Spinal Accessory Nerve
The Spinal Accessory nerve innervates the sternocleidomastoid and the trapezius.
Unilateral injury to the spinal accessory nerve usually does not produce an abnormal head position. However, when turning the head against resistance, the weakness of the sternocleidomastoid becomes more evident. If the injury is severe enough to unilaterally paralyze the trapezius, the patient will not be able to elevate (shrug) the shoulder, retract the shoulder, or raise the arm superior to the horizontal level. This is observed as a drooping of the shoulder.