pathway to give the brain a sense of the trapezius’s position during movement. The other nerves listed don’t supply the trapezius: the dorsal scapular nerve goes to the rhomboids and levator scapulae, the axillary nerve to the deltoid and teres minor, and the long thoracic nerve to the serratus anterior. So the spinal accessory nerve with proprioceptive input from C3-C4 best explains both the motor control and the sensory feedback for the trapezius.