What is the chief motor nerve that affects the muscles of the neck and back?

The chief motor nerve that affects the muscles of the neck and back is the Accessory Nerve. This cranial nerve, also known as cranial nerve XI, is responsible for innervating certain shoulder and neck muscles, specifically the sternocleidomastoid and trapezius muscles. These muscles play crucial roles in movements such as turning the head, shrugging the shoulders, and stabilizing the shoulder girdle.

Understanding the role of the Accessory Nerve helps in recognizing how this nerve's integrity is essential for various physical functions involving the neck and upper back. Damage or injury to this nerve can lead to weakness or paralysis in these muscles, significantly impacting a person's ability to perform activities that require head and shoulder movements.