The nerves of the lower neck are what mainly control the muscles of the upper back. What muscles are we referring to- your trapezius , rhomboids, shoulder muscles, and neck or upper back paraspinal muscles.
The paraspinals run along the spine up and down the length of it. Your rhomboids are basically from your spine to your scapula, otherwise known as your shoulder blade. Shoulder muscles that can often be affected are your deltoids and rotator cuff muscle. The trapezius muscle essentially covers the back of the neck and upper back.