The spinal accessory nerve (cranial nerve XI) is a motor nerve (special visceral efferent) that innervates the sternocleidomastoid and trapezius muscles. The trapezius, a broad, flat, triangular muscle, has fibers in its superior pa that originate from the external occipital protuberance and the superior nuchal line, the ligamentum nuchae, and the spinous processes of veebrae C-7 through T-4. These upper fibers pass laterally to inse onto the acromion and the spine of the scapula. Activation of this pa of the muscle results in elevation of the point of the shoulder (acromion moves superiorly), as in shrugging. The muscles that promote movement of the arm away from the midline of the body include the deltoid, subscapularis, supraspinatus, and infraspinatus muscles which are innervated by branches of the brachial plexus, not the spinal accessory nerve (CN XI). When there is damage to the spinal accessory nerve, they may also have difficulty elevating (abducting) the arm above a horizontal plane because the superior fibers of the trapezius, along with the serratus anterior, externally rotate the scapula about an anteroposterior axis, which is required for elevation of the arm beyond a horizontal plane. However, this is a question of "magnitude": loss of trapezius function would have greater effect on scapular elevation than rotation (i.e. serratus anterior is better able to compensate for loss of external rotation than levator scapulae can for elevation). The trapezius adducts the scapula, whereas the major arm adductors include the pectoralis major, teres major, latissimus dorsi, and coracobrachialis muscles. These muscles are innervated by branches of the brachial plexus. The muscles that act to laterally rotate the arm include the teres minor, infraspinatus, supraspinatus, and the dorsal poion of the deltoid muscles. These muscles are innervated by branches of the brachial plexus, not the spinal accessory nerve (CN XI). Ref: Ropper A.H., Samuels M.A. (2009). Chapter 47. Diseases of the Cranial Nerves. In A.H. Ropper, M.A. Samuels (Eds), Adams and Victor's Principles of Neurology, 9e