There are seven cervical(neck) vertebrae, numbered C1 through C7. When these vertebrae become misaligned, the result is a condition known as vertebral subluxation. Nerves become trapped between the bony surfaces of the vertebrae and leads to neurological dysfunction leading to pain and numbness/heaviness.
Dysfunction related to the cervical spine leads to stressed muscles not only in the neck, but also in the entire shoulder girdle. This can also lead to arm and shoulder, thus the site of pain isn't always the site of dysfunction.
The primary function of the shoulder girdle is to provide an attachment point for the numerous muscles that allow the shoulders and elbow joints to move. When nerves become trapped — due to vertebral subluxation the muscles cease to function properly.